Titanium Alloys-An Atlas of Structures and Fracture Features - Joshi

TITANIUMALLOYS

An Atlas of Structuresand Fracture Features

A CRC title, part of the Taylor & Francis imprint, a member of theTaylor & Francis Group, the academic division of T&F Informa plc.

TITANIUMALLOYS

An Atlas of Structuresand Fracture Features

Vydehi Arun Joshi

Boca Raton London New York

Published in 2006 byCRC PressTaylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742

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Library of Congress Cataloging-in-Publication Data

Joshi, Vydehi Arun.Titanium alloys : an atlas of structures and fracture features / Vydehi Arun Joshi.

p. cm.Includes bibliographical references and index.ISBN 0-8493-5010-7 (9780849350108)1. Titanium alloys--Fracture. I. Title.

TA480.T54J67 2006620.1'893226--dc22 2005022915

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and the CRC Press Web site at http://www.crcpress.comTaylor & Francis Group is the Academic Division of T&F Informa plc.

Dedication

To my husband Arun S. Joshi;my brother Dr. N. Narakanti Rao; andmy sisters Shanta, Saroja, and Neeraja.

Foreword

Titanium and its alloys find application in aerospace, thechemical and power industries, transportation, armament,and sports, but it is primarily the former that has driventhe development of this material. This wonderful bookdocuments the fractography of a set of aeronautical-gradetitanium alloys that failed in the laboratory under a widevariety of controlled testing conditions. The compositionsspan the range of different classes of titanium alloys —alpha, alpha/beta, and beta — as well as the titaniumaluminides that have been researched at the Defence Met-allurgical Research Laboratory (DMRL) in India over theyears or that have been produced at Mishra Dhatu NigamLimited (Midhani). The book, therefore, covers thebreadth and the range of titanium alloys that are used inIndia, as well as internationally. However, the book is alsomore than a simple fractographic handbook, since it doc-uments the underlying microstructure and test conditionsthat produced the fracture, together with a brief descrip-tion of the special fractographic features associated witheach combination of alloy, microstructure, and test con-dition. It is unique in that there is no document in theliterature today that captures the fractographic features of awide range of titanium alloys together with the conditions

that produced failure. It is, therefore, a book that will beused by researchers as well as those engaged in failureanalysis of titanium components in the aeronautical industry.

The author combines wide experience in the use ofscanning electron microscopy in fracture analysis with aneye for detail that has led to several award-winning micro-graphs and fractographs over the years. The book, there-fore, combines scientific detail with aesthetic appeal. Assomeone who has been involved in developing the under-standing of physical metallurgy of titanium and its alloys,I am very happy to see the publication of this edition. Itis important that the database that constitutes this book beupdated periodically as new alloys and applications oftitanium emerge. I am sure that the DMRL and Midhaniwill be at the forefront of these efforts in India, and Iwould love to see a Web edition of the book that reachesa wide audience and that could be updated in a regularmanner.

Dipankar Banerjee, Ph.D.Chief Controller Research & Development (AMS)Defence Research & Development OrganisationNew Delhi, India

The Author

Vydehi Arun Joshi graduated from Osmania University in1967 and joined the Defence Metallurgical Research Labo-ratory (DMRL) in the same year. She has been working atDMRL for the past 38 years. She completed her postgrad-uate studies and holds an M.Tech. degree from the Instituteof Technology, Banaras Hindu University, Varanasi. After adecade of service in the Optical Metallography Group, sheswitched to the field of transmission and scanning electron

microscopy, where she still works as a scientist. She haswide experience in microstructural characterization and frac-tography of different metals and alloys (titanium, nickel-based superalloys, aluminum, steels, and so forth) pertainingto various DMRL projects. She has also won numerousprizes in the metallography contests that are held annuallyby the Indian Institute of Metals, and she has several nationaland international publications to her credit.

Acknowledgments

Over the past few decades, I have been carrying out thescanning electron microscopy of different metals andalloys as a part of my regular work at the Defence Met-allurgical Research Laboratory (DMRL), Hyderabad,India. During this period I have had occasion to interactwith many scientists who have specialized in differentareas of metallurgy. The foremost among them is Dr. A.K.Gogia, scientist G, project director of the DMRL ProjectOffice (Materials), Kanchanbagh, Hyderabad, with whomI have had constant interaction while carrying out thescanning electron microscopy of titanium alloys. In fact,the idea of taking up the compilation work for this atlaswas initiated at his suggestion. I am exceedingly gratefulto him for the continuous technical support and advicethat he has provided me ever since, particularly in theinterpretation of the fractographs. Although Dr. Gogia hasnot formally coauthored this book, he has virtually ful-filled that role. He has also permitted me to make abundantuse of his doctoral thesis and other publications, and I amvery happy to acknowledge his contribution. In this con-text, I thank the Journal of Metallurgical and MaterialsTransactions and the authors — A.K. Gogia, D. Banerjee,and T.K. Nandy — of the paper entitled “Structure, tensiledeformation and fracture of Ti3Al-Nb alloy” for accordingme permission to use the SEM micrographs.

It was only after taking up the actual work of compi-lation that I realized the enormous magnitude of the activ-ities involved and the difficulties in executing the task. Iwas extremely fortunate in getting the support of my col-league Dr. K. Satyaprasad in this arduous stage of theproject. His constant support and presence while scanningand storing the entire set of photographs, titles, and cap-tions and then rendering them computer compatible forpublication, not only lightened my burden but also madethis tedious work pleasant and enjoyable.

When a sufficient number of fractographs and micro-structures were ready, they were bound together for ourpersonal in-house use. Dr. D. Banerjee, distinguishedscientist and chief controller of Research & Develop-ment, AMS, New Delhi, made me realize that this ref-erence work could be exhibited in a larger and widergallery, and could serve a more worthwhile purpose, ifdescriptive text were appended to each photograph andthe resulting work then brought out in book form. I am

extremely grateful to Dr. Banerjee for this suggestionand for his subsequent interest and follow-up. I particu-larly wish to thank him for readily agreeing to write theforeword to this book.

I am grateful to Dr. A.M. Sriramamurthy, director,Defence Metallurgical Research Laboratory, Hyderabad,for granting me permission to publish this book and alsofor allowing me to use the laboratory infrastructure andfacilities.

I am extremely thankful to the team that carried outthe failure investigation of a high-pressure compressorblade, namely, Dr. A.K. Gogia, Dr. K. Muraleedharan, Dr.D. Banerjee, and the Metallography and Electron ProbeMicro Analysis Groups of DMRL and Mishra DhatuNigam Limited (Midhani), Hyderabad, for the case studyreported in Chapter 9 of this book.

I am also thankful to Dr. A. Venugopal Reddy, regionaldirector, Regional Centre for Military Airworthiness(Materials), Hyderabad, for sharing his publishing expe-riences with me. I have benefited greatly from his advice,and I vividly recall my long association with him andremember the bygone days when he inducted me into thescience of fractography.

A work of this nature could never have been possiblewithout the active cooperation of scientists and other exec-utives who are working on titanium alloys and in alliedfields. I wish to thank Mr. S.N. Jha and Dr. T.V.L.Narasimha Rao of the Aeronautical Material Testing Lab-oratory, Hyderabad & Midhani, Hyderabad, for providingfractured test specimens. Likewise, I am thankful to Dr.K. Muraleedharan, Dr. T.K. Nandy, T. Raghu, Amit Bhat-tacharjee, G.S. Sharma, A.G. Paradkar, Dr. A.K. Singh,Dr. P.K. Sagar, H. Mishra, and S.M. Gupta for providingme with fractographs and specimens for scanning electronmicroscopy. I am also indebted to Dr. P. Ghosal, Dr. R.Balamuralikrishnan, and D.V. Sridhara Rao for helpingme prepare the disk book. Thanks are also due to theMetallography Group for the support given.

Lastly, I wish to thank my husband, Arun S. Joshi, forhis patience while I worked late hours, after which hedrove me safely home in one piece, despite the sustainedefforts of the local city drivers to test his braking skills.A thrilling narration of our narrow escapades might wellbe the subject of my next book!

Preface

Titanium is a newcomer among the metals that have gainedwidespread industrial importance. Alloys of titanium havefound a niche market even in the aerospace sector, wherematerial requirements are very demanding. The reason forthis proliferation of applications is its excellent blend of lowdensity and high strength, superior corrosion resistance, andstrength at moderately high temperatures.

The properties of any material depend upon its micro-structure, which in turn is defined by its composition andprocessing history. Hence, in the alloy development stage,it becomes mandatory to test the material and study themicrostructure and fracture features of the tested speci-mens. During this development, a comprehensive compi-lation of micrographs of different alloys of one metal canhelp in understanding the related experimental work. Thisatlas is intended to fulfill this need for titanium alloys byserving as a ready reference source of detailed fracto-graphic and microstructural analyses.

Chapter 1 provides an introduction to fractography,with typical fractographs of ductile, fatigue, intergranular,and cleavage fractures in general and of titanium alloysin particular. Chapter 2 covers the physical metallurgy oftitanium alloys and the evolution of their microstructures,while Chapter 3 presents the compositions of some com-mercially used titanium alloys. Chapters 4 through 8 are

compilations of more than 300 photographic illustrations,with accompanying descriptions, of the microstructuresand fracture features of α-, α+β-, and β-titanium alloysand Ti3Al- and TiAl-based titanium aluminides that weretested under various conditions. The concluding chapter(Chapter 9) of this atlas deals with the case study of afailed titanium blade. A CD with the included imagesaccompanies this atlas.

Thus, this compilation — arguably the largest collec-tion of microstructures and fractographs of titanium alloysever assembled within a single book — provides exhaus-tive information for engineers and researchers working inthese areas. This atlas is an outgrowth of the alloy devel-opment work carried out by the Titanium Alloy Group andthe Electron Microscopy Group in the Defence Metallur-gical Research Laboratory, Hyderabad, India. I wouldlike to thank all of those who have helped in the makingof this book. I hope that this combined effort will helpin understanding the rich insights into the microstructureand fracture features of titanium alloys that have beengained by optical and scanning electron microscopicobservations.

Vydehi Arun JoshiHyderabad, India

AbbreviationsAC Air cooled

bcc Body-centered cubic

BSE Backscattered electron

EPMA Electron-probe microanalysis

FC Furnace cooled

fcc Face-centered cubic

hcp Hexagonal close packed

OQ Oil quenched

ppm Parts per million

RT Room temperature

SE Secondary electron

SEM Scanning electron microscope

ST Solution treated

STA Solution treated and aged

TEM Transmission electron microscope

WQ Water quenched

Contents

Chapter 1 Introduction to Fractography.........................................................................................................................1

1.1 Dimple Rupture .........................................................................................................................................................11.2 Cleavage.....................................................................................................................................................................11.3 Fatigue........................................................................................................................................................................21.4 Intergranular...............................................................................................................................................................3

Chapter 2 Physical Metallurgy of Titanium Alloys .......................................................................................................7

2.1 Introduction................................................................................................................................................................72.2 Application of Titanium Alloys.................................................................................................................................72.3 Effect of Alloying Elements ......................................................................................................................................92.4 Types of Titanium Alloys ..........................................................................................................................................9

2.4.1 Alpha (α) Alloys..........................................................................................................................................102.4.2 Near α Alloys ..............................................................................................................................................102.4.3 α+β Alloys...................................................................................................................................................102.4.4 Metastable β Alloys.....................................................................................................................................102.4.5 Beta Alloys...................................................................................................................................................102.4.6 Titanium Aluminides ...................................................................................................................................10

2.5 The Microstructure of Titanium Alloys ..................................................................................................................102.5.1 Conventional Titanium Alloys.....................................................................................................................112.5.2 Titanium Aluminides ...................................................................................................................................15

Chapter 3 Chemical Compositions...............................................................................................................................17

Chapter 4 Alpha Alloys ................................................................................................................................................19

Chapter 5 Near-Alpha Alloys .......................................................................................................................................23

Chapter 6 Alpha + Beta Alloys ....................................................................................................................................59

Chapter 7 Beta Alloys...................................................................................................................................................97

Chapter 8 Titanium Aluminides .................................................................................................................................111

8.1 Ti3Al-Based Alloys ................................................................................................................................................1118.2 TiAl-Based Alloys .................................................................................................................................................111

Chapter 9 Case Study: Failure Investigation Report of IMI 550 High-Pressure Compressor (HPC-I) Aero Engine Blade ....................................................................................................................................203

9.1 Introduction............................................................................................................................................................2039.2 Investigation...........................................................................................................................................................203

9.2.1 Chemical Analysis .....................................................................................................................................2039.2.2 Microstructure............................................................................................................................................2039.2.3 Fractography ..............................................................................................................................................2039.2.4 Stress-Concentration Effects of a Notch...................................................................................................2049.2.5 Analysis of the Deposits............................................................................................................................205

9.3 Conclusion .............................................................................................................................................................205

References.......................................................................................................................................................................219

Index ...............................................................................................................................................................................221

1

1

Introduction to Fractography

Materials fracture either by transgranular (through grains)or intergranular (along grain boundaries) fracture paths.There are basically four principal modes of fracture:

1. Dimple rupture2. Cleavage3. Fatigue4. Intergranular

The detailed features of the above modes of fracture aregiven below.

1.1 DIMPLE RUPTURE

Most of the structural alloys fail by a mechanism known asmicrovoid coalescence when fractured under continuallyincreasing loads. Microvoids nucleate at the interfacesbetween matrix and inclusions, second-phase particles,grain boundaries, or imperfections such as microcracksand microporosity. As the load increases, microvoids growand coalesce and eventually fracture. This mode of frac-ture is called dimple rupture.

The shape and depth of the dimples or microvoids canbe related to the size of and spacing between initiatingparticles, to the applied stress (tension, shear, or torsion),and to the fracture toughness of the specimen. Whennucleation sites are few and widely spaced, the microvoidsgrow to large dimples. Small dimples are formed whennumerous nucleation sites are activated and individualmicrovoid growth is limited. Some very ductile materialshave deep conical dimples.

The increase in free surface resulting from microvoidnucleation can be great. Because the growth of free surfaceoccurs by plastic deformation, strain markings are occa-sionally evident on the walls of some large dimples. Thesemarkings include serpentine glide, ripples, and stretching.A typical ductile fracture of titanium alloy is shown inFigure 1.1.

1.2 CLEAVAGE

This type of fracture occurs on well-defined crystallo-graphic planes and is a low-energy fracture. Generallymetals with body-centered cubic (bcc) and hexagonalclose-packed (hcp) crystal structure only fracture by

cleavage mechanism. However, even metals with face-centered cubic (fcc) crystal structure like Al also have beenobserved to cleave on contact with mercury. In brass,cleavage by stress corrosion cracking is observed.

Cleavage may not indicate the relative ductility of thematerial. It describes only the fracture mechanism. Acleaved fracture surface shows features like cleavagesteps, river markings, feather markings, herringbone struc-tures, and tongues, since the materials are polycrystallineand contain imperfections, precipitates, inclusions, etc.Flat featureless cleavage surfaces are very rarely seen.

River marking

is one of the main cleavage featuresand is usually observed within a grain. This is astep between a cleavage crack-segment on thecleavage planes. The branches of the river patternjoin in the crack-propagation direction and canthus be used to find the fracture direction. Atypical cleavage fracture in titanium alloy isshown in Figure 1.2.

Feather markings

resemble a chevron pattern inthat they point back in the direction of localcrack. They are an array of very fine cleavagesteps on a cleavage facet.

Herringbone structure

forms as a result of theinteraction of an advancing cleavage crack withdeformation twins. This is generally seen in bccmaterials.

Tongues

are formed by the local deviation of acleavage plane crack as it intersects a boundarybetween a deformation twin and the matrix.

Wallner lines

are occasionally seen on brittlephases. They are parallel cleavage steps creatinga rippled pattern. They cross each other and aredifferent from fatigue striations, which do notcross each other. They are believed to resultfrom the interaction of a simultaneously prop-agating crack front and an elastic shock wavein the material.

Quasicleavage

is a mechanism involving a mixtureof both microvoid coalescence and cleavage. Inquasicleavage, there is no apparent boundarybetween a cleavage facet and a dimpled area bor-dering the facet. This mode of fracture is com-mon in high-strength materials.

2

Titanium Alloys: An Atlas of Structures and Fracture Features

1.3 FATIGUE

This type of fracture occurs by damage from cyclicstresses, i.e., fatigue. The crack growth due to fatigueleaves clear fractographic evidence known as fatigue stri-ations. Each fatigue striation has been shown to be theresult of a single stress cycle. Stage 1 is the initial stageof fatigue fracture and is attributed to slip-plane fracturefrom repeated reversing of the operative slip system.Fatigue striations are not generally seen during the firststage. Striations are formed during stage 2 fatigue cracking.Fatigue fracture in a titanium alloy is shown in Figure 1.3.

High-cycle fatigue generally has closely spaced, well-defined fatigue striations. In low-cycle fatigue, striationsappear to be broad and widely spaced and often discon-tinuous. Large second-phase particles and inclusions canchange the local crack growth rate and resulting fatigue-striation spacing. A fatigue crack approaching a particlecan briefly retard if the particle remains intact or accelerateif the particle cleaves. Small particles have little effect onthe striation spacing or crack growth.

FIGURE 1.1

Typical ductile fracture in a Ti alloy showing fine equiaxed

dimples.

10 μm


3

Tire cracks are fracture features associated with high-stress, low-cycle fatigue. These are seen on steep slopesof the fracture surface. Tire cracks are caused either bymechanical damage to the fracture caused by repeatedimpact and the relative motion of two mating surfaces orby loose particles caught between mating surfaces. Theyare not fatigue striations, but they indicate fatigue.

Fatigue striations usually bow outward in the directionof local crack propagation. Fatigue striations are best seenon the fracture surface of moderately hard alloys.

1.4 INTERGRANULAR

As the name implies, intergranular fracture occurs bygrain-boundary separation, i.e., between grains. Intergran-ular fracture is very clearly distinguishable from othertypes of fracture. The causes for this type of fracture arethe presence of weak or brittle grain boundary phases aswell as environmental or mechanical factors, such as stresscorrosion, hydrogen damage, or a triaxial state of stress.Elevated-temperature creep-to-rupture fractures are oftenintergranular.

FIGURE 1.2

Typical transcrystalline cleavage fracture in titanium aluminide.

100 μm

4


Sometimes a small layer of microvoid coalescence isseen at the grain interfaces. Some of these fracture featureshave a “rock candy” appearance. A typical intercrystallinefracture in a titanium alloy is shown in Figure 1.4.

The failure of a component in service is not alwaysdue to one type of fracture mode; it could be the result of

a mixed-mode fracture, i.e., the operation of two or moreintermingled mechanisms of fracture. The shape, size,cross section of the component, and the conditions pre-vailing during failure have an effect on the fracture fea-tures and the mode of fracture.

FIGURE 1.3

Typical fatigue fracture showing striations.

1 μm


5

FIGURE 1.4

Typical intercrystalline fracture in a Ti alloy.

1 μm

7

2

Physical Metallurgy of Titanium Alloys

2.1 INTRODUCTION

Titanium metal was first discovered by the English chem-ist William Gregor in 1971 in the black magnetic sandilmenite, and the metal was named “titanium” after thetitans of Greek mythology, a symbol of power and strength[1]. Titanium is the fourth-most-abundant metal in theEarth’s crust, the other three being aluminum, iron, andmagnesium. Titanium has low density and high strength,good corrosion and erosion resistance to different media,good oxidation resistance, and moderate strength at hightemperatures, making it attractive for industrial applica-tions. It has a number of features that distinguish it fromother light metals and that make its physical metallurgyboth complex and interesting.

The yield strength, fracture toughness, and creep prop-erties of titanium alloys can be increased tremendously.These alloys can also be tailored to achieve a desiredcombination of properties by changing the alloying andprocessing parameters. A change in the alloy compositionand processing modifies the microstructure. This changeis due to the phase transformation of various equilibriumand nonequilibrium phases present in the alloy system.The resultant mechanical properties depend on the natureof deformation, fracture of the microstructural constitu-ents, and the interaction between constituents. The phys-ical and electronic properties of the titanium atom,because of its position in the periodic table, make it suit-able for alloying with other elements to produce a widerange of alloys. Titanium has allotropic phase transforma-tion from high-temperature

β

phase with body-centeredcubic structure to the room-temperature

α

phase having aclosely packed hexagonal crystal structure. There is astrong dependence of the transformation temperature onthe alloy composition, and a variety of phase transforma-tions are possible. All these allow a wide variety of micro-structures, which can be optimized by controlling the ther-momechanical processing.

2.2 APPLICATION OF TITANIUM ALLOYS

Titanium and its alloys are used for aerospace, chemical,general engineering, and biomedical applications becausethey show an astonishing range of mechanical properties

(Figure 2.1). The unique high strength-to-weight ratio,easy formability, and fatigue resistance led to the intro-duction of titanium in aerospace applications like rocketengine parts, fuel tank, gas bottles, etc. It is also used inthe airframe structures, such as landing-gear beams,hydraulic tubings, wing boxes, spacers, bolts, etc. Tita-nium alloys are used in fan-jet engines for which largefront fans are required. The high specific strength of tita-nium along with the metallurgical stability at high tem-peratures and low creep rates make it favorable for jetengine components like blades and discs in the low andintermediate sections of compressors.

The next important area of application of titaniumalloys is chemical and general engineering. The out-standing corrosion resistance of titanium in many envi-ronments is the prime reason for its use in these industries.For low-stress applications, commercially pure (CP) tita-nium is generally used, and for high-strength applica-tions Ti-6Al-4V or Ti-13Nb-13Zr alloys are used. Inthe petrochemical industries, CP titanium grades andTa- or Pd-containing alloys are utilized for outstandingcorrosion resistance. Titanium alloys are used in marineand offshore applications for their excellent corrosionresistance in seawater and in sour hydrocarbon atmo-spheres. Figure 2.2 illustrates typical applications of tita-nium alloys.

In the field of biomedical applications, titanium isused for prosthetic devices for bone and joint implants,heart valves, and dental implants. These are made fromCP titanium, Ti-6Al-4V, or recently developed alloys suchas Ti-6Al-7Nb.

In the automobile sector, titanium engine valves havebeen used by Toyota in Japan. Titanium products likesprings are also used in racing cars and motorcycles. Amore recent application of titanium is in architecture, aconcept first used in Japan. The Guggenheim Museum inBilbao, Spain, is the most spectacular titanium building.Besides these applications, titanium is also used in sportsequipment, such as spikes for running shoes used bysprinters, tennis rackets, mountain crampons, ice axes,bicycle frames, etc. Titanium is also finding increasinguse in jewelry and fashion industries.

8


FIGURE 2.1

Range of yield strength and toughness in titanium alloys at room temperature.

FIGURE 2.2

General characteristics and typical applications of titanium alloys.

20 40 60 80 100Fracture toughness, KICMPa√m

200

400

600

800

1000

1200

1400

TiAl AlloysTiAl Alloys

Ti 6246IMI 579 IMI 550

CP Ti

Ti-3-2

6-2-4

Ti 215

IMI 685 IMI 829 Ti-SAl-2.5 Sn

Ti-6-4

0.2 %

Y.S

. MPa

Ti 10-2-3

BETA III

Characteristics of Titanium Alloys

Creep strength

Oxidation resistance

Microstructural stability

Corrosion resistance

Low-densityhigh-strength

easy formability

Fatigue resistance

- Fan discs and blades - Compressor discs and blades - Casings, after burner cowlings - Flange rings, spacers, bolts, etc.

- Landing gear beams - Hydraulic tubings, wing boxes - Spacers, bolts, etc.

- Submarine hulls - Propellers, pumps - Deep drilling pipes

- Heat exchangers- Reaction vessles- Tanks, pumps- Valves, tubes- Anodes for electrolysis cells- Consumer goods and jewelry

Applications of Titanium Alloys

Chemical and Process Industry

Marine Applications

Airframe Structures Jet Engines

Biomedical Applications

- Bone and joint implants- Heart valves- Dental implants

Automotive Applications

- Springs, fasteners, piston valves - Rocket engine parts - Fuel tanks, gas bottles


9

2.3 EFFECT OF ALLOYING ELEMENTS

Titanium is one of the transition metals and has an atomicnumber of 22 and atomic weight of 47.90. Table 2.1 sum-marizes the important physical properties [2]. Titaniumexists in two allotropic modifications, a high-temperature

β

that is stable between 882˚C and its melting point of1668˚C. The

α

modification of titanium exists at temper-atures below 882˚C.

Titanium has an incomplete shell in its electronicstructure, which enables the formation of solid solutionswith most substitutional elements having a size factorwithin ±20%. Elements like carbon, oxygen, etc. forminterstitials. The stabilization of

α

or

β

phase depends onthe number of electrons per atom of the alloying element(or the group number). Alloying elements with an elec-tron/atom (e/a) ratio of less than 4 stabilize the

α

phase, andelements having a ratio greater than 4 stabilize the

β

phase.Elements with an e/a ratio of 4 are neutral [3]. Kornilov [4]classified the elements in the periodic chart into four majorgroups, depending on their interaction with titanium.

1.

Continuous solid-solution-forming elementswith

α

or

β

titanium

: Zirconium and hafniumhave an outer-shell electronic configurationidentical to that of titanium. The structure isalso isomorphic to titanium. Thus the phasediagrams with these elements show continuous

α

and

β

solid solutions. Vanadium, niobium,tantalum, and molybdenum are isomorphic to

β

-titanium and form a continuous solid solutionwith the

β

allotrope of titanium. These elementshave limited solubility in

α

phase.2.

Limited solid-solution-forming elements with

α

and

β

titanium

: Chromium, manganese, iron,cobalt, nickel, and copper undergo eutectoidtransformation and lower the

β

transus. Withthe increase in the group number, the maximumsolubility in

β

titanium decreases and eutectoidtemperature increases. Aluminum, gallium, andindium show a peritectoid reaction and raise the

β

transus. These elements have higher solubilityin

α

titanium.3.

Ionic and covalent compound-forming ele-ments

: Fluorine, chlorine, bromine, iodine, sul-fur, selenium, tellurium, and phosphorous formionic and covalent compounds with titanium.They do not go into solid solution in

α

or

β

titanium.4.

Elements not interacting with titanium

: Exceptberyllium, which has limited solubility in

β

tita-nium, no other alkali or alkaline earth metalinteracts with titanium.

Boron, carbon, oxygen, nitrogen, and hydrogen forminterstitial solid solutions because of the large size differ-ence between the atoms of titanium and these elements.There is a difference in solubility of these elements in

α

and

β

titanium. Hydrogen is more soluble in

β

phase andreacts eutectoidally.

2.4 TYPES OF TITANIUM ALLOYS

Titanium has two allotropic modifications:

α

, which hasa closely packed hexagonal structure, and

β

, having abody-centered cubic structure. Various elements formingsolid solution with titanium are classified on the basis oftheir effect on the solubility of

α

or

β

phases. Elementsstabilizing

α

phase are known as

α

stabilizers (Al, Ga, O,N, C), and elements stabilizing

β

phase are known as

β

stabilizers (V, Mo, Nb, Fe, Cr, Ni, etc.). Some of theelements like Sn and Zr are neutral, as they do not stabilizeeither

α

or

β

phase, though they enter into solid solutionwith titanium.

Aluminum is the only

α

stabilizer of commercialimportance and forms a constituent of most of the com-mercial titanium alloys. The aluminum content is normallyrestricted to 7% or aluminum equivalent to 9% in thecommercial titanium alloys to avoid precipitation of Ti

3

Alphase, which leads to severe embrittlement.

Aluminum equivalent (Rosenberg criterion [5]) = Al + Sn/3 + Zr/6 + 10(O + C + N) (2.1)

TABLE 2.1Physical Properties of Unalloyed Titanium

Property Value

Atomic number 22Atomic weight 47.9Crystal structure:

α

-hcp

β

-bcc

c = 4.6832

±

0.0004 Åa = 2.9504

±

0.0004 Åc/a = 1.5873a = 3.28

±

0.003 ÅDensity 4.54 g/cm

3

Compressibility 0.8

×

10

−

6

cm

2

/kgCoeff. of thermal expansion at 20˚C 8.4

×

10

−

6

cm/cm/KThermal conductivity 0.041 Cal/cm/s/KSpecific heat 0.125 Cal/K/g

α

to

β

transus 882˚C (1155.5K)Latent heat of transformation 1050 Cal/moleHeat of fusion 5020 Cal/moleMelting point 1668˚C (1941K)Heat of vaporization 112,500 Cal/moleBoiling point 3260˚C (3533K)Electrical resistivity:

High purityCommercial purity

42

μΩ

-cm55

μΩ

-cmModulus of elasticity 11.6

×

10

11

dyne/cm

2

10


Recent developments in Ti alloys exploit the high-temperature properties of intermetallics Ti

3

Al (

α

2

) andTiAl (

γ

). Molybdenum, vanadium, niobium, and tantalum(isomorphous with titanium), which are the

β

-stabilizingelements, are generally the preferred alloying additions incommercial Ti alloys. Iron and chromium are also addedin limited amounts, although they are eutectoid-forming

β

stabilizers.Based on the alloying additions and phases present in

the microstructures, Ti alloys are classified as follows.

2.4.1 A

LPHA

(

αααα

) A

LLOYS

These are single-phase alloys, solid solution strengthenedby the addition of

α

stabilizers or neutral alloying ele-ments. Alpha alloys have good stability and good high-temperature properties but are not amenable to heat treat-ment for microstructural property modifications.

2.4.2 N

EAR

αααα

A

LLOYS

Small additions (1 to 2%) of

β

stabilizers improve thestrength and workability and are a good compromisebetween the higher strength of

α

+

β

alloys and the creepresistance of simple

α

alloys. The most widely used com-mercial high-temperature Ti alloys for aero-engine appli-cation belong to this class. They are primarily

α

alloyscontaining some amount of retained

β

in the final micro-structure.

2.4.3

αααα

+

ββββ

A

LLOYS

These alloys contain larger amounts of beta stabilizers (4to 6%). Beta alloys can be heat-treated to develop a varietyof microstructures and mechanical property combinations.Ti-6Al-4V, the most widely used alloy, belongs to thisclass.

2.4.4 M

ETASTABLE

ββββ

A

LLOYS

In alloys containing 10 to 15% of

β

stabilizers,

β

phaseis retained at room temperature in a metastable condition.This metastable

β

can be aged to form very fine Widman-stätten

α

in the matrix of enriched

β

. These alloys havehigh strength, toughness, excellent hardenability, andforgeability over a wide range of temperatures. Thesealloys contain small amounts of

α

-stabilizing elements asstrengthening agents. Beta and near-

β

alloys offerincreased fracture toughness over

α

+

β

alloys at a givenstrength level. They are finding increasing use in aircraftstructural applications.

2.4.5 B

ETA

A

LLOYS

Very large addition (30%) of

β

stabilizers results in retain-ing

β

as a stable phase at room temperature. Beta alloys

resemble refractory metals in their high densities and poorductility and are used for highly specialized burn-resis-tance and corrosion-resistance applications.

2.4.6 T

ITANIUM

A

LUMINIDES

A number of attractive intermetallic alloys have beendeveloped with useful ductility and strength. These alloysare based on the intermetallic compounds Ti

3

Al (

α

2

) andTiAl (

γ

). The alloys based on Ti

3

Al usually contain a largeamount of Nb additions and other

β

stabilizers (10–25 at.%, 20–45 wt.%). They consist of

β

phase or B2 phase toimprove their ductility. TiAl is also alloyed with

β

stabi-lizers such as Nb, V, Mn, and Cr in limited amounts.Recently, alloys based on compound Ti

2

AlNb (O-phase)are also under evaluation. All these alloys possess excel-lent high-temperature properties and good oxidation resis-tance but have poor ductility and fracture toughness. Thishas limited their use in commercial applications. Com-mercial titanium alloys use a very narrow compositionrange of

α

stabilizers (as dictated by the Rosenberg cri-terion in Equation 2.1). This has been a restraining factorin the development of titanium alloys. The situation maychange if the alloys based on Ti

3

Al and TiAl find com-mercial application. The range of compositions availablein titanium alloys may then expand substantially.

2.5 THE MICROSTRUCTURE OF TITANIUM ALLOYS

Titanium alloys can exhibit a wide variety of microstruc-tures depending upon alloy chemistry, processing, andheat treatment. This is made possible because titaniumand its alloys exhibit a broad range of phase transforma-tions. Some of these transformations are related to the

α

to

β

allotropic transformations, while others are precipi-tation reactions that involve the formation of metastabletransition phases and equilibrium that occur during thedecomposition of the metastable

α

or

β

phases. The sym-bols and definitions of various phases are listed in Table2.2 [2]. The latter transformation usually occurs in morehighly alloyed situations, and the resulting microstructuresare very complex in such alloys. The phase transforma-tions and resultant microstructures in alloys based onordered intermetallics Ti

3

Al (

α

2

), Ti

2

AlNb (O), and TiAl(

γ

), while being similar to conventional titanium alloys inmany aspects, are still more complex. In addition to themicrostructural variations evolving through various phasetransformations, morphological changes of the constituentphases due to other metallurgical processes such as recrys-tallization, spheroidization, etc. can lead to many moretypes of structural variations. Some of the major micro-structural variations usually generated in titanium alloysare presented in this section.


11

2.5.1 C

ONVENTIONAL

T

ITANIUM

A

LLOYS

A central point in the evolution of microstructures in tita-nium alloys is the

α→β

transformation temperature, gen-erally referred to as the

β

transus temperature, since itseparates the single-phase

β

field from the two-phase

α

+

β

field. A schematic pseudo binary phase diagram and atime, temperature, and transformation (TTT) diagram [6]for titanium alloy are shown in Figure 2.3 and Figure 2.4,respectively, to illustrate the fact. Titanium alloys, whenheat-treated above the

β

transus (specific temperature is afunction of the alloy chemistry), are in single-phase

β

. Oncooling through the

β

transition temperature,

β

can trans-form to various equilibrium or nonequilibrium phases,depending upon the cooling rate and alloying content. Onfaster cooling (like water quenching or oil quenching), theβ phase can transform martensitically (Figure 2.5) to α′(hcp) or α″ (orthorhombic); with increasing β stabilizingelement, there is an increasing tendency to form α″ inpreference to α′. On slower cooling, β can transform bynucleation and growth to Widmanstätten α phase (Figure2.6). The morphology of Widmanstätten α phase maychange from a colony of similarly aligned α laths to abasket-weave arrangement with an increase in cooling rateor alloying content. Moreover, lamellar structure becomesfiner as the cooling rate is increased. On slower cooling,α phase is also present on prior β grain boundaries. Inaddition to the transformation products (α, α′, α″), themicrostructure may retain small amounts of β phase,depending upon the alloying content. The amount ofretained β in the microstructure on cooling from β goeson increasing as the solute content is increased. The α′ orα″ martensite decomposes upon subsequent aging to pre-cipitate fine β, which leads to useful increments instrength. In some alloys, additional intermetallic phasessuch as silicides may form upon aging of martensites.

In highly β stabilized alloys, the β phase may beretained completely as a metastable phase on fast cooling.However, on slow cooling, the α phase can precipitate outat the grain boundaries and within the grain, the amountof α being a function of cooling rate and β-stabilizercontent. In some compositions, athermal ω may form in

TABLE 2.2Phases Observed in Titanium Alloys

Phase Symbol Description

α Low-temperature allotropic form of titanium with an hcp structure; exists below β transus

β High-temperature allotropic form with bcc structure; exists at low temperature as metastable or stable phase in highly enriched alloys

α2 Ti3Al; exists over a wide range of Al content; has an ordered hexagonal structure DO19

B2/β2 Ordered bcc phase with CsCl structure; Ti2AlNb ordered version of high-temperature bcc allotrope; can exist at low temperature as a metastable phase

O Ti2AlNb with orthorhombic structure; can exist over a broad range of Al and Nb content

γ TiAl with L10 structure; extends over a wide range of Al content

α′ Nonequilibrium phase due to martensitic transformation; hcp structure

α″ Martensite with orthorhombic structureω A high-pressure allotrope of titanium with

hexagonal structure; occurs as a transition phaseIntermetallic precipitates

Several intermetallic precipitates can occur, depending upon the alloy (TiZr)5Si3; Ti2Cu are prominent precipitates

B1/β′ bcc phases of different composition than the matrix; occurs as a result of phase separation in β-stabilized alloys

FIGURE 2.3 Pseudo binary schematic phase diagram of α+βtitanium alloys.

β stabilisers

β

ωS

MSαTe

mpe

ratu

re

FIGURE 2.4 TTT curve of a typical α+β titanium alloy.

α + β

β transusβ

Martensite

Time

Tem

pera

ture

12 Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 2.5 Ti-6Al-4V, β heat-treated at 1020˚C/20min/WQ. Optical micrograph shows martensitic structure with prior β boundaries.

FIGURE 2.6 Ti-6Al-4V, β heat-treated at 1020˚C/20min/FC. Optical micrograph shows Widmanstätten α structure with α phasepresent on prior β grain boundaries.

Physical Metallurgy of Titanium Alloys 13

the β phase during quenching. The athermal ω phase formsas very fine precipitates (2 to 5 nm).

The metastable β phase decomposes upon subsequentaging to precipitate fine α phase. The aging leads to asignificant increase in strength, while ductility registers adecline. However, strength and ductility combination inthese alloys can be optimized by selecting the appropriatecombination of aging temperature and time. Two otherdecomposition reactions in metastable β may occur at lowtemperatures: ω formation in lean β alloys and a phase-separation reaction ω phase → β1+β2 in richer alloys. Theformation of ω phase is considered undesirable becauseits presence can cause severe embrittlement of the alloyconcerned and should be avoided by controlling the agingcondition. β-phase-separation reaction has not receivedmuch attention because it is not considered in commercialalloys. Both ω phase reaction and β separation reactionmay affect morphology and distribution of α phase insome alloys, since α phase may form indirectly fromeither the ω or β1 phases.

The microstructure resulting from the solution treat-ment above the β transus and transformation of β phaseare generally referred as “transformed β” or β heat-treatedstructure, irrespective of the finer details of the microstruc-ture. In addition to the α-phase morphologies that resultfrom martensitic transformation or nucleation and growth ofthe α phase (generally termed as secondary α), thermome-chanical processing at temperatures in the two-phase α+βregion has an important effect on α-phase morphology. Hotworking below the β transus (in α+β-phase field) results in

recrystallization of α phase to equiaxed morphology(referred to as primary α). The aspect ratio of primary αphase is determined by temperature, strain rate, and extentof hot working in the two-phase region. Solution heattreatment of α+β-worked alloys permits control over thefinal duplex microstructure. The relative volume fractionof primary α and transformed β can be controlled bysolution-treatment temperature in the two-phase field andcooling rate from the solution-treated temperature. Theeffect of cooling rate on the microstructure from a givensolution-treatment temperature is shown in Figure 2.7, Fig-ure 2.8, and Figure 2.9. The β phase present at the solution-treated temperature undergoes transformation to α/α′/α″,depending upon the cooling rate and β-phase chemistry, asdescribed earlier. These types of microstructures are com-monly known as α+β structures or equiaxed α + trans-formed-β structures. The α+β structures exhibit much finerβ grain size than β heat-treated structures. Due to anoma-lously high diffusion rate in the β phase, solution-treatedtimes for β heat treatments are generally very short. In α+βheat treatments, β grain growth is restricted by the presenceof second phase (α) at the solution-treatment temperatures.

Apart from the distribution of α and β phases in themicrostructures as discussed above, there are other struc-tural features that occur on a much finer scale. Precipita-tion of α2 (Ti3Al) in the α phase of some alloys afterprolonged thermal exposure and precipitation of other inter-metallics, such as Ti2Cu (in Ti-Cu alloy) and silicides, areexamples of fine-scale microstructural features. Precipitation

FIGURE 2.7 Ti-6Al-4V, α+β heat-treated at 960˚C/1h/WQ. Optical micrograph shows equiaxed α and transformed β microstructure.


FIGURE 2.8 Ti-6Al-4V, α+β heat-treated at 960˚C/1h/AC. Optical micrograph shows equiaxed α and transformed β microstructure.

FIGURE 2.9 Ti-6Al-4V, α+β heat-treated at 960˚C/1h/FC. Optical micrograph shows equiaxed α and transformed β microstructure.The volume fraction of α increases with a decrease in cooling rate, and transformed β becomes coarser.

Physical Metallurgy of Titanium Alloys 15

of α phase in fact imposes an upper limit on α stabilizercontent in commercial titanium alloys, as described earlier.

2.5.2 TITANIUM ALUMINIDES

The microstructural evolution in titanium aluminide alloysexhibits startling similarities to conventional titaniumalloys. Apart from the fact that α and β undergo orderingtransformations to α2 (Ti3Al)/O (Ti2AlNb) and B2 struc-tures, the morphological changes on transformation fromβ or B2 phases in Ti3Al/Ti2AlNb-based alloys to α2/Ophases are very similar to those observed in β → α trans-formation in conventional titanium alloys. The Ti3Al/Ti2AlNb-based alloys can also be processed and heat-treated below the β/B2 transus temperature to achieveequiaxed α2/O + transformed β (B2) structures. Also thearrangement of α2/O laths, on cooling from the β/B2 heat-treatment temperature, changes from basket-weave to col-ony structure as in the conventional titanium alloys. How-ever, much more complex microstructures, especially inthe finer scale, can be generated in these alloys due to theretention of B2 phase upon quenching and subsequentdecomposition to α2/O laths in a variety of transformationsand α2 → O transformations.

The alloys based on TiAl (γ) consist of α2 and γ phasesas alternate lamellae in the microstructure. Similar to othertitanium alloys, the morphology of the γ phase can bemodified to equiaxed shape by thermal and/or thermome-chanical processing, and a mixture of equiaxed γ + lamel-lae (α2+γ) can be achieved in the microstructure. Hotworking at temperatures below the α transus generally

results in a fine-grained microstructure. Postworking heattreatment in a single-phase α field results in fully lamellarstructures, while heat treatment in the two-phase α+γ fieldresults in a mixture of equiaxed and lamellar γ structure.Therefore, the α transus temperature is of particularimportance in these alloys and has the same significanceas the β transus temperature in conventional titaniumalloys.

The various microstructures have a strong influenceon the deformation and fracture behavior and conse-quently on the mechanical properties of titanium alloys.Finer microstructural features lead to increased strengthand ductility. They also retard crack nucleation. Coarsemicrostructures, on the other hand, are more resistant tocreep and fatigue-crack growth. Equiaxed structures ingeneral exhibit high ductility and high fatigue strength,while lamellar structures possess high fracture strengthand show superior resistance to creep and fatigue-crackgrowth. Bimodal structures combine the advantages oflamellar and equiaxed structures and show a balancedprofile of properties. These general observations regardingthe structure–property relationship not only apply to con-ventional titanium alloys, but also hold true for titaniumaluminide alloys. The influence of microstructure on frac-ture features in titanium alloys will be easily perceived asyou glance through the fractographs in this atlas. Thefractographs presented are for different titanium alloys,including titanium aluminides in a variety of microstruc-tural conditions.

17

3

Chemical Compositions

TABLE 3.1Typical

Composition of Titanium Alloys (At.

%)

Alloy Al Nb V Mo Ta Zr Sn Si Mn Cr FeOther

Elements Ti

αααα

Alloys

IMI260 — — — — — — — — — — — 0.2 Pd Bal.IMI317 5.0 — — — — — 2.5 — — — — — Bal.

Near-

αααα

Alloys

Ti-811 8.0 — 1.0 1.0 — — — — — — — — Bal.Ti-6242 6.0 — — 2.0 — 4.0 2.0 — — — — — Bal.IMI679 2.25 — — 1.0 — 5.0 11.0 — — — — — Bal.TIMETAL685 6.0 — — 0.5 — 5.0 — 0.25 — — — — Bal.TIMETAL834 6.0 0.7 — 0.5 — 3.5 4.0 0.35 — — — 0.06 C Bal.

αααα

+

ββββ

Alloys

IMI318 6.0 — 4.0 — — — — — — –– — — Bal.Ti-662 6.0 — 6.0 — — — 2.0 — — — 0.7 — Bal.IMI550 4.0 — — 4.0 — — 2.0 0.5 — — — — Bal.IMI680 2.25 — — 4.0 — — 11.0 0.2 — — — — Bal.Ti-6246 6.0 — — 6.0 — 4.0 2.0 — — — — — Bal.

Metastable

ββββ

Alloys

Timet LCB 1.5 — — 6.8 — — — — — — 4.5 — Bal.Ti-10-2-3 3.0 — 10.0 — — — — — — — 2.0 — Bal.BetaIII — — — 11.5 — 6.0 4.5 — — — — — Bal.TIMETAL 21S 3.0 2.6 — 15.0 — — — 0.2 — — — — Bal.

ββββ

Alloys

Alloy C — — 35.0 — — — — — — 15.0 — — Bal.— — — 40.0 — — — — — — — — Bal.— — — 30.0 — — — — — — — — Bal.

Ti

3

Al (

αααα

2

) Alloys

Near

α

2

24.0 10.0 — — — — — — — — — — Bal.25.0 8.0 — 2.0 2.0 — — — — — — — Bal.

α

2

+O 25.0 10.0 3.0 1.0 — — — — — — — — Bal.25.0 17.0 — 1.0 — — — — — — — — Bal.

Ti

2

AlNb (Orthorhombic)

22.0 27.0 — — — — — — — — — — Bal.22.0 24.0 — — — — — — — — — — Bal.

TiAl (

γγγγ

) Alloys

48.0 — 1.0 — — — — — — — — — Bal.48.0 2.0 — — — — — — — 2.0 — — Bal.48.0 4.0 2.0 — — — — — — — — — Bal.48.0 4.0 — 1.0 — — — — — — — — Bal.48.0 2.0 — — — — — — 2.0 — — — Bal.

18


TABLE 3.2Chemical Composition of Alloys in This Book (wt.%)

Alloy Al V Mo Nb Ta Si Zr Sn Cu Fe C Mn Cr Ti

Commercial (ASTM-2) — — — — — — — — — 0.3 0.1 — — Bal.OT4-1 1.5 — — — — 0.15 0.3 — — 0.3 — 1.2 — Bal.IMI 685 6.0 — 0.5 — — 0.25 5.0 — — — — — — Bal.IMI 834 5.5 — 0.5 1.0 — 0.35 4.0 4.0 — 0.5 0.6 — — Bal.Ti-64 6.0 4.0 — — — — — 0.1 0.1 0.3 0.08 — — Bal.VT9 6.5 — 3.2 — — 0.25 1.8 — 0.05 0.06 0.01 — — Bal.Ti-10-2-3 3.0 10.0 — — — — — — — 2.0 — — — Bal.

β

-Ti alloy 1.65 9.8 — — — — — — — 4.95 0.03 — — Bal.Ti-24Al-11Nb 13.5 — — 21.3 — — — — — — — — — Bal.Ti-24Al-15Nb 13.0 — — 28.0 — — — — — — — — — Bal.Ti-24Al-20Nb 12.5 — — 36.0 — — — — — — — — — Bal.Ti-24Al-11Nb-4Ta 12.2 — — 19.2 13.6 — — — — — — — — Bal.Ti-27Al-14Nb-1Mo 14.8 — 2.0 26.5 — — — — — — — — — Bal.Ti-25Al-15Nb 13.6 — — 28.2 — — — — — — — — — Bal.Ti-24Al-27Nb 11.8 — — 45.6 — — — — — — — — — Bal.Ti-47Al-2Nb-2Cr 32.5 — — 5.0 — — — — — — — — 2.5 Bal.Ti-48Al-4Nb-1Mo 32.3 — 2.4 9.3 — — — — — — — — — Bal.

19

4

Alpha Alloys

Alpha titanium alloys are single-phased alloys. Alpha sta-bilizers like aluminum and oxygen stabilize the

α

phase.Tin and zirconium are neutral elements and have solidsolubility in both the

α

and

β

phases; they also strengthenthe

α

phase, along with aluminum and oxygen. Alphaalloys have high stability and good high-temperature prop-erties, but they cannot be heat-treated for modification ofmicrostructure for improving their properties.

Commercially pure (CP) titanium and Ti-5Al-2.5Snare the most important alloys of this type. CP titanium(ASTM grades 1–4) is usually hot rolled, forged, and heat-treated in the single

α

-phase field. Typical processingtemperature for CP titanium is 800˚C. Typical heat treat-ment is 675˚C/1h/AC. Alpha alloys are mainly used in thechemical and process-engineering industries, where cor-rosion resistance and deformability are the main concern.Microstructure and fractographs of CP titanium (ASTMgrade 2) are presented in this section.

FIGURE 4.1

Commercial titanium, 675°C/1h/AC tensile tested at room temperature. Optical micrograph shows equiaxed

α

grains.

20


FIGURE 4.2

Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. Low-magnification scanning electron micro-scope (SEM) fractograph shows the fracture surface.

FIGURE 4.3

Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. High-magnification SEM fractograph of Figure4.2 shows overload fracture with fine dimples.

Alpha Alloys

21

FIGURE 4.4

Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. SEM fractograph of a different area of Figure4.2 shows overload fracture with fine dimples and a few conical dimples.

FIGURE 4.5

Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. High-magnification SEM fractograph of theconical dimples shows serpentine glide (stretch-like regions) surrounded by fine dimples.

22


FIGURE 4.6

Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. High-magnification SEM fractograph of adifferent area shows the conical dimple.

23

5

Near-Alpha Alloys

These alloys contain 1 to 2 wt.% of

β

stabilizers, whichare added to improve their strength and workability. The

α

phase is predominant in these alloys, which are a goodcompromise between high-strength

α

+

β

alloys and creep-resistant

α

alloys.OT4-1 (Ti-1.5Al-1.2Mn-0.15Si-0.3Zr), a Russian

alloy, is normally used in

α

+

β

treated condition. This alloyis primarily used in structural components for applicationsup to 300˚C.

IMI685 (Ti-6Al-5Zr-0.5Mo-0.3Si) is another alloythat is aimed at higher-temperature applications in jetengines (up to 520˚C). This alloy is mainly used in the

β

heat-treated condition. The alloy is usually processed inthe

β

- or high in the

α

+

β

-phase field. This chapter presentsthe microstructure and fractography of heat-treated alloyIMI685, tensile tested at room temperature and at 520˚C.The fracture features of the room-temperature-tested spec-imen shows predominantly cleavage and dimples at thecolony and lath boundaries, whereas the specimen testedat 520˚C shows ductile fracture features with dimples.

IMI834 (Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si),the most advanced alloy of this class, can be used up to

550˚C. SEM fractographs of this alloy are also includedin this chapter. This alloy is typically heat-treated high inthe

α

+

β

-phase field to achieve 5 to 10% primary

α

, andit offers a good combination of fatigue and creep resis-tance. The solution-treatment temperature that determinesthe primary

α

volume fraction has a strong influence onthe properties. A study of IMI834 alloy, heat-treated atdifferent temperatures and creep-tested at 220 MPa and650˚C, shows the variation of dimple size with theincrease in the heat-treatment temperatures. The creep lifeincreased from 22 h to 220 h when the solution-treatmenttemperature was increased from 970˚C (70%

α

) to 1080˚C(0%

α

). This can be attributed to the increase in the volumefraction of transformed

β

phase, consisting of acicular

α

,with the increase in heat-treatment temperature. Fractog-raphy of the sample heat-treated at 1080

°

C/2h/AC +700

°

C/2h/AC clearly shows coarse

β

grains at low mag-nification, while ductile dimples are seen at higher mag-nification. A small area showing cleavage facets was alsoobserved at the center of the specimen.

24


FIGURE 5.1

Ti-1.5Al-1.2Mn-0.15Si-0.3Zr, 650˚C/0.5h/AC. Optical micrograph shows fine

α

grains with small amounts of

β

at thegrain boundaries.

FIGURE 5.2

Ti-1.5Al-1.2Mn-0.15Si-0.3Zr, 650˚C/0.5h/AC, sheet specimen, tensile tested at room temperature. Low-magnificationscanning electron microscope (SEM) fractograph shows the general fracture appearance.

Near-Alpha Alloys

25

FIGURE 5.3

Ti-1.5Al-1.2Mn-0.15Si-0.3Zr, 650˚C/0.5h/AC, sheet specimen, tensile tested at room temperature. SEM fractographshows overload fracture features with fine dimples.

FIGURE 5.4

Ti-1.5Al-1.2Mn-0.15Si-0.3Zr, 650˚C/0.5h/AC, sheet specimen, tensile tested at room temperature. High-magnificationSEM fractograph shows fine equiaxed dimples and a few conical dimples with serpentine glide (arrows).

26


FIGURE 5.5

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC. Optical micrograph shows transformed

β

structure withprior

β

grain boundaries.

FIGURE 5.6

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at room temperature. Macrograph showsrough-faceted fracture features.

Near-Alpha Alloys

27

FIGURE 5.7

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at room temperature. SEM fractograph showstranscrystalline fracture features and tear ridges (arrow marked).

FIGURE 5.8

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at room temperature. SEM fractograph showsfracture along the laths. Fine dimples and microvoids (arrows) are also seen in between the laths. This could be due to the fractureof the thin layer of the

β

phase in between the

α

laths.

28


FIGURE 5.9

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at room temperature. High-magnificationSEM fractograph shows transcystalline cracks along the colony boundaries. Dimples and tear ridges are also seen.

FIGURE 5.10

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. Optical micrograph of the gripof the tensile-tested specimen shows transformed

β

structure with prior

β

grain boundaries.

Near-Alpha Alloys

29

FIGURE 5.11

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. Macrograph shows ductile fea-tures. High-magnification SEM fractographs of regions A and B are shown in Figure 5.13 through Figure 5.16.

FIGURE 5.12

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. High-magnification SEM fracto-graph of Figure 5.11 shows ductile dimples and deep secondary cracks. A few coarse shallow dimples are also seen in the center ofthe fractograph.

30


FIGURE 5.13

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. SEM fractograph of region A inFigure 5.11 shows dimples of different sizes.

FIGURE 5.14

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. High-magnification SEM fracto-graph of region A shows large shallow dimples and voids.

Near-Alpha Alloys

31

FIGURE 5.15

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. SEM fractograph of region B inFigure 5.11 shows mixed-size dimples and tear ridges.

FIGURE 5.16

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. High-magnification SEM fracto-graph of region B shows large shallow dimples and voids. The dimple size is coarser as compared with that of region A.

32


FIGURE 5.17

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC. Optical micrograph shows acicular

α

within prior

β

grains.

FIGURE 5.18

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. Low-magnification SEM fractograph shows the general fracture appearance.

Near-Alpha Alloys

33

FIGURE 5.19

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. SEMfractograph shows the origin of the fracture.

FIGURE 5.20

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. High-magnification SEM fractograph shows fatigue striations.

34


FIGURE 5.21

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. Low-magnification SEM fractograph shows final overload fracture with dimples.

FIGURE 5.22

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. High-magnification SEM fractograph of Figure 5.21 shows final overload fracture with dimples and tear ridges.

Near-Alpha Alloys

35

FIGURE 5.23

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC + creep tested at 520˚C for

100 h followed by tensiletesting. Optical micrograph of the grip of the tested sample shows Widmanstätten

α

within prior

β

grains.

FIGURE 5.24

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing.SEM fractograph shows rough fracture and prior

β

boundaries.

36


FIGURE 5.25

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing.SEM fractograph shows transgranular fracture along the colonies.

FIGURE 5.26

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing.High-magnification SEM fractograph shows cleavage facets and fine dimples at the lath boundaries. Tear ridges are also seen.

Near-Alpha Alloys

37

FIGURE 5.27

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing.High-magnification SEM fractograph of another region of Figure 5.24 shows cleavage facets. Fine dimples at the lath boundariesare due to the fracture of the

β

phase.

FIGURE 5.28

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing.SEM fractograph of a different region of the fracture surface shows the effect of Widmanstätten microstructure (Figure 5.23) on the fracture.

38


FIGURE 5.29

Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing.High-magnification SEM fractograph of Figure 5.28 shows microvoids along the lath and colony boundaries.

FIGURE 5.30

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/AC + 700˚C/2h/AC. Optical micrograph shows primary

α

andtransformed

β

.

Near-Alpha Alloys

39

FIGURE 5.31

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, tensile tested at room temperature. SEMfractograph shows rough fracture surface.

FIGURE 5.32

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, tensile tested at room temperature. Low-magnification SEM fractograph shows transcrystalline fracture features. Prior

β

boundaries are delineated.

40


FIGURE 5.33

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, tensile tested at room temperature. High-magnification SEM fractograph shows cleavage features and dimples. Tear ridges are also seen (arrow).

FIGURE 5.34

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, notch tensile tested. SEM fractographshows smooth fracture surface.

Near-Alpha Alloys

41

FIGURE 5.35

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, notch tensile tested. SEM fractographshows cleavage fracture features, dimples, and secondary cracks. The effect of the microstructure can be seen on the fracture.

FIGURE 5.36

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, notch tensile tested. High-magnificationSEM fractograph shows cleavage facets with river pattern. Dimples surrounding the cleavage facets are also seen.

42


FIGURE 5.37

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at astress of 475 MPa. SEM macrograph shows the fracture surface.

FIGURE 5.38

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at astress of 475 MPa. High-magnification SEM of Figure 5.37 shows origin.

Near-Alpha Alloys

43

FIGURE 5.39

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at astress of 475 MPa. SEM fractograph shows inclusion at the origin at higher magnification. Cleavage-like fracture features are also seen.

FIGURE 5.40

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at astress of 475 MPa. SEM fractograph shows fatigue striations and secondary cracks at higher magnification. Fissures at the roots offatigue striation are also seen.

44


FIGURE 5.41

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at astress of 475 MPa. SEM fractograph shows fatigue striations in a different region away from the origin. Secondary cracks are also seen.

FIGURE 5.42

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of550 MPa. SEM macrograph shows smooth fracture surface. Origin is indicated by arrow.

Near-Alpha Alloys

45

FIGURE 5.43

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of550 MPa. Low-magnification SEM fractograph of the origin shows secondary cracks and cleavage facets.

FIGURE 5.44

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of550 MPa. High-magnification SEM fractograph shows the origin and secondary cracks with cleavage facets.

46


FIGURE 5.45

SEM fractograph of Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatiguetested at RT a stress of 550 MPa. High-magnification view of a different region showing patches of fatigue striations separated bytear ridges (A). Cleavage facets (B) are also seen at places. Grains are favorably-oriented to the stress axis fracture by cleavage andthose oriented to relax the load by cyclic relaxation fracture by fatigue.

FIGURE 5.46

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of550 MPa. SEM fractograph of a different area shows cleavage facets and dimples.

Near-Alpha Alloys

47

FIGURE 5.47

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of550 MPa. SEM fractograph shows fatigue striations and fine dimples.

FIGURE 5.48

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, stress ruptured. SEM macrograph showsrough fracture surface.

48


FIGURE 5.49

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, stress ruptured. Low-magnification SEMfractograph shows fine dimples with voids and secondary cracks.

FIGURE 5.50

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, stress ruptured. SEM fractograph showsa mixture of fine and coarse dimples and large voids. These could be due to the

α

-phase pullout.

Near-Alpha Alloys

49

FIGURE 5.51

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, stress ruptured. High-magnification SEMfractograph shows coarse, deep dimples surrounded by fine dimples.

FIGURE 5.52


α

andtransformed

β

.

50


FIGURE 5.53


α

andtransformed

β

.

FIGURE 5.54


α

andtransformed

β

. The percentage of primary

α

decreased with increasing solution treatment temperature.

Near-Alpha Alloys

51

FIGURE 5.55

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1080˚C/2h/AC + 700˚C/2h/AC. Optical micrograph shows fully trans-formed

β

microstructure.

FIGURE 5.56

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 970˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 22-h life.SEM macrograph shows general fracture appearance.

52


FIGURE 5.57

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 970˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 22-h life.SEM fractograph shows ductile fracture features with voids and secondary cracks.

FIGURE 5.58

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 970˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 22-h life.High-magnification SEM fractograph shows dimples and voids. Tear ridges are also seen.

Near-Alpha Alloys

53

FIGURE 5.59

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1000˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 32.5-hlife. SEM macrograph shows rough fracture features.

FIGURE 5.60

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1000˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 32.5-hlife. SEM fractograph shows ductile fracture features with voids and secondary cracks.

54


FIGURE 5.61

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1000˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 32.5-hlife. High-magnification SEM fractograph shows dimples and large voids. Tear ridges are also seen.

FIGURE 5.62

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1045˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 54-hlife. SEM macrograph shows rough fracture features.

Near-Alpha Alloys

55

FIGURE 5.63

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1045˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 54-hlife. SEM fractograph shows ductile fracture features with voids and secondary cracks.

FIGURE 5.64

Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1045˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 54-hlife. High-magnification SEM fractograph shows slightly coarse dimples and large voids and secondary cracks. The fracture featuresof samples heat-treated at temperatures from 970˚C to 1045˚C are similar.


FIGURE 5.65 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1080˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 220-hlife. SEM macrograph shows coarse, granular fracture features with prior beta boundaries. The general appearance of the fracturesurface is different compared with that of Figure 5.56, Figure 5.59, and Figure 5.62.

FIGURE 5.66 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1080˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 220-hlife. SEM fractograph shows a mixture of coarse and fine dimples.

Near-Alpha Alloys 57

FIGURE 5.67 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1080˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 220-hlife. High-magnification SEM fractograph shows large dimples and a few cleavage facets (marked A). Creep life increased with theincrease in solution treatment temperature. At this heat-treatment temperature, the fracture seems to be changing to mixed mode.

59

6

Alpha + Beta Alloys*

Alloys containing 4 to 6% of beta stabilizers are called

α

+

β

alloys. Examples of this class of alloys are Ti-6Al-4V, Ti-6Al-6V-2Sn, etc. These alloys can be heat-treatedto develop a variety of microstructures and mechanicalproperties.

Ti-6Al-4V, the commonly used alloy of this class, isprimarily used in

α

+

β

condition. This alloy is used in theannealed condition or in the solution-treated and aged(STA) condition. Fractographs of this alloy in the STAcondition, tensile tested at room temperature, 200

°

C, and300

°

C, are shown in this chapter. Ti-6Al-4V is a very well-tested alloy and has a good balance of properties, andhence it is used in the aerospace industry.

VT9 (Ti-6.5Al-3.2Mo-1.8Zr-0.25Si) [7] is also an

α

+

β

alloy of Russian origin. This alloy can be used in the

α

+

β

or

β

heat-treated condition. Fractographs showing thefracture features of low-oxygen-content (

≈

600 ppm) VT9alloy tensile tested at room temperature and 500

°

C are

also shown in this chapter. The dimples in the 500

°

C-tested specimen are coarser and more equiaxed comparedwith the room-temperature-tested sample. The fracturefeatures are similar in the alloy containing higher oxygen(1300 ppm). The room-temperature impact-tested speci-men showed similar features in both the low- and high-oxygen-content specimens. However, the dimple size ofthe higher-oxygen-content specimen was slightly finer.The fracture features of the

−

70

°

C impact-tested specimenof the high-oxygen-content sample (not shown in thisbook) were also similar to the low-oxygen-content sample.The

α

+

β

heat-treated tensile-tested (500

°

C) specimenappears to be more ductile, with prominent necking show-ing cup-and-cone fracture with coarse equiaxed dimplescompared with the

β

heat-treated specimen tested at sim-ilar conditions. Other

α

+

β

alloys (Ti-6246, Ti-6222, Ti-17, etc.) are developed for high-temperature applicationsin gas-turbine engines up to 400

°

C.

* The micrographs shown in Figure 6.23 to Figure 6.73 are all from DMR Technical Report from Banerjee, D., Saha, R.L., Mukherjee, D., andMuraleedharan, K., Structure and Properties of Ti-6.5Al-3Mo-1.8Zr-0.25Si Alloy, DMRTR 8983, Defence Metallurgical Research Laboratory,Hyderabad, India, 1989. With permission.

FIGURE 6.1

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC. Optical micrograph of tensile-tested specimen shows primary

α

and trans-formed

β

.

60


FIGURE 6.2

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at room temperature. Low-magnification fractograph showingcup-and-cone fracture with a prominent shear lip. The high magnification photographs of regions A and B are shown in Figure 6.3and Figure 6.4, respectively.

FIGURE 6.3

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at room temperature. SEM fractograph shows dimples in thecentral region (A) at high magnification.

Alpha + Beta Alloys

61

FIGURE 6.4

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at room temperature. SEM fractograph of shear-lip region (B)shows equiaxed dimples.

FIGURE 6.5

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at room temperature. SEM fractograph of another area ofshear lip shows elongated dimples at high magnification.

62


FIGURE 6.6

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. Scanning electron microscope (SEM) macrographshows cup-and-cone fracture with a pronounced shear lip.

FIGURE 6.7

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. The central region of the SEM fractograph of Figure6.6 shows a mixture of coarse and fine dimples.

Alpha + Beta Alloys

63

FIGURE 6.8

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. SEM fractograph shows dimples at higher magni-fication. The dimple size seems to be coarser as compared with room-temperature-tested specimen.

FIGURE 6.9

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. Low-magnification SEM fractograph of the shear-lip region shows equiaxed dimples.

64


FIGURE 6.10

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. High-magnification SEM fractograph of the shear-lip region shows elongated dimples.

FIGURE 6.11

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 300˚C. SEM macrograph shows classical cup-and-conefracture with a prominent shear lip.

Alpha + Beta Alloys

65

FIGURE 6.12

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 300˚C. SEM fractograph of the central region of Figure6.11 shows a mixture of coarse and fine dimples at higher magnification. The dimple size seems to be increasing with increasingtest temperature, indicating greater ductility.

FIGURE 6.13

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 300˚C. SEM fractograph of the shear-lip region of Figure6.11 shows slightly elongated dimples at higher magnifications.

66


FIGURE 6.14

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–800 MPa. Low-magnificationSEM fractograph shows smooth fracture surface and the origin (arrow).

FIGURE 6.15

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–800 MPa. High-magnificationSEM fractograph shows fatigue striations at higher magnification away from the origin.

Alpha + Beta Alloys

67

FIGURE 6.16

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–800 MPa. SEM fractographshowing a different area of Figure 6.14 reveals fatigue striations and numerous secondary cracks. Fissures at the roots of some fatiguestriations are also seen.

FIGURE 6.17

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–800 MPa. The final fractureof the same specimen (Figure 6.14) shows dimpled overload fracture features.

68


FIGURE 6.18

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. SEM fractograph shows thefracture surface and origin (arrow).

FIGURE 6.19

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. Low-magnification SEMfractograph of Figure 6.18 shows origin.

Alpha + Beta Alloys

69

FIGURE 6.20

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. SEM fractograph shows fatiguestriations at higher magnification away from the origin.

FIGURE 6.21

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. High-magnification SEMfractograph of a different area of the specimen shows fatigue striations with secondary cracks. Fissures (arrow) at the roots of somestriations are also seen.

70


FIGURE 6.22

Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. SEM fractograph of the finaloverload fracture of the specimen shows dimples and tear ridges (arrow).

FIGURE 6.23

Ti-6.5Al-3.2Mo-1.8Zr-0.25Si,

α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC. Optical micrograph of low-oxygen-content (600 ppm) alloy shows primary

α

and transformed

β

. The transformed

β

microstructure is coarse with prominent acicularity.

Alpha + Beta Alloys

71

FIGURE 6.24

Ti-6.5Al-3.2Mo-1.8Zr-0.25Si,

α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC. Optical micrograph of high-oxygen-content (1300 ppm) alloy shows

α

+

β

microstructure. The percentage of primary

α

is greater in high-oxygen-content alloy comparedwith low-oxygen-content alloy (Figure 6.23).

FIGURE 6.25

Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content),

α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperaturetensile tested. Low-magnification SEM fractograph shows surface of a tensile specimen. The fracture consists of a flat fibrous centralregion and a shear lip.

72


FIGURE 6.26


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temper-ature tensile tested. SEM fractograph of central region of the specimen in Figure 6.25 shows dimples at low magnification. Tearridges are also seen.

FIGURE 6.27


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temper-ature tensile tested. High-magnification SEM fractograph of the central region of Figure 6.25 shows dimples.

Alpha + Beta Alloys

73

FIGURE 6.28


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, tensile testedat 500˚C. Low-magnification SEM fractograph shows classical cup-and-cone fracture with pronounced necking and lip formation.

FIGURE 6.29


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, tensile testedat 500˚C. SEM fractograph of the center of the specimen of Figure 6.28 shows a mixture of fine and coarse deeply rounded dimplesat higher magnification.

74


FIGURE 6.30


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, tensile testedat 500˚C. High-magnification SEM fractograph of the area in the rectangle in Figure 6.29 shows serpentine glide in a coarse dimple.

FIGURE 6.31

Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content),

α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature tensile tested. Low-magnification SEM fractograph shows smooth fracture.

Alpha + Beta Alloys

75

FIGURE 6.32


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature tensile tested. SEM fractograph of the central region of Figure 6.31 shows fine equiaxed dimples.

FIGURE 6.33


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature tensile tested. SEM fractograph of the central region of Figure 6.31 shows fine equiaxed dimples at higher magnification.

76


FIGURE 6.34


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temper-ature impact tested. Low-magnification SEM fractograph shows flat fracture.

FIGURE 6.35


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temper-ature impact tested. Higher-magnification SEM fractograph shows dimples and secondary cracks.

Alpha + Beta Alloys

77

FIGURE 6.36


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temper-ature impact tested. Higher-magnification SEM fractograph shows dimples and secondary cracks. Tear ridges (arrow) are also seen.

FIGURE 6.37


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, impact testedat

−

70˚C. Low-magnification SEM fractograph shows smooth fracture.

78


FIGURE 6.38


α

+

β


−

70˚C. Higher-magnification SEM fractograph shows dimples.

FIGURE 6.39


α

+

β


−

70˚C. Higher-magnification SEM fractograph shows dimples and secondary cracks. The fracture features of

−

70˚C-tested specimenare similar to those of room-temperature-tested specimen.

Alpha + Beta Alloys

79

FIGURE 6.40


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature impact tested. Low-magnification SEM fractograph shows fracture features.

FIGURE 6.41


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature impact tested. High-magnification SEM fractograph of the area in the rectangle in Figure

6.40 shows dimples.

80


FIGURE 6.42


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature impact tested. SEM fractograph shows fine and coarse dimples a little away from the notch.

FIGURE 6.43


α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature impact tested. SEM fractograph of the central region of the specimen shows fine equiaxed dimples. The fracture featuresare similar to the low-oxygen-content specimen, but the dimple size seems to be slightly finer.

Alpha + Beta Alloys

81

FIGURE 6.44

Ti-6.5Al-3.2Mo-1.8Zr-0.25Si,

α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stressrange of 0–895 MPa. Low-magnification SEM fractograph shows faceted fracture.

FIGURE 6.45

Ti-6.5Al-3.2Mo-1.8Zr-0.25Si,

α

+

β

heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stressrange of 0–895 MPa. SEM fractograph of fracture surface shows the origin (arrow).

82


FIGURE 6.46

Ti-6.5Al-3.2Mo-1.8Zr-0.25Si,

α

+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stressrange of 0–895 MPa. SEM fractograph shows cleavage-like features with fatigue striations at the center of the specimen.

FIGURE 6.47 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stressrange of 0–895 MPa. SEM fractograph shows cleavage-like features with fatigue striations away from the origin.

Alpha + Beta Alloys 83

FIGURE 6.48 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stressrange of 0–940 MPa. Low-magnification SEM fractograph shows smoother fracture with the absence of facets compared with low-stress range (Figure 6.44).

FIGURE 6.49 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stressrange of 0–940 MPa. High-magnification SEM fractograph of the area in the rectangle (Figure 6.48) shows the origin.


FIGURE 6.50 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stressrange of 0–940 MPa. High-magnification SEM fractograph of the center of the specimen shows fatigue striations (arrow marked).

FIGURE 6.51 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stressrange of 0–940 MPa. SEM fractograph shows fatigue striations and secondary cracks away from the origin.


FIGURE 6.52 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, high-cycle fatigue tested at 685MPa. Low-magnification SEM fractograph shows origin.

FIGURE 6.53 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, high-cycle fatigue tested at 685MPa. High-magnification SEM fractograph shows a view of the origin.


FIGURE 6.54 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, high-cycle fatigue tested at 685MPa. SEM fractograph at higher magnification shows fatigue striations and secondary cracks away from the origin.

FIGURE 6.55 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC. Optical micrograph of specimenshows transformed β microstructure with prior β boundaries. A thin continuous α film is present at the grain boundaries.


FIGURE 6.56 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-tem-perature tensile tested. SEM macrograph shows faceted fracture features.

FIGURE 6.57 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-tem-perature tensile tested. Low-magnification SEM fractograph shows dimples on the facets.


FIGURE 6.58 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-tem-perature tensile tested. High-magnification SEM fractograph shows dimples on the facets. Alignment of dimples along the α/βinterface is also seen.

FIGURE 6.59 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature tensile tested. Low-magnification SEM fractograph shows faceted fracture features.


FIGURE 6.60 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature tensile tested. High-magnification SEM fractograph shows dimples on the facets. Alignment of dimples along the α/βinterface is more prominent. The fracture features of both low- and high-oxygen-content alloys are similar.

FIGURE 6.61 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, tensile testedat 500˚C. Low-magnification SEM fractograph shows smoother fracture surface as compared with room-temperature-tested specimen.


FIGURE 6.62 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, tensile testedat 500˚C. High-magnification SEM fractograph shows equiaxed dimples and tear ridges.

FIGURE 6.63 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, tensile testedat 500˚C. High-magnification SEM fractograph shows both equiaxed and elongated dimples.


FIGURE 6.64 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-tem-perature impact tested. Low-magnification SEM fractograph shows mixed inter- and transgranular fracture modes. The fracture ismore intergranular in comparison with tensile fracture (Figure 6.56).

FIGURE 6.65 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-tem-perature impact tested. High-magnification SEM fractograph shows dimples and secondary cracks just below the notch.


FIGURE 6.66 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-tem-perature impact tested. High-magnification SEM fractograph shows dimples and secondary cracks away from the notch.

FIGURE 6.67 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature impact tested. Low-magnification SEM fractograph shows mixed inter- and transgranular fracture modes. The fractureis more intergranular compared with the low-oxygen-content specimen.


FIGURE 6.68 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature impact tested. High-magnification SEM fractograph shows dimples on the facets and secondary cracks just below thenotch.

FIGURE 6.69 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature impact tested. High-magnification SEM fractograph shows dimples on the facets and secondary cracks away from thenotch.


FIGURE 6.70 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, high-cycle fatigue tested at 465MPa. Low-magnification SEM fractograph shows fatigue fracture surface with flat origin crisscrossed by linear features.

FIGURE 6.71 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, high-cycle fatigue tested at 465MPa. SEM fractograph at higher magnification shows fatigue striations and secondary cracks away from the origin.


FIGURE 6.72 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, high-cycle fatigue tested at 588MPa. SEM fractograph at low magnification shows origin (arrow).

FIGURE 6.73 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, high-cycle fatigue tested at 588MPa. SEM fractograph at higher magnification shows fatigue striations and secondary cracks away from the origin.

97

7

Beta Alloys*

In beta alloys, the

β

phase is stabilized at room tempera-ture by the addition of approximately 30% of

β

stabilizerslike V, Mo, Nb, Ta, etc. The important alloy of this classis

β

-C, which is used in burn-resistant applications. Moreoften, the so-called

β

alloys are metastable

β

alloys inwhich the

β

phase can be retained on fast cooling fromthe

β

solution-treatment temperature. Titanium alloys, inthe solution-treated condition, generally have lowerstrength. However, these alloys can be aged to obtain veryhigh strength levels due to the precipitation of fine-

α

phaseupon aging. These alloys have an excellent combinationof fracture toughness and strength, which can be tailoredby selecting appropriate aging temperature and time.

Ti-10V-2Fe-3Al, Ti-13V-11Cr-3Al, and Ti-15Mo aresome of the prominent alloys of this class. Low-cost

β

alloys such as TIMETAL LCB (Ti-4.5Fe-6.8Mo-1.5Al)are attracting interest for industrial and automotive appli-cations. Ti-10V-4.5Fe-1.5Al is another low-cost

β

alloy

being studied in DMRL [8]. Metastable

β

alloys contain10 to 15% of

β

stabilizers. Beta alloys are normally usedin the

α

+

β

solution-treated and aged condition.This chapter presents microstructures and fracto-

graphs of the alloys Ti-10V-2Fe-3Al (Ti-10-2-3) and Ti-10V-4.5Fe-1.5Al in the solution-treated (ST) and ST +aged (STA) conditions. Fracture features of the solution-treated specimens showed only ductile dimples for bothalloys. The fracture surface of the

β

solution-treated andaged specimens (450˚C) revealed predominantly inter-granular fracture features with dimples on the grain facets,and the specimens aged at higher temperatures showedmore ductile features, with dimples interspersed withintergranular fracture features in both the alloys. The

α

+

β

solution-treated and aged specimens in both alloys showedductile fracture with dimples. These alloys have highstrength and good hardenability, and they are used inaircraft structural applications.

* All micrographs in this chapter are from Bhattacharjee, A., Joshi, V.A., Deshpande, D.G., Hussain, S.M., Nandy, T.K., and Gogia, A.K., Developmentof Low Cost Beta Titanium Alloys, I, DMR TR 200270, Defence Metallurgical Research Laboratory, Hyderabad, India, 2000. With permission.

FIGURE 7.1

Ti-10V-2Fe-3Al,

β

solution treated at 820˚C/8h/WQ. Optical micrograph of

β

alloy shows coarse equiaxed

β

grains.

98


FIGURE 7.2

Ti-10V-2Fe-3Al,

β

solution treated at 820˚C/8h/WQ and aged at 600˚C/8h/AC. Scanning electron microscope (SEM)backscattered electron image shows fine-

α

precipitates in

β

matrix.

FIGURE 7.3

Ti-10V-2Fe-3Al,

β

solution treated at 820˚C/8h/WQ, tensile tested at room temperature. SEM macrograph showstransgranular fracture and a few voids.

Beta Alloys

99

FIGURE 7.4

Ti-10V-2Fe-3Al,

β

solution treated at 820˚C/8h/WQ, tensile tested at room temperature. High-magnification SEMfractograph shows ductile fracture with dimples.

FIGURE 7.5

Ti-10V-2Fe-3Al,

β

solution treated at 820˚C/8h/WQ and aged at 450˚C/1h/AC, tensile tested at room temperature.SEM macrograph shows rough fracture features.

100


FIGURE 7.6

Ti-10V-2Fe-3Al,

β

solution treated at 820˚C/8h/WQ and aged at 450˚C/1h/AC, tensile tested at room temperature. High-magnification SEM fractograph shows mixed-mode, brittle, and ductile fracture with shallow dimples on the facets. Secondary cracks arealso seen.

FIGURE 7.7

Ti-10V-2Fe-3Al,

β

solution treated at 820˚C/8h/WQ and aged at 600˚C/1h/AC, tensile tested at room temperature.SEM macrograph shows rough fracture features with voids.

Beta Alloys

101

FIGURE 7.8

Ti-10V-2Fe-3Al,

β

solution treated at 820˚C/8h/WQ and aged at 600˚C/1h/AC, tensile tested at room temperature. High-magnification SEM fractograph shows mixed-mode, brittle, and ductile fracture with shallow dimples on the facets. Secondary cracks arealso seen.

FIGURE 7.9

Ti-10V-2Fe-3Al,

α

+

β

solution treated at 700˚C/8h/WQ and aged at 600˚C/4h/AC. SEM backscattered electron imageshows equiaxed primary

α

and acicular

α

.

102


FIGURE 7.10

Ti-10V-2Fe-3Al,

α

+

β

solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC, tensile tested at room temperature.SEM macrograph shows cup-and-cone fracture with a prominent shear lip.

FIGURE 7.11

Ti-10V-2Fe-3Al,

α

+

β

solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC, tensile tested at room temperature.High-magnification SEM fractograph shows ductile fracture with dimples.

Beta Alloys

103

FIGURE 7.12

Ti-10V-4.5Fe-1.5Al,

β

solution treated at 800˚C/8h/WQ. Optical micrograph of

β

alloy shows equiaxed

β

grains.

FIGURE 7.13

Ti-10V-4.5Fe-1.5Al,

β

solution treated at 800˚C /8h/WQ + 600/1h/AC. Secondary electron image of

β

alloy showsequiaxed

β

grains with fine-

α

precipitation.

104


FIGURE 7.14

Ti-10V-4.5Fe-1.5Al,

β

solution treated at 800˚C/8h/WQ, tensile tested at room temperature. Low-magnification SEMfractograph shows fully ductile fracture features.

FIGURE 7.15

Ti-10V-4.5Fe-1.5Al,

β

solution treated at 800˚C/8h/WQ, tensile tested at room temperature. SEM fractograph of thecenter of the specimen (Figure 7.14) at intermediate magnification shows fine dimples and tear ridges (arrow).

Beta Alloys

105

FIGURE 7.16

Ti-10V-4.5Fe-1.5Al,

β

solution treated at 800˚C/8h/WQ, tensile tested at room temperature. High-magnification SEMfractograph of a different area of the specimen (Figure 7.14) shows dimples and some inclusions (probably silicates [marked A]).

FIGURE 7.17

Ti-10V-4.5Fe-1.5Al,

β

solution treated at 800˚C/8h/WQ, tensile tested at room temperature. High-magnification SEMfractograph of the surface of the specimen (Figure 7.14) shows fine and coarse dimples.

106


FIGURE 7.18

Ti-10V-4.5Fe-1.5Al,

β

solution treated and aged at 800˚C/8h/WQ + 450˚C/1h/AC, tensile tested at room temperature.SEM macrograph shows crystalline fracture features.

FIGURE 7.19

Ti-10V-4.5Fe-1.5Al,

β

solution treated and aged at 800˚C/8h/WQ + 450˚C/1h/AC, tensile tested at room temperature.SEM fractograph shows mixed-mode fracture of intergranular and ductile fracture features at intermediate magnification.

Beta Alloys

107

FIGURE 7.20

Ti-10V-4.5Fe-1.5Al,

β

solution treated and aged at 800˚C/8h/WQ + 450˚C/1h/AC, tensile tested at room temperature.Higher-magnification SEM fractograph shows fine dimples on the grain facets.

FIGURE 7.21

Ti-10V-4.5Fe-1.5Al,

β

solution treated and aged at 800˚C/8h/WQ + 500˚C/1h/AC, tensile tested at room temperature.Low-magnification SEM fractograph shows crystalline fracture features.

108


FIGURE 7.22

Ti-10V-4.5Fe-1.5Al,

β

solution treated and aged at 800˚C/8h/WQ + 500˚C/1h/AC, tensile tested at room temperature.SEM fractograph shows mixed mode of intergranular- and ductile-fracture features at intermediate magnification. Ductile mode ismore prominent, and intergranular fracture is less compared with the specimen aged at 450˚C (Figure 7.19).

FIGURE 7.23

Ti-10V-4.5Fe-1.5Al,

β

solution treated and aged at 800˚C/8h/WQ + 500˚C/1h/AC, tensile tested at room temperature.SEM fractograph shows fine dimples near the surface of the specimen (Figure 7.21) at higher magnification.

Beta Alloys

109

FIGURE 7.24

Ti-10V-4.5Fe-1.5Al,

α

+

β

solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC. SEM backscattered electronimage shows globular primary

α

and acicular

α

.

FIGURE 7.25

Ti-10V-4.5Fe-1.5Al,

α

+

β

solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC, tensile tested at room temper-ature. SEM macrograph shows ductile fracture with shear lip.

110


FIGURE 7.26

Ti-10V-4.5Fe-1.5Al,

α

+

β

solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC, tensile tested at room temper-ature. High-magnification SEM fractograph shows dimples and a big void.

111

8

Titanium Aluminides*

8.1 Ti

3

Al-BASED ALLOYS

These are intermetallics possessing excellent high-temper-ature creep and stress-rupture properties. A Ti-Al systemshows two intermetallics on the Ti-rich side: Ti

3

Al andTiAl. Both of these have excellent high-temperature prop-erties but suffer from poor room-temperature ductility.Over the years, several alloys with ternary and quaternaryadditions have been investigated in an attempt to achieveadequate room-temperature ductility. The alloys based onTi

3

Al contain large amounts of niobium and small addi-tions of other

β

stabilizers like Ta, Mo, etc., and they mayconsist of a B2 phase in addition to

α

2

(Ti

3

Al) or a deriv-ative of

α

2

known as O (Ti

2

AlNb), which has an ortho-rhombic lattice. The alloys show microstructural varia-tions similar to those observed in conventional titaniumalloys and have similar response to thermomechanicalprocessing. However, microstructure in these alloys canbe more complex due to the presence of the O phase andseveral other phase transformations in the Ti-Al-Nb system.

Microstructures and fractographs of the Ti

3

Al-basedalloys are presented in this chapter, including Ti-24Al-11Nb [6, 9], Ti-24Al-20Nb [10], Ti-24Al-27Nb [10], andTi-25Al-15Nb [11], with and without Ta and Mo additions[11]. All of the alloys were tensile tested after coolingfrom different temperatures using different cooling rates.

In the alloy Ti-24Al-20Nb,

α

+

β

water-quenched(WQ) specimens, the fractographs show a mixture ofcleavage facets and dimpled regions. The extent of thedimpled regions, which are more prominent in the high-

temperature solution-treated specimens, can be related tothe fracture of the B2 phase, and the cleavage facets canbe ascribed to the fracture of the

α

2

or O phase.In the Ti-24Al-27Nb alloy, cleavage features dominate

the fracture surface at all solution-treated (ST) tempera-tures, although a few dimples are apparent in the speci-mens that were solution treated at 980˚C and 1020˚C. Thesize of the cleavage facets is very large to be associatedwith the particle size of the O phase in the microstructure.This could be due to the fracture of the large unrecrystal-lized B2 grains, while the dimpled regions are due to thefracture of the fine recrystallized B2 grains

.

Fractographs of alloy Ti-24Al-20Nb in the

β

-treatedcondition show transcrystalline features at all coolingrates. At high cooling rates, quasicleavage fracture fea-tures are seen. At low cooling rates, large cleavage facetswith cleavage of similarly oriented laths are observed. Inthe

β

-treated Ti-24Al-27Nb alloy, a mixture of transcrys-talline and intercrystalline fracture is seen at high coolingrates. Large cleavage facets with river markings are seenat this cooling rate.

8.2 TiAl-BASED ALLOYS

Microstructures and fractographs of TiAl-based

γ

alu-minides of two alloys with composition Ti-47Al-2Nb-2Crand Ti-48Al-4Nb-1Mo are presented. Both alloys weretensile tested at room temperature and the fractographsshow transcrystalline fracture with cleavage facets.

* The micrographs shown in Figure 8.55 through Figure 8.105 have all been taken from Gogia, A.K., Nandy, T.K., Muraleedharan, K., Joshi, V.A.,Sagar, P.K., and Banerjee, D., Development of Advanced High Temperature Ti-alloys, VI, Effect of Nb in Ti

3

base alloys, DMR TR 94175, DefenceMetallurgical Research Laboratory, Hyderabad, India, 1994. With permission. The micrographs shown in Figure 8.106 through 8.159 are from Gogia, A.K., Nandy, T.K., Muraleedharan, K., Joshi, V.A., Kamat, S.V., andBanerjee, D., Development of an Advanced High Temperature Titanium Alloy, V, DMR TR 93166, Defence Metallurgical Research Laboratory,Hyderabad, India, 1993. With permission.

112


FIGURE 8.1

Ti-24Al-11Nb,

α

2

+

β

heat-treated, 1020˚C/8h/WQ. Optical micrograph shows equiaxed

α

2

and B2. (From Gogia, A.K.,Microstructure, Tensile Deformation and Fracture in Ti

3

Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy,Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.2

Ti-24Al-11Nb, 1020˚C/8h/WQ, tensile tested at room temperature. Scanning electron microscope (SEM) macrographshows smooth fracture features. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K.,

Metallurgical Transactions A

, 21A, 1990.With permission.)

Titanium Aluminides

113

FIGURE 8.3

Ti-24Al-11Nb, 1020˚C/8h/WQ, tensile tested at room temperature. SEM fractograph at intermediate magnificationshows cleavage fracture features with secondary cracks.

(Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K.,

MetallurgicalTransactions A

, 21A, 1990. With permission.)

FIGURE 8.4

Ti-24Al-11Nb, 1020˚C/8h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows cleav-age fracture features in the center of the specimen with secondary cracks. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K.,



114


FIGURE 8.5

Ti-24Al-11Nb,

α

2

+

β

heat-treated at 1060˚C/4h/WQ. Optical micrograph shows

α

2

and B2.

α

2

has a higher aspect ratiothat resulted due to processing at high temperature close to the

β

transus of the alloy. (From Gogia, A.K., Microstructure, TensileDeformation and Fracture in Ti

3

Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology,Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.6

Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. Low-magnification SEM fractograph shows rough-faceted transcrystalline fracture features.

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115

FIGURE 8.7

Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM fractograph of central region of the specimen(Figure 8.6) shows cleavage steps.

FIGURE 8.8

Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM fractograph of central region of the specimen(Figure 8.6) shows shallow dimples on the cleavage facets.

116


FIGURE 8.9

Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows dimpleson the facets and fluting (A) associated with

α

2

failure.

FIGURE 8.10

Ti-24Al-11Nb,

α

2

+

β

heat-treated at 1060˚C/4h/WQ. Optical micrograph shows equiaxed

α

2

and B2. Equiaxed

α

2

hasresulted due to processing at temperature lower than the

β

transus of the alloy (compare with Figure 8.5). (From Gogia, A.K.,Microstructure, Tensile Deformation and Fracture in Ti

3


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117

FIGURE 8.11

Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture features.The difference in fracture features as compared with Figure 8.6 is due to the difference in processing temperature. (Courtesy ofGogia, A.K., Banerjee, D., and Nandy, T.K.,



FIGURE 8.12

Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM fractograph shows cleavage fracture featuresand a few dimples.

118


FIGURE 8.13

Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM fractograph at intermediate magnificationshows cleavage facets with secondary cracks. The facet size is of the order of primary

α

2

size.

FIGURE 8.14

Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. High-magnification SEM fractograph showsfeather markings on the cleavage facets (A). Dimples (B) around the facets are also seen at places.

Titanium Aluminides

119

FIGURE 8.15

Ti-24Al-11Nb, 1060˚C/4h/AC, tensile tested at room temperature. SEM macrograph shows radial fracture with originat the center.

FIGURE 8.16

Ti-24Al-11Nb, 1060˚C/4h/AC, tensile tested at room temperature. Low-magnification SEM fractograph shows qua-sicleavage fracture features with secondary cracks.

120


FIGURE 8.17

Ti-24Al-11Nb, 1060˚C/4h/AC, tensile tested at room temperature. High-magnification SEM fractograph of Figure8.16 shows cleavage facets and secondary cracks.

FIGURE 8.18

Ti-24Al-11Nb, 1060˚C/4h/AC, tensile tested at room temperature. High-magnification SEM fractograph of a differentarea shows fan-shaped cleavage facets (A) and a few dimples (arrow).

Titanium Aluminides

121

FIGURE 8.19

Ti-24Al-11Nb,

α

2

+

β


α

2

and B2.

α

2

has a higher aspectratio that resulted due to processing at higher temperature close to the

β

transus of the alloy. The volume fraction of

α

2

decreaseswith an increase in solution-treatment temperature. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti

3

AlBase Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi,1990. With permission.)

FIGURE 8.20

Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. Low-magnification SEM fractograph shows coarsecleavage facets and secondary cracks.

122


FIGURE 8.21

Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. SEM fractograph shows river pattern on cleavagefacets. Cleaved steps (arrow) and secondary cracks are also seen.

FIGURE 8.22

Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows finedimples on some cleavage facets.

Titanium Aluminides

123

FIGURE 8.23

Ti-24Al-11Nb,

α

2

+

β

heat-treated at 1100˚C/2h/WQ. Optical micrograph shows equiaxed

α

2

and B2. (From Gogia,A.K., Microstructure, Tensile Deformation and Fracture in Ti

3


FIGURE 8.24

Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture. (FromGogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti

3

Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. ofMetallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

124


FIGURE 8.25

Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. High-magnification SEM fractograph showstransgranular fracture with cleavage of equiaxed

α

2

(arrow) and dimples. (From Gogia, A.K., Microstructure, Tensile Deformationand Fracture in Ti

3

Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras HinduUniversity, Varanasi, 1990. With permission.)

FIGURE 8.26

Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. High-magnification SEM fractograph showsfaceted failure with shallow dimples on facets. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K.,

Metallurgical TransactionsA


Titanium Aluminides

125

FIGURE 8.27

Ti-24Al-11Nb, 1100˚C/4h/FC. Optical micrograph shows equiaxed

α

2

and retained B2. (Courtesy of Gogia, A.K.,Banerjee, D., and Nandy, T.K.,



FIGURE 8.28

Ti-24Al-11Nb, 1100˚C/4h/FC, tensile tested at room temperature. SEM macrograph shows rough fracture features.Origin is shown by the arrow.

126


FIGURE 8.29

Ti-24Al-11Nb, 1100˚C/4h/FC, tensile tested at room temperature. SEM fractograph of the center of the specimen(Figure 8.28) at intermediate magnification shows cleavage fracture features with secondary cracks.

FIGURE 8.30

Ti-24Al-11Nb, 1100˚C/4h/FC, tensile tested at room temperature. SEM fractograph of a different region of thespecimen (Figure 8.28) at high magnification shows cleavage and secondary cracks.

Titanium Aluminides

127

FIGURE 8.31

Ti-24Al-11Nb, 1100˚C/4h/FC, tensile tested at room temperature. SEM fractograph of a different region of thespecimen (Figure 8.28) at high magnification shows cleavage facets with feathery fracture features and secondary cracks.

FIGURE 8.32

Ti-24Al-11Nb, 1130˚C/1h/OQ. Optical micrograph shows very fine

α

2

microstructure within B2 grains. (Courtesy ofGogia, A.K., Banerjee, D., and Nandy, T.K.,



128


FIGURE 8.33

Ti-24Al-11Nb, 1130˚C/1h/OQ, tensile tested at room temperature. Low-magnification SEM fractograph shows coarsetransgranular fracture features. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K.,


, 21A, 1990.With permission.)

FIGURE 8.34

Ti-24Al-11Nb, 1130˚C/1h/OQ, tensile tested at room temperature. SEM fractograph shows cleavage facets andsecondary cracks.

Titanium Aluminides

129

FIGURE 8.35

Ti-24Al-11Nb, 1130˚C/1h/OQ, tensile tested at room temperature. SEM fractograph shows a grain with triple pointand cleavage facets.

FIGURE 8.36

Ti-24Al-11Nb, 1130˚C/1h/OQ, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.35shows faceted transgranular fractures with shallow dimples in region A and intergranular ductile fracture features with coarse and finedimples in region B. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K.,



130


FIGURE 8.37

Ti-24Al-11Nb, 1150˚C/1h/AC. Optical micrograph shows acicular

α

2

in B2 grains. (Courtesy of Gogia, A.K., Banerjee,D., and Nandy, T.K.,



FIGURE 8.38

Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. SEM macrograph shows mixed-mode fracture withcoarse cleavage and intergranular features. Origin and intergranular facets are shown by white and black arrows, respectively. (FromGogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti

3

Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. ofMetallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

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131

FIGURE 8.39

Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. Low-magnification SEM fractograph of the originshows grain-boundary-initiated failure. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti

3

Al Base AlloysContaining Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. Withpermission.)

FIGURE 8.40

Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. SEM fractograph of Figure 8.39, seen at highmagnification, shows the origin and grain-boundary-initiated failure in detail.

132


FIGURE 8.41

Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. SEM fractograph at low magnification shows propa-gation of transcrystalline failure on the macroscopic facets extending on the B2 grains. Shallow dimples on the facets are also apparent.

FIGURE 8.42

Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. SEM fractograph of Figure 8.41 at high magnifi-cation shows propagation of transcrystalline failure on the macroscopic facets extending on the B2 grains.

Titanium Aluminides

133

FIGURE 8.43

Ti-24Al-11Nb, 1150˚C/1h/FC. Optical micrograph shows coarse colony structure of

α

2

and B2. (Courtesy of Gogia,A.K., Banerjee, D., and Nandy, T.K.,

Metallurgical Transactions A, 21A, 1990. With permission.)

FIGURE 8.44 Ti-24Al-11Nb, 1150˚C/1h/FC, tensile tested at room temperature. SEM macrograph shows faceted transgranularfracture. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)


FIGURE 8.45 Ti-24Al-11Nb, 1150˚C/1h/FC, tensile tested at room temperature. SEM fractograph shows cleavage fracture withfeathery features and secondary cracks. The orientation of cleavage facet changes across the colony boundaries.

FIGURE 8.46 Ti-24Al-11Nb, 1150˚C/1h/FC, tensile tested at room temperature. High-magnification SEM fractograph of individualfacets shows feathery cleavage of α2 plates, which are separated by tear ridges. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy,T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

Titanium Aluminides 135

FIGURE 8.47 Ti-24Al-11Nb, 1060˚C/4h/WQ + 800˚C/24h/AC. Optical micrograph shows α2 + transformed B2 microstructure.

FIGURE 8.48 Ti-24Al-11Nb, 1060˚C/4h/WQ + 800˚C/24h/AC, tensile tested at room temperature. Low-magnification SEM frac-tograph shows a fracture surface with secondary cracks.


FIGURE 8.49 Ti-24Al-11Nb, 1060˚C/4h/WQ + 800˚C/24h/AC, tensile tested at room temperature. SEM fractograph of center ofFigure 8.48 shows quasicleavage fracture features.

FIGURE 8.50 Ti-24Al-11Nb, 1060˚C/4h/WQ + 800˚C/24h/AC, tensile tested at room temperature. High-magnification SEM frac-tograph shows terraced fracture surface (arrow) with smooth plateau and ridges.


FIGURE 8.51 Ti-24Al-11Nb, 1150˚C/1h/AC + 750˚C/24h/AC. Optical micrograph shows α2 + transformed B2 microstructure.(From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept.of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.52 Ti-24Al-11Nb, 1150˚C/1h/AC + 750˚C/24h/AC, tensile tested at room temperature. Low-magnification SEM fracto-graph shows radial transgranular fracture and origin (arrow).


FIGURE 8.53 Ti-24Al-11Nb, 1150˚C/1h/AC + 750˚C/24h/AC, tensile tested at room temperature. SEM fractograph shows quasi-cleavage fracture features with prior beta boundary.

FIGURE 8.54 Ti-24Al-11Nb, 1150˚C/1h/AC + 750˚C/24h/AC, tensile tested at room temperature. High-magnification SEM fracto-graph shows quasicleavage fracture features.


FIGURE 8.55 Ti-24Al-20Nb, α2+β solution treated at 900˚C/1h/WQ. SEM backscattered electron image shows α2 (dark) + O (gray)+ B2 (bright) phases.

FIGURE 8.56 Ti-24Al-20Nb, α2+β solution treated at 980˚C/1h/WQ. SEM backscattered electron image shows α2 (dark) + O (darkgray needles) + B2 (light gray, matrix) phases. The volume fraction of the O phase is less compared with the 900˚C solution-treatedsample.


FIGURE 8.57 Ti-24Al-20Nb, α2+β solution treated at 1060˚C/1h/WQ. SEM backscattered electron image shows α2 (dark) + B2(gray, matrix) phases. The volume fraction of the B2 phase increases with increasing solution-treatment temperature.

FIGURE 8.58 Ti-24Al-20Nb, α2+β solution treated at 900˚C/1h/WQ, tensile tested at room temperature. SEM macrograph showsorigin (arrow) and secondary cracks.


FIGURE 8.59 Ti-24Al-20Nb, α2+β solution treated at 900˚C/1h/WQ, tensile tested at room temperature. SEM fractograph of Figure8.58 at intermediate magnification shows cleavage facets and a few dimples.

FIGURE 8.60 Ti-24Al-20Nb, α2+β solution treated at 900˚C/1h/WQ, tensile tested at room temperature. High-magnification SEMfractograph shows clear cleavage facets and a few dimples. Cleavage facets are attributed to the fracture of the α2 or O phase, anddimples are due to fracture of the β phase.


FIGURE 8.61 Ti-24Al-20Nb, α2+β solution treated at 980˚C/1h/WQ, tensile tested at room temperature. SEM macrograph showsfracture surface with secondary cracks.

FIGURE 8.62 Ti-24Al-20Nb, α2+β solution treated at 980˚C/1h/WQ, tensile tested at room temperature. SEM fractograph shows amixture of dimples and cleavage facets. Secondary cracks are also apparent.


FIGURE 8.63 Ti-24Al-20Nb, α2+β solution treated at 980˚C/1h/WQ, tensile tested at room temperature. High-magnification SEMfractograph of Figure 8.62 shows a mixture of dimples and cleavage facets. The dimpled region is related to the fracture of the βphase, and cleavage facets are related to the fracture of the α2 or O phase.

FIGURE 8.64 Ti-24Al-20Nb, α2+β solution treated at 1060˚C/1h/WQ, tensile tested at room temperature. SEM macrograph showssmooth fracture surface.


FIGURE 8.65 Ti-24Al-20Nb, α2+β solution treated at 1060˚C/1h/WQ, tensile tested at room temperature. SEM fractograph showsa mixture of dimples and cleavage facets.

FIGURE 8.66 Ti-24Al-20Nb, α2+β solution treated at 1060˚C/1h/WQ, tensile tested at room temperature. High-magnification SEMfractograph of Figure 8.65 shows a mixture of dimples and cleavage facets. The dimpled region is related to the fracture of the βphase, and the cleavage facets are related to the fracture of the α2 or O phase. A dimpled region surrounding the cleavage facets isalso apparent.


FIGURE 8.67 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.016˚C/sec. SEM backscattered electron image shows coarsecolony structure comprising the α2, O, and B2 phases.

FIGURE 8.68 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.1˚C/sec. SEM backscattered electron image (Figure 8.67)shows comparatively fine colony structure comprising the α2, O, and B2 phases.


FIGURE 8.69 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 2.5˚C/sec. SEM backscattered electron image shows fine basket-weave structure. The structure is very fine due to the faster cooling rate.

FIGURE 8.70 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. Low-magnifi-cation SEM fractograph shows rough transcrystalline fracture features.


FIGURE 8.71 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. High-magnifi-cation SEM fractograph shows transcrystalline fracture with large cleavage facets, with cleavage of similarly oriented laths in acolony and a few dimpled areas (arrow).

FIGURE 8.72 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. SEM macrographshows fine features with smooth appearance.


FIGURE 8.73 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. High-magnificationSEM fractograph shows transcrystalline fracture with large cleavage facets with cleavage of similarly oriented laths in a colony anda few dimpled areas.

FIGURE 8.74 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. Low-magnificationSEM fractograph shows coarse transgranular quasicleavage fracture features.


FIGURE 8.75 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. High-magnificationSEM fractograph shows transgranular quasicleavage with cleavage steps.

FIGURE 8.76 Ti-24Al-20Nb, β treated at 1140˚C/1h/WQ, fracture-toughness tested (K1c). Low-magnification SEM fractographshows classic cleavage fracture.


FIGURE 8.77 Ti-24Al-20Nb, β treated at 1140˚C/1h/WQ, fracture-toughness tested (K1c). High-magnification SEM fractograph ofFigure 8.76 shows river pattern on the cleaved facets and fine dimples in between the facets.

FIGURE 8.78 Ti-24Al-27Nb, α2+β solution treated at 900˚C/24h/WQ. SEM backscattered electron image shows two phases, B2(bright), in the matrix of O phase.


FIGURE 8.79 Ti-24Al-27Nb, α2+β solution treated at 940˚C/16h/WQ. SEM backscattered electron image shows two phases, B2(bright), in the matrix of O phase.

FIGURE 8.80 Ti-24Al-27Nb, α2+β solution treated at 980˚C/8h/WQ. SEM backscattered electron image shows two phases, O (dark),in the matrix of B2.


FIGURE 8.81 Ti-24Al-27Nb, α2+β solution treated at 1000˚C/6h/WQ. SEM backscattered electron image shows two phases, O(dark), in the matrix of B2. The volume fraction of B2 seems to be increasing with increasing solution-treatment temperature.



FIGURE 8.83 Ti-24Al-27Nb, α2+β solution treated at 900˚C/24h/WQ, tensile tested at room temperature. Low-magnification SEMfractograph of Figure 8.82 shows predominantly cleavage fracture.

FIGURE 8.84 Ti-24Al-27Nb, α2+β solution treated at 900˚C/24h/WQ, tensile tested at room temperature. High-magnification SEMfractograph of Figure 8.83 shows fine cleavage facets.



FIGURE 8.86 Ti-24Al-27Nb, α2+β solution treated at 940˚C/16h/WQ, tensile tested at room temperature. Low-magnification SEMfractograph shows predominantly cleavage fracture.


FIGURE 8.87 Ti-24Al-27Nb, α2+β solution treated at 940˚C/16h/WQ, tensile tested at room temperature. High-magnification SEMfractograph shows cleavage facets, which are slightly coarse as compared with samples treated at 900˚C (Figure 8.84).

FIGURE 8.88 Ti-24Al-27Nb, α2+β solution treated at 980˚C/8h/WQ, tensile tested at room temperature. SEM macrograph showsfracture surface and numerous secondary cracks.


FIGURE 8.89 Ti-24Al-27Nb, α2+β solution treated at 980˚C/8h/WQ, tensile tested at room temperature. Low-magnification SEMfractograph shows predominantly cleavage fracture features and some dimples. Dimpled region (A) indicates failure of fine recrys-tallized B2 grains, and the large cleavage facets (B) are due to unrecrystallized B2 grains.

FIGURE 8.90 Ti-24Al-27Nb, α2+β solution treated at 980˚C/8h/WQ, tensile tested at room temperature. High-magnification SEMfractograph of Figure 8.89 shows a large cleavage facet with river markings.


FIGURE 8.91 Ti-24Al-27Nb, α2+β solution treated at 1000˚C/6h/WQ, tensile tested at room temperature. SEM macrograph showsrough fracture surface.

FIGURE 8.92 Ti-24Al-27Nb, α2+β solution treated at 1000˚C/6h/WQ, tensile tested at room temperature. SEM fractograph showspredominantly coarse cleavage fracture features with secondary cracks.


FIGURE 8.93 Ti-24Al-27Nb, α2+β solution treated at 1000˚C/6h/WQ, tensile tested at room temperature. High-magnification SEMfractograph of Figure 8.92 shows coarse cleavage facets and a few dimples.

FIGURE 8.94 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.016˚C/sec. SEM backscattered electron image shows coloniesof O and B2 phases. The dark phase seen between the laths and at the prior β boundary is an Al-rich and Nb-lean phase.


FIGURE 8.95 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.1˚C/sec. SEM backscattered electron image shows colonies ofO and B2 phases. The dark phase seen between the laths and at the prior β boundary is an Al-rich and Nb-lean phase.

FIGURE 8.96 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 2.5˚C/sec. SEM backscattered electron image shows fine basket-weave structure (not resolved) of O + B2 due to higher Nb content and a dark phase (Al rich and Nb lean) at the boundaries.Microstructure is similar to Figure 8.69 but is very fine.


FIGURE 8.97 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. SEM macrographshows transgranular fracture.

FIGURE 8.98 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. Low-magnifi-cation SEM fractograph shows predominantly fine cleavage fracture with secondary cracks.


FIGURE 8.99 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. High-magnifi-cation SEM fractograph of Figure 8.98 shows cleavage facets with secondary cracks.

FIGURE 8.100 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. SEM macrographshows transgranular fracture features.


FIGURE 8.101 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. Low-magnificationSEM fractograph shows relatively coarse cleavage fracture with secondary cracks as compared with Figure 8.98.

FIGURE 8.102 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. High-magnificationSEM fractograph shows coarse cleavage facets with river markings and secondary cracks.


FIGURE 8.103 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. SEM macrographshows very coarse transgranular cleavage fracture.

FIGURE 8.104 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. SEM fractographshows very coarse cleavage facets with river markings.


FIGURE 8.105 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. SEM fractographshows cleavage facets with river markings. The fracture features coarsened with the increase in cooling rate from 0.016˚C/sec to2.5˚C/sec.

FIGURE 8.106 Ti-25Al-15Nb, α2+β heat-treated at 900˚C/16h/WQ. Optical micrograph shows α2 and B2 phases. (Transmissionelectron microscope [TEM] observations of this sample revealed small amounts of O phase.)


FIGURE 8.107 Ti-25Al-15Nb, α2+β heat-treated at 980˚C/8h/WQ. Optical micrograph shows α2 and B2 phases.

FIGURE 8.108 Ti-25Al-15Nb, α2+β heat-treated at 1020˚C/6h/WQ. Optical micrograph shows α2 and B2 phases.


FIGURE 8.109 Ti-25Al-15Nb, α2+β heat-treated at 1060˚C/4h/WQ. Optical micrograph shows α2 and B2 phases. A change in thevolume fraction of α2 is observed with the increase in solution-treatment temperature.

FIGURE 8.110 Ti-25Al-15Nb, α2+β heat-treated at 900˚C/16h/WQ, tensile tested at room temperature. SEM macrograph showssmooth fracture surface.


FIGURE 8.111 Ti-25Al-15Nb, α2+β heat-treated at 900˚C/16h/WQ, tensile tested at room temperature. High-magnification SEMfractograph shows cleavage features and secondary cracks.



FIGURE 8.113 Ti-25Al-15Nb, α2+β heat-treated at 980˚C/8h/WQ, tensile tested at room temperature. High-magnification SEMfractograph shows cleavage features and secondary cracks.


Titanium Aluminides

169

FIGURE 8.115

Ti-25Al-15Nb,

α

2

+

β

heat-treated at 1020˚C/6h/WQ, tensile tested at room temperature. High-magnification SEMfractograph shows coarse cleavage fracture features with river markings and secondary cracks.

FIGURE 8.116

Ti-25Al-15Nb,

α

2

+

β

heat-treated at 1060˚C/4h/WQ, tensile tested at room temperature. SEM macrograph showsrough fracture surface.

170


FIGURE 8.117

Ti-25Al-15Nb,

α

2

+

β

heat-treated at 1060˚C/4h/WQ, tensile tested at room temperature. High-magnification SEMfractograph shows coarser cleavage fracture features with river markings and secondary cracks. Cleavage facet size increases withincreasing the solution-treatment temperature.

FIGURE 8.118

Ti-25Al-15Nb,

β

heat-treated at 1170˚C/1h and cooled at 4˚C/sec. Optical micrograph shows extremely fine lathstructure of

α

2

with small amounts of primary

α

2

.

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171

FIGURE 8.119

Ti-25Al-15Nb,

β

heat-treated at 1170˚C/1h and cooled at 0.7˚C/sec. Optical micrograph shows basket-weave structureof

α

2

laths. Equiaxed primary

α

is present at the prior

β

grain boundaries and within the grains.

FIGURE 8.120

Ti-25Al-15Nb,

β

heat-treated at 1170˚C/1h and cooled at 4˚C/sec, tensile tested at room temperature. SEM macro-graph shows rough cleavage fracture features.

172


FIGURE 8.121

Ti-25Al-15Nb,

β

heat-treated at 1170˚C/1h and cooled at 4˚C/sec, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.120 shows cleavage steps and river markings.

FIGURE 8.122

Ti-25Al-15Nb,

β

heat-treated at 1170˚C/1h and cooled at 0.7˚C/sec, tensile tested at room temperature. SEMmacrograph shows rough transcrystalline fracture features.

Titanium Aluminides

173

FIGURE 8.123

Ti-25Al-15Nb,

β

heat-treated at 1170˚C/1h and cooled at 0.7˚C/sec, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.122 shows coarse leaf-like cleavage facets with river markings.

FIGURE 8.124

Ti-24Al-11Nb-4Ta,

α

2

+

β


α

2

and B2 phases.

174


FIGURE 8.125

Ti-24Al-11Nb-4Ta,

α

2

+

β


α

2

and B2 phases.

FIGURE 8.126

Ti-24Al-11Nb-4Ta,

α

2

+

β


α

2

and B2 phases.

Titanium Aluminides

175

FIGURE 8.127

Ti-24Al-11Nb-4Ta,

α

2

+

β


α

2

and B2 phases. The volumefraction of

α

2

decreases with increasing solution temperature.

FIGURE 8.128

Ti-24Al-11Nb-4Ta,

α

2

+

β

heat-treated at 900˚C/16h/WQ, tensile tested. SEM macrograph shows smooth fracture.

176


FIGURE 8.129

Ti-24Al-11Nb-4Ta,

α

2

+

β

heat-treated at 900˚C/16h/WQ, tensile tested. High-magnification SEM fractograph showspredominantly cleavage fracture features.

FIGURE 8.130

Ti-24Al-11Nb-4Ta,

α

2

+

β

heat-treated at 980˚C/8h/WQ, tensile tested. SEM macrograph shows smooth fracture.

Titanium Aluminides

177

FIGURE 8.131

Ti-24Al-11Nb-4Ta,

α

2

+

β

heat-treated at 980˚C/8h/WQ, tensile tested. High-magnification SEM fractograph showspredominantly cleavage fracture features with secondary cracks.

FIGURE 8.132

Ti-24Al-11Nb-4Ta,

α

2

+

β

heat-treated at 1060˚C/4h/WQ, tensile tested. Low-magnification SEM fractograph showstranscrystalline fracture features.

178


FIGURE 8.133

SEM fractograph of Ti-25Al-11Nb-4Ta,

α

2

+

β

heat-treated at 1060˚C/4h/WQ, tensile tested. High-magnificationfractograph showing predominantly cleavage fracture features (A) and a few dimpled areas (B).

FIGURE 8.134

SEM fractograph of Ti-24Al-11Nb-4Ta,

α

2

+

β

heat-treated at 1100˚C/2h/WQ, tensile tested. Low-magnification fracto-graph showing coarser transcrystalline fracture features as compared with samples solution treated at temperatures 900˚C to 1060˚C.

Titanium Aluminides

179

FIGURE 8.135

Ti-24Al-11Nb-4Ta,

α

2

+

β

heat-treated at 1100˚C/2h/WQ, tensile tested. High-magnification SEM fractograph showspredominantly large cleavage fracture features and some shallow dimples.

FIGURE 8.136

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 10˚C/sec. Optical micrograph shows fine lath structureof

α

2

.

180


FIGURE 8.137

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 2.5˚C/sec. Optical micrograph shows coarser

α

2

lathsas compared with Figure 8.136.

FIGURE 8.138

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 0.1˚C/sec. Optical micrograph showing coarse colonystructure of

α

2

laths. Coarse

α

2

laths are observed at prior

β

grain boundaries also. As the cooling rate decreases coarsening of thelaths occurs.

Titanium Aluminides

181

FIGURE 8.139

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 10˚C/sec, tensile tested. SEM macrograph shows coarsecleavage fracture features. The fracture initiation is at the prior

β

boundary, and propagation in the cleavage mode is apparent acrossthe entire prior

β

grain.

FIGURE 8.140

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 10˚C/sec, tensile tested. SEM fractograph shows roughcleavage facets with secondary cracks.

182


FIGURE 8.141

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 10˚C/sec, tensile tested. High-magnification SEMfractograph of Figure 8.140 shows cleavage fracture initiating at the prior

β

boundary.

FIGURE 8.142

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 2.5˚C/sec, tensile tested. SEM macrograph showstranscrystalline fracture with rough cleavage facets.

Titanium Aluminides

183

FIGURE 8.143

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 2.5˚C/sec, tensile tested. SEM fractograph shows roughcleavage facets.

FIGURE 8.144

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 2.5˚C/sec, tensile tested. High-magnification SEMfractograph of Figure 8.143 shows grain-boundary-initiated fracture (arrow).

184


FIGURE 8.145

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 0.1˚C/sec, tensile tested. SEM macrograph showstranscrystalline fracture with cleavage facets.

FIGURE 8.146

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 0.1˚C/sec, tensile tested. Low-magnification SEMfractograph shows cleavage facets traversing the colony of similarly oriented laths.

Titanium Aluminides

185

FIGURE 8.147

Ti-24Al-11Nb-4Ta,

β

solution treated at 1170˚C/1h/cooled at 0.1˚C/sec, tensile tested. High-magnification SEMfractograph shows feathery cleavage facets with secondary cracks.

FIGURE 8.148

Ti-25Al-14Nb-1Mo,

α

2

+

β


α

2

in the matrix of B2.

186


FIGURE 8.149

Ti-25Al-14Nb-1Mo,

α

2

+

β


α

2


FIGURE 8.150

Ti-25Al-14Nb-1Mo,

α

2

+

β


α

2


Titanium Aluminides

187

FIGURE 8.151

Ti-25Al-14Nb-1Mo,

α

2

+

β


α

2

in the matrix of B2. Thevolume fraction of

α

2

decreases with increasing solution-treatment temperature.

FIGURE 8.152

Ti-25Al-14Nb-1Mo,

α

2

+

β

heat-treated at 900˚C/16h/WQ, tensile tested at room temperature. SEM macrographshows rough fracture features.

188


FIGURE 8.153

Ti-25Al-14Nb-1Mo,

α

2

+

β

heat-treated at 900˚C/16h/WQ, tensile tested at room temperature. High-magnificationSEM fractograph shows fine cleavage fracture features with secondary cracks. Dimples are also seen at places.

FIGURE 8.154

Ti-25Al-14Nb-1Mo,

α

2

+

β

heat-treated at 980˚C/8h/WQ, tensile tested at room temperature. SEM macrograph showsrough fracture features. Secondary cracks are also seen.

Titanium Aluminides

189

FIGURE 8.155

Ti-25Al-14Nb-1Mo,

α

2

+β heat-treated at 980˚C/8h/WQ, tensile tested at room temperature. High-magnificationSEM fractograph shows slightly coarser cleavage fracture features compared with Figure 8.153.

FIGURE 8.156 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1020˚C/6h/WQ, tensile tested at room temperature. Low-magnificationSEM fractograph shows transcrystalline fracture features with the origin near to the center (arrow).


FIGURE 8.157 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1020˚C/6h/WQ, tensile tested at room temperature. High-magnificationSEM fractograph shows large cleavage facets and secondary cracks.

FIGURE 8.158 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1060˚C/4h/WQ, tensile tested at room temperature. SEM macrographshows transcrystalline fracture features and the origin (arrow). Secondary cracks are also seen.


FIGURE 8.159 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1060˚C/4h/WQ, tensile tested at room temperature. High-magnificationSEM fractograph shows very large cleavage facets with river markings.

FIGURE 8.160 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC. Optical micrograph shows fine recrystallized α grainssurrounding large grains with twins.


FIGURE 8.161 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC. Optical micrograph shows fine recrystallized α grainswith twins surrounding the large lamellae of α2+γ.

FIGURE 8.162 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC. SEM backscattered electron image of Figure 8.161shows lamellae of α2+γ within the recrystallized grains.


FIGURE 8.163 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC. SEM backscattered electron image of the arrow-markedarea in Figure 8.162 shows lamellae of α2+γ and massive γ phase at high magnification.

FIGURE 8.164 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC. Optical micrograph shows fine lamellar structure of α2+γ.


FIGURE 8.165 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC. Optical micrograph shows fine lamellar structure ofα2+γ and a thin layer of very fine recrystallized α grains at the boundaries.

FIGURE 8.166 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 950˚C/6h/AC. Optical micrograph shows coarse colonies of finelamellae of α2+γ.


FIGURE 8.167 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 950˚C/6h/AC. Optical micrograph shows lamellar structure of α2+γat higher magnification.

FIGURE 8.168 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC, tensile tested at room temperature. SEM macrographshows rough fracture surface with transgranular crack.


FIGURE 8.169 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC, tensile tested at room temperature. SEM fractographshows cleavage fracture features with secondary cracks.

FIGURE 8.170 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC, tensile tested at room temperature. High-magnificationSEM fractograph shows cleavage facets with river markings.


FIGURE 8.171 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM macrographshows rough fracture surface with facets.

FIGURE 8.172 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM fractographshows faceted fracture with secondary cracks. The effect of the microstructure is also seen on the fracture surface.


FIGURE 8.173 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. High-magnificationSEM fractograph of the arrow-marked region in Figure 8.172 shows fine dimples on a cleavage facet.

FIGURE 8.174 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM fractographof Figure 8.172 at higher magnification shows cleavage facets and delamination (arrow).


FIGURE 8.175 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM fractographof a different area at high magnification shows feathery cleavage facets.

FIGURE 8.176 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM macrographshows rough fracture surface.


FIGURE 8.177 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. High-magnifica-tion SEM fractograph of Figure 8.176 shows deep secondary cracks and cleavage facets.

FIGURE 8.178 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. High-magnifica-tion SEM fractograph of a different area shows fan-like cleavage facets with river markings.


FIGURE 8.179 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM fractographshows feathery pattern on the cleavage facet.

203

9

Case Study: Failure Investigation Report of IMI 550 High-Pressure Compressor (HPC-I) Aero Engine Blade*

9.1 INTRODUCTION

A failed high-pressure compressor (HPC) stage-1 aeroengine rotor blade of titanium alloy IMI 550 was receivedto assess the cause of premature failure [12]. The failureof the blade occurred during the trial run of the engine.The blade failed at the aerofoil hub and caused secondarydamage to the blades in the remaining stages.

9.2 INVESTIGATION

A schematic top view of the failed blade sample is shownin Figure 9.1. The fracture surface showed a smooth region(marked A), a comparatively rough region (marked B),and a shear-lip region (marked C). Low- and high-mag-nification photographs of the smooth region and shear lipare shown in Figure 9.2. The origin of the fracture wasseen in the smooth region (marked by an arrow). Adetailed fractography study was conducted using the scan-ning electron microscope (SEM). A small sample cut fromthe blade (as shown by the arrow in Figure 9.1) wasutilized for microstructural observation. Electron probemicroanalysis (EPMA) was used for chemical analysis.No coloration was observed on the fracture surface or onthe rest of the blade, indicating that there was no thermaldamage.

9.2.1 C

HEMICAL

A

NALYSIS

The specified chemistry and the chemistry of the sampleevaluated by EPMA are presented in Table 9.1. The alloychemistry is within the specification limits of IMI 550.

9.2.2 M

ICROSTRUCTURE

The alloy IMI 550 is generally used in the

α

+

β

heat-treated condition (900˚C/1h/AC + 500˚C/24h/AC). Themicrostructure of the sample showed equiaxed

α

andtransformed

β

of approximately equal volumes (Figure9.3). The microstructure appeared to be consistent withthe heat treatment. Large

α

stringers (>100

μ

m) alignedalong the blade axis were observed at many locations.Because the blades were machined from bar stock, no

oxygen enrichment or alpha casing was expected, andnone was observed.

9.2.3 F

RACTOGRAPHY

The failed sample was seen in the SEM for a detailedobservation of the fracture surface. The apparent origin ofthe fracture observed at low magnification is shown inFigure 9.2. The same is shown at higher magnification inFigure 9.4. The origin appeared to be associated withmachining grooves, as seen Figure 9.4b. A prominentbeach mark was observed on the fracture surface (Figure9.5).

Observation of the fracture surface at higher magnifi-cations (Figure 9.6, Figure 9.7, and Figure 9.8) showedvoids in the thumbnail-shaped origin. Flat smooth features(marked A in Figure 9.7) and tear ridges were observedthroughout the fracture surface. Secondary cracking wasalso observed, and the extent of the secondary crackingwas observed to increase at locations farther from theorigin. These secondary cracks appeared to be aligned.The thumbnail shape of the origin, presence of prominentbeach mark, alignment of secondary cracking parallel tothe origin and beach mark, and smooth appearance of thefracture surface in region A are indicative of fatigue fail-ure. Extensive tearing also suggests that the blade samplemight have undergone fatigue deformation closer to low-cycle fatigue conditions. However, the fracture featuresdid not provide conclusive and unambiguous evidence forthe fatigue conditions, and the observed features shouldonly be used in conjunction with other data such as stressanalysis.

Samples that fail under fatigue conditions usuallyexhibit fatigue striations on the fracture surface. However,the sample under investigation exhibited no such features.The appearance of striations in fatigue-tested samples,however, depends upon material and microstructural con-ditions. Alpha + beta heat-treated titanium alloys have notbeen found to exhibit these striations (Figure 9.9), whilein

β

-treated conditions, titanium alloys usually showprominent striations (Figure 9.10). The fracture featuresof an

α

+

β

heat-treated laboratory-tested specimen shown

* All the figures in this chapter are from Gogia, A.K., Muraleedharan, K., Joshi, V.A., and Banerjee, D., Failure Investigation of HPC Blade, unpublishedreport, 2000. With permission.

204


in Figure 9.9 are otherwise similar to those observed inthe sample under investigation and are very distinct fromthose resulting from the other modes of failure, such astensile overload.

Region B revealed dimpled failure due to tensile over-load, as shown in Figure 9.11. The tensile failure in regionB can occur once the crack in region A has grown suffi-ciently large so that the stress level exceeds the tensilestrength of the material in the remaining load-bearingsection of the blade.

Prima facie, the failure appeared to have occurredunder fatigue conditions. The origin of the fatigue crackappeared to be associated with machine marks. To inves-tigate further, the failed blade sample was sectionedthrough the origin (Figure 9.12) to observe underlyingdefects that might have initiated the crack. The sectioningwas done by marking the origin precisely and cutting witha low-speed diamond saw. The section was polished andetched for observation under SEM. Figure 9.13 shows the

section through the origin and the presence of several deepmachining notches (

≈

10 to 20

μ

m). The fracture appearsto have initiated at one such notch. Upon tilting the sam-ple, it was seen clearly that the fracture originated at themachining notch (Figure 9.14). Large numbers of smallvoids were observed near the fracture surface (Figure9.13b). The density of these voids was observed todecrease with increasing distance from the fracture sur-face, as indicated by the computer-processed image shownin Figure 9.13c. These voids may be due to stressconcentration ahead of the fatigue crack. The absence ofthese voids away from the fracture surface suggests thatthey were not present beforehand and are not due to mate-rial defect. The presence of machining notches in the filletradius region (Figure 9.15) suggests the use of an impropermachining and polishing process on the blades. This isalso supported by the fact that these notches were seeneven in an unused blade (Figure 9.16).

9.2.4 S

TRESS

-C

ONCENTRATION

E

FFECTS

OF

A

N

OTCH

A geometrical discontinuity in the blade, such as amachining notch, can result in stress concentration at thediscontinuity. For a small elliptical notch, the maximumstress at the ends of a notch is given by the equation

σ

max

=

σ

(1+2)

a

/

b

where

a

and

b

are axes of the elliptical notch. In the presentcase, the notch can be approximated as a semicircle andthus

a

≈

b

, in which case

σ

max

= 3

σ

. The stress concentrationdue to machining can result in stresses close to yield, evenat low applied stress, that can cause fatigue failure.

TABLE 9.1Specified and Analyzed Composition of Alloy IMI 550

Element Specifications (wt.%) EPMA (wt.%)

Al 3.0–5.0 4.2Sn 1.5–2.5 2.5Mo 3.0–5.0 4.3Si 0.3–0.7 0.6

Source

: From Gogia, A.K., Muraleedharan, K., Joshi, V.A., and Banerjee,D., Failure Investigation of HPC Blade, unpublished report, 2000. Withpermission.

FIGURE 9.1

Schematic sketch of the failed blade.

Fracture surface

B

A

C Polished for microstructure

Case Study: Failure Investigation Report of IMI 550 High-Pressure Compressor (HPC-I) Aero Engine Blade

205

9.2.5 A

NALYSIS

OF

THE

D

EPOSITS

A small sample was cut from the blade area showing athicker deposit and analyzed by electron probe microanal-ysis (EPMA). The morphology of the deposit is shown inFigure 9.17. The EPMA results showed that the depositcontains Al, Si, and O (Figure 9.18). The deposit couldbe oxides of aluminum and silicon.

9.3 CONCLUSION

• The microstructure and chemistry of the mate-rial were within specifications.

• Machining notches on the blade, observedmainly at the fillet radius region, were the resultof an improper machining and polishing process.

• A deposit on the remaining blades — a result ofthe blade failure — showed oxides of Al and Si.

• Blade failure appeared to be due to fatigue. Thefatigue crack was initiated at the site of amachining notch on the blade surface.

FIGURE 9.2

Macrograph of the origin and the shear-lip regions A and C of Figure

9.1 shown at (a) low magnification and (b) highmagnification.

Origin

a C

2 mm

A

1 mm

Origin

b

C

Shear lip

A

206


FIGURE 9.3

Optical micrographs from the failed blade: (a) low-magnification image showing primary

α

+ transformed

β

micro-structure with elongated

α

stringers and (b) high-magnification micrograph showing primary

α

+ transformed

β

microstructure withlarge

α

stringers.


207

FIGURE 9.4

SEM micrographs of the fracture surface near the origin: (a) low magnification and (b) high magnification. The machinemarks on the surface are clearly seen in (b).

Origin

Origin

a

b

208


FIGURE 9.5

SEM micrograph shows distinct features of the fracture. A prominent beach mark, as shown, may be related to a stressjump in the life cycle of the failed blade. The final failure region, marked with a shear lip, is shown at the bottom of the micrograph.

FIGURE 9.6

Fracture features at the origin at a higher magnification. Voids are seen at the origin.

Origin

Shear lip

Beach mark


209

FIGURE 9.7

Fracture features away from the origin, in between the origin and the beach mark, at a higher magnification. Flatsmooth regions, marked A, and tear ridges are seen.

FIGURE 9.8

Fracture features outside of the beach mark showing secondary cracks at a higher magnification.

210


FIGURE 9.9

Fatigue features on the fracture surface of

α

+

β

heat-treated Ti-6.5Al-3.2Mo-1.8Zr-0.25Si alloy tested under HCFcondition.

FIGURE 9.10

Fatigue striations on the fracture surface of

β

heat-treated Ti-6.5Al-3.2Mo-1.8Zr-0.25Si alloy tested under HCFcondition.


211

FIGURE 9.11

Region B of Figure 9.1 shows ductile overload fracture features with dimples at higher magnification.

FIGURE 9.12

Micrograph of the blade sample showing the section through the origin that was sampled to investigate the micro-structure of the blade underneath the fracture origin.

212


FIGURE 9.13

Micrographs of the blade sample at the section through the origin: (a) secondary electron micrograph showingmachining marks as indicated (the origin and the fracture surface are also marked); (b) backscattered electron micrograph showingthe microstructure underneath the fracture surface; and (c) distribution of voids in the material below the fracture surface in the samearea as in (b). The image in (c) is a binary image processed by an image-analysis routine to reveal only the darkest (void) areas inthe micrograph in (b).

Fracturesurface

Origin

a

Fracturesurface

b50 µm


213

FIGURE 9.13 (continued)

FIGURE 9.14

Same sample as in Figure 9.13, in tilted condition, showing the origin and machine marks on the fillet radius asindicated.

c50 µm

Fracturesurface

Origin

214


FIGURE 9.15

Micrographs showing machine marks (arrows) at the fillet radius sectioned through the origin.


215

FIGURE 9.16

Micrographs of an unused blade: (a) aerofoil section and (b) root radius section. Arrows show machine marks.

a

b

216


FIGURE 9.17

SEM micrograph shows morphology of the deposit on the blades.


217

FIGURE 9.18

EPMA images show backscattered electron micrograph and x-ray elemental images from the same region taken acrossthe blade with a deposit. The thickness of the deposit is about 100

μ

m, and the elemental images indicate the presence of Si, Al,and O. A copper adhesive tape strip (seen on the top part of the BSE

micrograph) was attached to the outer surface of the blade topreserve the deposit during metallographic preparation of the cross section.

Cu Strip

Deposit

Blade

50 µm

BSE

Ti

Si O

Al

219

References

1. Titanium, Technology Information, Forecasting andAssessment Council, Department of Science and Tech-nology, New Delhi, July 1991.

2. Gogia, A.K., Titanium (in-house publication), Vol. 2,Nos. 3 and 4, Defence Metallurgical Research Labora-tory, Hyderabad, India, 1997.

3. Jaffee, R.I.,

Titanium Science and Technology

, Vol. 3,Jaffee, R.I. and Burte, H.M., Eds., Plenum Press, NewYork, 1973, p. 1665.

4. Kornilov, I.I.,

The Science, Technology and Applicationsof Titanium

, Jaffee, R.I. and Promisel, N.E., Eds., Per-gamon Press, Oxford, 1970, p. 407.

5. Rosenberg, H.W.,

The Science, Technology and Appli-cations of Titanium

, Jaffee, R.I. and Promisel, N.E.,Eds., Pergamon Press, Oxford, 1970, p. 851.

6. Gogia, A.K., doctoral thesis, Banaras Hindu University,Varanasi, India, 1990.

7. Banerjee, D., Saha, R.L., Mukherjee, D., and Mura-leedharan, K., DMRL technical report, DMRL TR 8983,Defence Metallurgical Research Laboratory, Hydera-bad, India, 1989.

8. Bhattacharjee, A., Joshi, V.A., Deshpande, D., Hussain,S.M., Nandy, T.K., and Gogia, A.K., DMRL technicalreport, DMRL TR 2000270, Defence MetallurgicalResearch Laboratory, Hyderabad, India, 2000.

9. Gogia, A.K., Banerjee, D., and Nandy, T.K.,

Metal.Trans. A.

, 21A, 609, 1990.

10. Gogia, A.K., Nandy, T.K., Muraleedharan, K., Joshi,V.A., Sagar, P.K., and Banerjee, D., DMRL technicalreport, DMRL TR 94175, Defence MetallurgicalResearch Laboratory, Hyderabad, India, 1994.

11. Gogia, A.K., Nandy, T.K., Muraleedharan, K., Joshi,V.A., Kamath, S.V., and Banerjee, D., DMRL technicalreport, DMRL TR 93166, Defence MetallurgicalResearch Laboratory, Hyderabad, India, 1993.

12. Gogia, A.K., Muraleedharan, K., Joshi, V.A., and Ban-erjee, D., Failure Investigation of HPC Blade, unpub-lished report, 2000.

FURTHER READING

1. ASM,

Metals Handbook: Fractography and Atlas of

Fractographs

, Vol. 9, American Society for Metals, Met-als Park, OH, 1974.

2. Leyens, C. and Peters, M., Eds.,

Titanium and Titanium

Alloys

, Wiley-VCH GmbH & Co. KgaA, Germany,2003.

3. Polmear, I.J.,

Light Alloys: Metallurgy of the Light Met-

als

, Edward Arnold, London, 1989.4. Donachie, M.J., Jr., Titanium, A Technical Guide, ASM

International, Metals Park, OH, 1989.

221

Index

A

Acicular

αβ

alloysTi-10V-2Fe-3Al (Ti-10-2-3), 101Ti-10V-4.5Fe-1.5Al, 109

near-

α

alloys, 23, 32Ti-24Al-11Nb, 130

Acicular

β

, VT9, 70Aero engine rotor blade failure,

see

Failure investigation report, HPC-I aero engine blade

Aerospace applications, 7–8, 10Aging conditions,

β

alloys,

see

β

alloys, microstructure and fractographyAllotropic modifications, 9, 11Alloy C, chemical composition, 17Alloying element effects, 9

α

, primaryaero engine blade failure analysis, 206

α

+

β

alloysTi-6Al-4V, 59VT9, 70

near-

α

alloys, IMI 834, 38, 49–50Ti-25Al-15Nb, 171

α

alloys, 19–23;

see also

Near-

α

alloyschemical composition, 17crystal structure, 9metallurgy, 9–10phases observed in, 11

α

phaseaging and, 97physical metallurgy, 11

α

+

β

alloyschemical composition, 17metallurgy, 10microstructure, evolution of, 11, 13–14microstructure and fractography, 59–95

Ti-6Al-4V, 59, 60–70VT9, 59, 70–95

α

+

β

solution/heat treatmentaero engine blade failure analysis, 203–204

α

+

β

alloys, VT9, 71–86

β

alloys, 97Ti-10V-2Fe-3Al (Ti-10-2-3), 102Ti-10V-4.5Fe-1.5Al, 109–110

titanium aluminides, 111Ti-24Al-11Nb, 114, 116, 121, 123Ti-24Al-11Nb-4Ta, 173–179Ti-24Al-20Nb, 139–144Ti-24Al-27Nb, 150–158, 164Ti-25Al-14Nb-1Mo, 185–191Ti-25Al-15Nb, 165–170

α

precipitates,

β


α

stringers, aero engine blade failure analysis, 206

α

2

(Ti

3

Al), 10chemical composition, 17microstructure, evolution of, 13, 15phases observed in, 11

α

2

failureTi-24Al-11Nb, 116Ti-24Al-20Nb, 141, 143

α

2

phaseTi-24Al-11Nb, 114, 121, 125, 127, 133–135, 137Ti-24Al-11Nb-4Ta, 173–175, 179–180Ti-24Al-20Nb, 141, 145Ti-25Al-14Nb-1Mo, 185–187Ti-25Al-15Nb, 164–166, 170

α

2

+

γ

Ti-47Al-2Nb-2Cr, 192Ti-48Al-4Nb-1Mo, 194

α

volume fraction, near-

α

alloys, 23Aluminides,

see

Titanium aluminidesAluminum

α

alloys, 19chemical composition, 17–18effects of alloying elements, 9–10

Aluminum equivalent (Rosenberg criterion), 9–10Aluminum-rich phase, Ti-24Al-27Nb, 158–159Analysis of deposits, HPC-I aero engine blade failure investigation, 205,

216–217Applications of titanium alloys, 7–9Architecture, applications of titanium alloys, 7Automotive applications, 7–8

B

B1/

β′, 11

B2/

β

2

microstructure, evolution of, 15phases observed in alloys, 11

B2 grains, Ti-24Al-27Nb, 156B2 phase

titanium aluminides, 111Ti-24Al-11Nb, 114, 116, 121, 125, 127, 130, 132–133Ti-24Al-11Nb-4Ta, 173–175Ti-24Al-20Nb, 145Ti-24Al-27Nb, 150–152, 158Ti-25Al-14Nb-1Mo, 185–187Ti-25Al-15Nb, 164–166

Basket-weave structure, 15Ti-24Al-20Nb, 146Ti-24Al-27Nb, 159Ti-25Al-15Nb, 171

bcc phases, 11Beach mark, aero engine blade failure analysis, 203, 208–209

β

alloyschemical composition, 17crystal structure, 9metallurgy, 9–10metastable, 10microstructure, evolution of, 11–12microstructure and fractography, 97–110

Ti-10V-2Fe-3Al (Ti-10-2-3), 97–102Ti-10V-4.5Fe-1.5Al, 102–110

phases observed in, 11

β

boundariesnear-

α

alloys, IMI 834, 39

222


titanium aluminidesTi-24Al-11Nb-4Ta, 181–182Ti-24Al-27Nb, 158

β−

C, 97

β

microstructure, aero engine blade failure analysis, 206

β

phase fracturenear-

α

alloys, 37Ti-24Al-20Nb, 141, 143–144

β

solution/heat treatment

α

+

β

alloys, VT9, 91–92, 95

β

alloys, 97Ti-10V-2Fe-3Al (Ti-10-2-3), 97–101Ti-10V-4.5Fe-1.5Al, 103–108

and striation, 203titanium aluminides

Ti-24Al-11Nb-4Ta, 179–185Ti-24Al-20Nb, 145–150Ti-24Al-27Nb, 159–164Ti-25Al-15Nb, 170–173

β

stabilizers,

see

α

+

β

alloys

β

structure/microstructure

α

+

β

alloys, VT9, 86near-

α

alloysIMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 26, 28IMI 834, 51

β

-Ti alloy, chemical composition, 18Bilbao, Guggenheim Museum, 7Biomedical applications, 7–8Body-centred cubic structure,

β

alloys, 9Boron, effects of alloying elements, 9Boundary initiated failure

titanium aluminidesTi-24Al-11Nb, 131Ti-24Al-11Nb-4Ta, 183

Boundary regionsgrain, 12

intergranular fracture, 3near-

α

alloys, OT4-1, 24Ti-25Al-15Nb, 171

IMI 685, 38titanium aluminides

Ti-24Al-11Nb, 131Ti-24Al-11Nb-4Ta, 180, 183Ti-24Al-27Nb, 158–159

Brittle fracture, Ti-10V-2Fe-3Al (Ti-10-2-3), 100–101Bromine, effects of alloying elements, 9

C

Carbon, effects of alloying elements, 9Chemical analysis, HPC-I aero engine blade failure investigation,

203–204Chemical applications, 7–8Chemical composition of alloys, 17–18Chlorine, effects of alloying elements, 9Chromium

chemical composition of alloys, 17–18effects of alloying elements, 9–10

Cleavage, fractography, principles of, 1Cleavage facets

near-

α

alloys, 23IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 36, 37IMI 834, 41, 45–46, 57

titanium aluminides, 111Ti-24Al-11Nb, 118, 120–121, 128–129Ti-24Al-11Nb-4Ta, 181, 183–185

Ti-24Al-20Nb, 141–144, 147Ti-24Al-27Nb, 153, 155–156, 158, 161, 164Ti-25Al-14Nb-1Mo, 190–191Ti-25Al-15Nb, 170, 173Ti-47Al-2Nb-2Cr, 196, 198Ti-48Al-4Nb-1Mo, 200–201

Cleavage fracturesnear-

α

alloys, IMI 834, 40–41, 43titanium aluminides

Ti-24Al-11Nb, 113, 115, 117, 126Ti-24Al-11Nb-4Ta, 176, 178–179, 181Ti-24Al-20Nb, 149Ti-24Al-27Nb, 153–154, 156–157, 160, 163Ti-25Al-14Nb-1Mo, 188–189Ti-25Al-15Nb, 167–168Ti-47Al-2Nb-2Cr, 196

Cleavage steps, titanium aluminidesTi-24Al-20Nb, 149Ti-25Al-15Nb, 172

Closely packed hexagonal structure,

α

alloys, 9Coalescence, intergranular fracture, 3Coarse cleavage, titanium aluminides

Ti-24Al-11Nb, 130Ti-24Al-11Nb-4Ta, 181Ti-24Al-27Nb, 162Ti-25Al-14Nb-1Mo, 189Ti-25Al-15Nb, 169–170

Coarse colony structure, Ti-24Al-20Nb, 145Coarse granular fracture, IMI 834, 56Cobalt, effects of alloying elements, 9Colony boundaries

IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 38Ti-24Al-11Nb, 134

Colony structure, titanium aluminidesTi-24Al-11Nb, 133Ti-24Al-11Nb-4Ta, 180Ti-24Al-20Nb, 145Ti-24Al-27Nb, 158Ti-48Al-4Nb-1Mo, 194

Commercially pure (CP) titaniumapplications, 7chemical composition, 18microstructure and fractography, 19–22

Conical dimple,

α

alloys, 22Continuous solid solution formation, 9Conventional alloys, microstructure evolution, 11–15Cooling rate,

see also

Specific heat treatmentsmicrostructure evolution, 11titanium aluminides

Ti-24Al-20Nb, 146–148Ti-24Al-27Nb, 163–164

Copper, effects of alloying elements, 9Corrosion resistance, 7–8Covalent bonding, 9CP titanium,

see

Commercially pure (CP) titaniumCreep strength, 8Creep testing, near-

α

alloys, 23IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 35–37, 38IMI 834, 52–57

Creep-to-rupture fractures, intergranular, 3Crystal structure, 3

cleavage fracture, 1microstructure, 11types of titanium alloys, 9

Crystalline fracture, Ti-10V-4.5Fe-1.5Al, 106–107

Index

223

Cup-and-cone fracture

α

+

β

alloys, Ti-6Al-4V, 59–60, 62, 64

β

alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 102Cyclic relaxation fracture, IMI 834, 46

D

Delamination, Ti-47Al-2Nb-2Cr, 198Deposits, aero engine blade failure analysis, 205, 217Dimple rupture

aero engine blade failure analysis, 204fractography, principles of, 1–2

Dimplesaero engine blade failure analysis, 210

α

alloys, 20–22

α

+

β

alloys, 59Ti-6Al-4V, 60–65, 67VT9, 72–73, 75, 77–80, 90–91, 93

β

alloys, 97Ti-10V-2Fe-3Al (Ti-10-2-3), 100–102Ti-10V-4.5Fe-1.5Al, 104–105, 107–108, 110

near-

α

alloys, 23IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 30–31, 34, 37IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 27–28IMI 834, 40–41, 46–48, 52, 54–57OT4-1, 25

titanium aluminides, 111Ti-24Al-11Nb, 115–116, 118, 120, 122, 124, 129, 132Ti-24Al-11Nb-4Ta, 178–179Ti-24Al-20Nb, 141–144, 147, 150Ti-24Al-27Nb, 156, 158Ti-25Al-14Nb-1Mo, 188Ti-47Al-2Nb-2Cr, 198

Ductile fracture

β

alloys, 97Ti-10V-2Fe-3Al (Ti-10-2-3), 99–102Ti-10V-4.5Fe-1.5Al, 104, 106, 108

dimple rupture, 1–2near-

α

alloysIMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 29IMI 834, 52–53, 55

titanium aluminides, Ti-24Al-11Nb, 129Ductile overload, aero engine blade failure analysis, 210

E

Electron probe microanalysis (EMPA), aero engine blade failure analysis, 203, 205, 217

Engine rotor blade failure,

see


Engine valves, applications of titanium alloys, 7Equiaxed

α

, primary

β

alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 101Ti-25Al-15Nb, 171

Equiaxed

α

2, Ti-24Al-11Nb, 124Equiaxed

α

2 and B2, Ti-24Al-11Nb, 116, 123Equiaxed dimples,

α

+

β

alloysTi-6Al-4V, 59, 61, 63VT9, 75, 80, 90

Equiaxed grains,

β


Eutectoid transformation, 9–10

F

Faceted failureTi-24Al-11Nb, 124, 133Ti-47Al-2Nb-2Cr, 197

Facets, VT9, 93Failure investigation report, HPC-I aero engine blade,

203–217analysis of deposits, 205, 216–217chemical analysis, 203–204conclusion, 205fractography, 203–204, 205–215microstructure, 203, 206stress-concentration effects of notch, 204

Fan-like cleavage facet, Ti-48Al-4Nb-1Mo, 200Fatigue

aero engine blade failure analysis, 203, 210fractography, principles of, 2–3, 4

Fatigue resistance properties, 7–8Fatigue testing

α

+

β

alloysTi-6Al-4V, 66–69VT9, 81–86, 94

near-

α

alloys, IMI 834, 42–46Feathery features, 1

Ti-24Al-11Nb, 118, 127, 134Ti-24Al-11Nb-4Ta, 185Ti-48Al-4Nb-1Mo, 199, 201

Fine cleavage fractureTi-24Al-27Nb, 160Ti-25Al-14Nb-1Mo, 188

Fissures

α

+

β

alloys, Ti-6Al-4V, 67near-

α

alloys, IMI 834, 43Flat fracture, VT9, 76Fluorine, effects of alloying elements, 9Fluting, Ti-24Al-11Nb, 116Forge temperature

Ti-47Al-2Nb-2Cr, 191–198Ti-48Al-4Nb-1Mo, 194–195, 199–201

Fractography, 1–5

α

alloys, 19–22cleavage, 1dimple rupture, 1–2fatigue, 2–3, 4HPC-I aero engine blade failure investigation, 203–204,

205–215intergranular, 3–5

Fracture behaviormicrostructure and, 15near-

α

alloysIMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 26–27OT4-1, 24

Fracture toughness testing, Ti-24Al-20Nb, 149

G

Gallium, effects of alloying elements, 9Gamma (TiAl), 10, 173–185

chemical composition, 17microstructure, evolution of, 15phases observed in, 11Ti-24Al-11Nb-4Ta, 111, 173–185Ti-25Al-14Nb-1Mo, 185–191Ti-47Al-2Nb-2Cr, 111, 191–194, 195–199Ti-48Al-4Nb-1Mo, 111, 194–195, 199–201

224


Gamma phaseTi-47Al-2Nb-2Cr, 192–193Ti-48Al-4Nb-1Mo, 194

Globular

α

, Ti-10V-4.5Fe-1.5Al, 109Grain boundaries, Ti-24Al-11Nb-4Ta, 180Grain boundary failure, titanium aluminides

Ti-24Al-11Nb, 131Ti-24Al-11Nb-4Ta, 183

Grain boundary phases, 12intergranular fracture, 3near-

α

alloys, OT4-1, 24Ti-25Al-15Nb, 171

Grain size, evolution of microstructure, 13Granular fracture,

see

Intergranular fractureGuggenheim Museum, Bilbao, 7

H

Hafnium, effects of alloying elements, 9Herringbone structure, 1High-cycle fatigue, 2High-pressure compressor (HPC-I) aero engine blade failure,

see


Hydrogen, effects of alloying elements, 9Hydrogen damage, intergranular fracture, 3

I

Image processing, aero engine blade failure analysis, 212IMI 550

chemical composition, 17engine rotor blade failure,

see


IMI 685chemical composition, 18microstructure and fractography

Ti-6Al-5-Zr-0.5Mo-0.25Si, 29–38Ti-6Al-5-Zr-0.5Mo-0.30Si, 23, 26–28

IMI 834chemical composition, 18microstructure and fractography, 23, 38–57

Impact testing, VT9, 76–79, 91–92Inclusions, Ti-10V-4.5Fe-1.5Al, 105Indium, effects of alloying elements, 9Intergranular facets, Ti-24Al-11Nb, 130Intergranular fracture

α

+

β

alloys, VT9, 91–92

β

alloys, Ti-10V-4.5Fe-1.5Al, 97, 106, 108fractography, principles of, 3–5near-

α

alloys, IMI 834, 56titanium aluminides, Ti-24Al-11Nb, 129

Intermetallics (Ti

3

Al, Ti

2

AlNb, TiAl),

see also

Titanium aluminideshigh-temperature properties, 10microstructure, 10phases, precipitates, 11

Interstitial solid solution formation, 9Iodine, effects of alloying elements, 9Ionic bonding, effects of alloying elements, 9Iron

chemical composition, 17–18effects of alloying elements, 9–10

L

LamellaeTi-47Al-2Nb-2Cr, 192–193Ti-48Al-4Nb-1Mo, 194–195

Lathsnear-

α

alloysIMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 36, 38IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 27

titanium aluminides, 111Ti-24Al-11Nb-4Ta, 179–180, 184Ti-24Al-20Nb, 147–148Ti-24Al-27Nb, 158–159Ti-25Al-15Nb, 170

Leaf-like cleavage facets, Ti-25Al-15Nb, 173Limited solid solution formation, 9

M

Machining grooves, aero engine blade failure analysis, 203–204, 207, 212–215

Manganesechemical composition, 17–18effects of alloying elements, 9

Marine applications, 7–8Martensitic transformation, 11–13Mechanical properties, microstructure and, 15Metallurgy, physical,

see

Physical metallurgyMetastable

β

alloys, 10–11

α

+

β

solution treatment and aging, 97chemical composition, 17

Microstructure,

see also

Specific alloys and alloy classesaero engine blade failure analysis, 212

α

alloys, 19–22applications of titanium alloys, 8HPC-I aero engine blade failure investigation, 203, 206physical metallurgy, 10–16

conventional alloys, 11–15titanium aluminides, 15

Microvoids,

see

Voids/microvoidsMixed mode fracture

β

alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 100–101near-

α

alloys, IMI 834, 57Ti-24Al-11Nb, 130

Molybdenum

β

alloys, 97chemical composition, 17–18effects of alloying elements, 9–10titanium aluminides, 111

N

Near-

α

alloyschemical composition, 17metallurgy, 10microstructure and fractography, 23–57

IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 29–38IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 23, 26–28IMI 834, 23, 38–57OT4-1, 23–25

Near-

β

alloys, 10Necking,

α

+

β

alloys, 59Nickel, effects of alloying elements, 9Niobium

β

alloys, 97chemical composition, 17–18effects of alloying elements, 9–10

Niobium-lean phase, Ti-24Al-27Nb, 158–159Nitrogen, effects of alloying elements, 9Notch, stress-concentration effects of, 204

Index

225

Notch tensile testing, IMI 834, 40–41Nucleation, 1, 11

O

O phasemicrostructure, evolution of, 15titanium aluminides, 111

Ti-24Al-20Nb, 139, 141, 145Ti-24Al-27Nb, 150–151, 158Ti-25Al-15Nb, 164

O phase fracture, Ti-24Al-20Nb, 141, 143–144Offshore applications, 7Omega, 11, 13Orthorhombic structure

chemical composition, 17microstructure, 11titanium aluminides, 111

OT4-1chemical composition, 18microstructure and fractography, 23–25

Overload fracture

α

alloys, 20–21

α

+

β

alloys, Ti-6Al-4V, 67near-

α

alloys, IMI 685, 34Oxidation resistance, 8Oxide deposit, aero engine blade failure analysis, 205, 216–217Oxygen

α

alloys, 19effects of alloying elements, 9

Oxygen content, VT9, 70–80, 87–93

P

Peritectoid transformation, 9Phase diagrams, alloys, 11Phosphorus, effects of alloying elements, 9Physical metallurgy, 7–15

alloying element effects, 9applications of titanium alloys, 7–9microstructure, evolution of, 10–16

conventional alloys, 11–15titanium aluminides, 15

phases observed in alloys, 11physical properties of unalloyed titanium, 9types of titanium alloys, 9–10

Physical properties, unalloyed titanium, 9Plastic deformation, 1Precipitates

aging and, 97

β


microstructure, 11Primary

α

,

see

α

+

β

solution/heat treatment;

α

, primary

Q

Quasicleavage, 1, 111Ti-24Al-11Nb, 119, 136, 138Ti-24Al-20Nb, 148–149

Quaternary additions, titanium aluminides, 111

R

Radial fracture, Ti-24Al-11Nb, 119, 137Recrystallization, 10, 111

Ti-24Al-27Nb, 156Ti-47Al-2Nb-2Cr, 191–192, 194

Ripples, dimple rupture, 1River markings, 1, 111

Ti-24Al-20Nb, 150Ti-24Al-27Nb, 156, 162–164Ti-25Al-14Nb-1Mo, 191Ti-25Al-15Nb, 169–170, 172–173Ti-47Al-2Nb-2Cr, 196Ti-48Al-4Nb-1Mo, 200

Rock candy appearance, intergranular fracture, 3Rosenberg criterion, 9–10Rotor blade failure,

see


Rough fracturenear-

α

alloysIMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 35IMI 834, 39

titanium aluminidesTi-24Al-11Nb, 125Ti-24Al-11Nb-4Ta, 183Ti-24Al-27Nb, 157Ti-25Al-14Nb-1Mo, 187–188Ti-25Al-15Nb, 169, 171Ti-47Al-2Nb-2Cr, 195Ti-48Al-4Nb-1Mo, 199

S

Secondary

α

, 13Secondary cracks

aero engine blade failure analysis, 203, 209

α

+

β

alloysTi-6Al-4V, 69VT9, 76–78, 84, 86, 91, 93–95

β

alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 100–101near-

α

alloys, IMI 834, 41, 43–45, 48, 53, 55titanium aluminides

Ti-24Al-11Nb, 113, 118–122, 126–128, 134–135Ti-24Al-11Nb-4Ta, 177, 181, 185Ti-24Al-20Nb, 142Ti-24Al-27Nb, 155, 157, 160–162Ti-25Al-14Nb-1Mo, 188, 190Ti-25Al-15Nb, 167–170Ti-47Al-2Nb-2Cr, 196–197Ti-48Al-4Nb-1Mo, 200

Selenium, effects of alloying elements, 9Serpentine glide

α

alloys, 21dimple rupture, 1near-

α

alloys, OT4-1, 25Silicides, microstructure evolution, 13Silicon, chemical composition, 17–18Slip-plane fracture, 2Smooth fracture

α

+

β

alloys, VT9, 77titanium aluminides

Ti-24Al-11Nb, 123Ti-24Al-11Nb-4Ta, 175–176Ti-24Al-20Nb, 143Ti-24Al-27Nb, 154Ti-25Al-15Nb, 166–168

226


Solid solution formation, 9Spheroidization, 10Stabilizers, effects of alloying elements, 9Strain markings, dimple rupture, 1Stress-concentration effects of notch, 204Stress corrosion, intergranular fracture, 3Stress rupture, IMI 834, 47–49Stretching

α

alloys, 21dimple rupture, 1

Striationsaero engine blade failure analysis, 203, 210

α

+

β

alloysTi-6Al-4V, 69VT9, 82, 84, 86, 94–95

fracture mechanics, 2–3near-

α

alloysIMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 33IMI 834, 43–44, 47

Sulfur, effects of alloying elements, 9

T

Tantalum

β

alloys, 97chemical composition, 17–18effects of alloying elements, 9–10titanium aluminides, 111

Tear ridges

α

+

β

alloys, VT9, 77

β

alloys, Ti-10V-4.5Fe-1.5Al, 104near-

α

alloysIMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 31, 34, 36IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 27, 28IMI 834, 40, 52, 54

Ti-24Al-11Nb, 72, 134Tellurium, effects of alloying elements, 9Temperature,

see also Specific alloys and testing regimescreep-to-rupture fractures, 3high-temperature properties of intermetallics, 10Ti-24Al-27Nb solution treatment, 152, 155

Tensile failure, aero engine blade failure analysis, 204Tensile testing

α + β alloysTi-6Al-4V, 59, 60VT9, 73–75, 87–89

β alloysTi-10V-2Fe-3Al (Ti-10-2-3), 98–102Ti-10V-4.5Fe-1.5Al, 104–110

near-α alloysIMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 26, 28–29, 35–36, 38 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 25IMI 834, 39–41

titanium aluminides, 111Ti-24Al-11Nb, 112–138Ti-24Al-11Nb-4Ta, 175–178, 181–185Ti-24Al-20Nb, 140, 142–144, 146–149Ti-24Al-27Nb, 152–157, 160–164Ti-25Al-14Nb-1Mo, 187–190Ti-25Al-15Nb, 166–169, 171–173Ti-47Al-2Nb-2Cr, 195–199Ti-48Al-4Nb-1Mo, 199–201

Ternary additions, titanium aluminides, 111Terraced fracture surface, Ti-24Al-11Nb, 136Ti2AlNb

chemical composition, 17microstructure, evolution of, 15phases observed in, 11

Ti2Cu, 11, 13Ti3Al, see α2 (Ti3Al)Time, temperature, and transformation (TTT) diagram, 11TIMETAL LCB, 17, 97Tin

α alloys, 19chemical composition, 17–18

Tire cracks, 3Titanium aluminides, 111–201

chemical composition, 17–18 metallurgy, 10, 11, 15microstructure, evolution of, 15 microstructure, TiAl-based (γ), 111, 173–201; see also γ phase

Ti-24Al-11Nb-4Ta, 111, 173–185Ti-25Al-14Nb-1Mo, 185–191Ti-47Al-2Nb-2Cr, 111, 191–194, 195–199Ti-48Al-4Nb-1Mo, 111, 194–195, 199–201

microstructure, Ti3Al-basedTi-24Al-11Nb, 111–112, Ti-24Al-20Nb, 111, 139–150Ti-24Al-27Nb, 111, 150–164Ti-25Al-15Nb, 111, 164–173

Ti-5Al-2.5Sn, 19Ti-6Al-4V, 10, 12–14

Titanium-vanadium alloyschemical composition, 17–18microstructure and fractography, 97–102

Ti-10V-4.5Fe-1.5Al, 102–110Tongue formation, 1Toughness, 8Transcrystalline fractures, 111

Ti-24Al-11Nb, 114, 132Ti-24Al-11Nb-4Ta, 177–178, 182, 184Ti-24Al-20Nb, 146, 148Ti-24Al-27Nb, 111Ti-25Al-14Nb-1Mo, 189–190Ti-25Al-15Nb, 172

Transformed B2, Ti-24Al-11Nb, 135, 137Transformed β, 13

α + β alloysTi-6Al-4V, 59VT9, 70

near-α alloys, IMI 834, 49–50Transgranular crack, Ti-47Al-2Nb-2Cr, 195Transgranular fracture

β alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 98near-α alloys

IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 36IMI 834, 39

titanium aluminidesTi-24Al-11Nb, 124, 128–129, 133, 137Ti-24Al-20Nb, 149Ti-24Al-27Nb, 160–161, 163

Twinning, Ti-47Al-2Nb-2Cr, 1, 191–192

V

Vanadiumβ alloys, 97chemical composition, 17–18effects of alloying elements, 9–10

Voids/microvoids

Index 227

aero engine blade failure analysis, 204, 208β alloys

Ti-10V-2Fe-3Al (Ti-10-2-3), 98, 100Ti-10V-4.5Fe-1.5Al, 110

dimple rupture, 1intergranular fracture, 3near-α alloys

IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 31, 38IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 27IMI 834, 52–54

VT9chemical composition, 18microstructure and fractography, 59, 70–95

W

Wallner lines, 1Water quenching, titanium aluminides, 111Widmanstätten α structure, 11–12, 35, 37

Z

Zirconiumα alloys, 19chemical composition, 17–18effects of alloying elements, 9

Titanium Alloys-An Atlas of Structures and Fracture Features - Joshi

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