Hot Deformation And Microstructural Characteristics of Al...

HOT DEFORMATION AND

MICROSTRUCTURAL

CHARACTERISTICS OF Al AND Si

CONTAINING Mg-3Sn-2Ca (TX32)

ALLOYS: CORRELATION WITH

PROCESSING MAPS

CHALASANI DHARMENDRA

DOCTOR OF PHILOSOPHY

CITY UNIVERSITY OF HONG KONG

AUGUST 2013

CITY UNIVERSITY OF HONG KONG

香港城市大學香港城市大學香港城市大學香港城市大學

Hot Deformation and Microstructural

Characteristics of Al and Si Containing

Mg-3Sn-2Ca (TX32) Alloys:

Correlation with Processing Maps

含 Mg-3Sn-2Ca (TX32) 合金的

鋁與硅之熱加工微型結構特性:

加工效果圖相互關係硏究

Submitted to

Department of Mechanical and Biomedical Engineering 機械與生物醫藥工程系

in Partial Fulfillment of the Requirements

for the Degree of Doctor of Philosophy 哲學博士學位

by

Chalasani Dharmendra

August 2013 二零一三年八月

i

ABSTRACT

The worldwide need to reduce energy consumption has pushed the emergence of light-

weighting technologies and, among them, research towards developing new alloys of

Mg - the lightest of all structural metals - is of great interest for structural applications.

Mg and Mg alloys suffer from poor plasticity due to the hexagonal close packed (HCP)

crystal structure, which results in limited number of individual slip systems for active

deformation. Due to this reason, most Mg alloys develop strong textures during thermo-

mechanical processes such as rolling and extrusion, resulting in pronounced anisotropy.

Formability can be improved by randomizing or weakening the texture either by

modifying the existing alloys with minor additions of other elements or developing new

alloy systems. The response of the intrinsic nature of material to the imposed processing

parameters, namely temperature, strain rate and strain, is of significant importance to

the workability. The knowledge of interrelation between process parameters,

microstructure and mechanical properties will help in achieving reliable wrought

products for longer service. From this view point, the development of a

‘processing map’ is of great significance, which defines ‘safe’ window(s) to process a

material within certain temperature and strain rate range(s).

Among several Mg-Sn-Ca alloys, TX32 alloy (Mg-3Sn-2Ca) is found to be the best

compromise between corrosion resistance and creep strength due to Sn and Ca,

respectively, through the formation of CaMgSn and Mg2Ca intermetallic phases. For

further improvement of strength and/or weakening of texture, additions of aluminum

(0.4 and 1 wt.%) and silicon (0.2 - 0.8 wt.%) are made to develop a set of six cast

alloys. Very limited information is available in the literature on hot workability studies

of these alloys to achieve an irreversible change in the microstructure which is essential

for processing. Metallurgical phenomena are complex and metallic alloys are rate-

sensitive during high temperature deformation, which necessitates metal forming

processes to be carried out within correct ranges of parameters. The technique of

processing map, which is based on dynamic materials model (DMM) involving

irreversible thermodynamics, has proved to be highly successful in accurately

identifying ‘safe’ processing windows. This approach has been adopted by several

researchers in obtaining critical information towards optimizing hot workability and

achieving microstructural control for bulk forming of several metallic materials.

ii

The main aim of the present investigation is to study the hot deformation behavior of

the selected set of TX32 series cast alloys through the development of their processing

maps utilizing the interrelationships between flow stress and a wide range of process

parameters (temperature and strain rate). The emphasis of this study is to establish the

effect of Al and Si additions on the features of processing map of TX32 base alloy with

respect to domains of dynamic recrystallization (DRX), optimum deformation

conditions, flow instability and cracking regimes. Another aim of the study is to identify

the dominant mechanisms of hot deformation through kinetic approach and to establish

an interrelation between the process parameters, microstructure and evolving texture

during compression. The effect of process parameters on the activation of important slip

systems along the compression direction needs to be analyzed in terms of their relative

orientations.

Cylindrical specimens of 10 mm diameter and 15 mm height were machined from the

as-cast billets and uni-axial compression tests were performed in the temperature range

300 ○C to 500

○C at constant true strain rates in the range 0.0003 s

-1 to 10 s

-1 using

computer controlled servo-hydraulic test system. The microstructure and microtexture

characterizations after deformation were carried out using optical microscopy (OM) and

scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS)

and an electron back scattered diffraction (EBSD) facility. Pole figures and Schmid

factors are used to analyse the activity of individual slip systems at various deformation

conditions. Transmission electron microscopy (TEM) was used for select specimens to

supplement the microstructural features. Tensile tests were carried out in the case of

TX32-1Al alloy to correlate the characteristics of different domains by observing the

fracture surfaces of the tested samples in SEM.

The major conclusions drawn from the present study are listed below.

(i) The processing map of cast TX32 alloy exhibits two domains of DRX in the

temperature and strain rate ranges in hot deformation: (a) 300 ○C to 350

○C and

0.0003 s-1

to 0.001 s-1

(Domain 1) and (b) 390 ○C to 500

○C and 0.005 s

-1 to 0.6 s

-1

(Domain 2). Texture evolution as characterized by EBSD analysis indicates that

specimens deformed under conditions in Domain 1 exhibit a basal texture with a

maximum intensity of basal poles located at about 35○ to 45

○ with the

compression direction. At temperatures higher than 400 ○C (Domain 2), texture

was randomized due to increase in the activity of second-order pyramidal slip.

iii

While CaMgSn particles in the matrix contribute to significant back stress to

dislocation moment, the grain boundary phase Mg2Ca reduces the grain boundary

sliding.

(ii) The homogenization treatment of cast TX32 alloy has negligible influence on its

hot deformation behavior and texture evolution, implying that homogenization

step may be eliminated in hot deformation schedules. Compared with the most

widely used AZ31 magnesium alloy, TX32 alloy can be hot worked over a broad

temperature and strain rate range.

(iii) With the addition of 0.4 wt.% Al, the ultimate compressive strength (UCS) of

TX32 alloy has improved in the testing temperature range of 25 ○C to 250

○C,

which may be attributed to the effect of solid solution strengthening. However, the

addition of Al did not significantly change its hot working behavior (300 ○C to

500 ○C) as the basic features of the processing map remain unchanged. At low

temperatures, the alloy exhibited flow instability in the form of flow localization

at intermediate strain rates and adiabatic shear bands at high strain rates.

(iv) The addition of 1 wt.% Al promoted prismatic slip at intermediate temperatures

(between 350 ○C to 400

○C), causing changes to the features of processing map

compared to TX32 alloy. The processing map revealed three workable DRX

domains and a fourth domain related to grain boundary sliding at high temperature

and low strain rate range (430 ○C to 500

○C and 0.0003 s

-1 to 0.002 s

-1) which is

unsuitable for processing. The specimens deformed in lower temperature and

strain rate (Domain 1) exhibited basal textures whereas second-order pyramidal

slip randomized the texture in specimens deformed in high temperature and

intermediate strain rate range (Domain 3).

(v) The alloy with 0.4 wt.% Al and 0.2 wt.% Si has exhibited UCS closer to that of

the TX32 alloy between 25 ○C to 250

○C. However, increased additions of Si

(from 0.4 to 0.8 wt.%) significantly decreased UCS at higher temperatures

(100 ○C to 250

○C), likely due to the differences in intermetallic phases formed

(CaMgSi and Ca2Sn in Si containing alloys vs. CaMgSn and Mg2Ca in TX32) and

the increase of their volume fraction. Moreover, both CaMgSi and Ca2Sn are

distributed in the matrix compared to the presence of Mg2Ca at the grain

boundaries. All the Si-containing alloys have exhibited pronounced ductility at

250 ○C indicating the beginning of hot workability temperature range.

iv

(vi) The processing map of TX32-0.4Al-0.4Si alloy showed a shift of DRX domains

to high temperatures and reduced flow instability regime particularly at high

temperatures as compared to TX32. The addition of 0.4% Si is favorable for

enhancing the hot workability since it widens the processing windows (domains).

The basal poles are spread out from the compression axis and the (0001) <11 2 0>

slip dominated as DRX grains have high Schmid factors for basal slip at low

temperatures (300 and 350 ○C) and low strain rates (0.0003 and 0.001 s

-1). The

texture got randomized at ≥450 ○C at intermediate strain rates in Domain 2.

(vii) The apparent activation energy values obtained through kinetic analyses for these

alloys indicate that the deformation in the low strain rate DRX domain is

controlled by climb and recovery process, whereas the deformation in the high

strain rate DRX domain is attributed to cross-slip since the stacking fault energy

on the pyramidal slip systems is high.

(viii) For 0.6% Si addition to TX32-0.4Al alloy, an additional DRX domain (Domain 3)

occurs at high temperatures and high strain rates. Domain 1 is characterized as

cracking domain, whereas in Domains 2 and 3, DRX is occurring predominantly

by basal slip with climb as a recovery process.

(ix) With further increase in Si (TX32 with 0.4 wt.% Al and 0.8 wt.% Si), the first

DRX domain at low strain rates has shifted further towards high temperature and

the second DRX domain at high temperature shifted to high strain rates.

Deformation is basal slip dominated and the recovery is by climb in both the

domains.

(x) When the volume fraction of intermetallic particles increased steeply (in 0.6 and

0.8% Si-containing alloys), the back stress increases significantly and thus, the

activation of basal slip required considerably high temperatures for its extensive

participation in plastic flow.

Key words: Mg-Sn-Ca (TX) alloy, Hot compression, Flow curves, Processing map,

Kinetic analysis, Microstructure, Dynamic recrystallization, Microtexture,

EBSD, TEM.

vii

TABLE OF CONTENTS

Page

ABSTRACT i

ACKNOWLEDGEMENTS v

LIST OF TABLES xii

LIST OF FIGURES xiii

CHAPTER 1 INTRODUCTION

1.1 Motivation 1-1

1.2 Research problem 1-1

1.3 Scope of the thesis 1-3

CHAPTER 2 LITERATURE REVIEW

2.1 Magnesium and its alloys

2.1.1 Magnesium characteristics 2-1

2.1.2 Alloying elements 2-2

2.1.3 Mg alloys development 2-5

2.1.4 ASTM standard magnesium alloy designations 2-5

2.2 Deformation of magnesium

2.2.1 Schmid factor 2-6

2.2.2 Slip systems in magnesium 2-7

2.2.3 Deformation of polycrystals 2-10

2.2.4 Effect of temperature on the deformation mechanisms 2-12

2.2.5 Effect of alloying addition on deformation 2-13

2.2.6 Creep resistance 2-14

2.3 Hot working

2.3.1 Flow curves and mechanical testing 2-15

2.3.2 Materials modelling in hot deformation

2.3.2.1 Kinetic model 2-17

2.3.2.2 Atomistic model (Ashby and Raj maps) 2-18

2.3.2.3 Dynamic materials model (Processing map) 2-19

2.4 Hot deformation mechanisms

2.4.1 Dynamic recovery 2-21

2.4.2 Dynamic recrystallization (DRX) 2-21

2.4.3 Superplastic deformation 2-23

2.4.4 Void formation 2-23

2.4.5 Flow instability processes 2-24

viii

2.5 Texture (Preferred orientation) 2-24

2.5.1 Texture measurement by X-ray technique 2-24

2.5.2 Neutron diffraction 2-24

2.5.3 Electron backscatter diffraction 2-25

2.5.3.1 Pole figure and inverse pole figure 2-25

2.6 Mg-Sn-Ca (TX) system 2-26

CHAPTER 3 EXPERIMENTAL DETAILS

3.1 Flow chart of the study 3-1

3.2 Preparation of alloys 3-2

3.3 Initial material characterization

3.3.1 X-ray diffraction 3-2

3.3.2 Differential scanning calorimetry (DSC) 3-2

3.3.3 Electron probe micro-analyzer 3-3

3.3.4 Volume fraction of intermetallic particles 3-3

3.4 Compression test 3-3

3.4.1 Data analysis 3-5

3.4.2 Computational procedure 3-5

3.5 Tensile testing 3-6

3.6 Microstructure investigation

3.6.1 Optical microscopy 3-6

3.6.2 Scanning electron microscopy 3-7

3.6.3 Transmission electron microscopy 3-7

3.7 Micro-texture analysis 3-8

CHAPTER 4 HOT WORKABILITY ANALYSIS AND TEXTURE

CHARACTERISTICS OF A TX32 MAGNESIUM ALLOY

IN AS-CAST AND HOMOGENIZED CONDITIONS

4.1 TX32 alloy in an as-cast condition

4.1.1 Initial microstructure and texture of the as-cast material 4-1

4.1.2 Mechanical strength at low temperatures 4-1

4.1.3 Compressive stress-strain behavior under hot working

conditions 4-3

4.1.4 Processing map and microstructures 4-5

4.1.5 Kinetic analysis 4-6

4.1.6 Mechanisms of hot deformation 4-9

4.1.7 Texture evolution – Domain 1 4-10

4.1.8 Texture randomization – Domain 2 4-15

4.1.9 Summary 4-18

ix

4.2 Effect of homogenization on hot deformation behavior of a cast

TX32 magnesium alloy

4.2.1 Cast and homogenized (CH) alloy microstructure 4-19

4.2.2 Flow behavior 4-21

4.2.3 Processing map for CH TX32 alloy 4-22

4.2.4 Comparison of processing maps for TX32 alloy in

AC and CH conditions 4-25

4.2.5 Pole figures and Schmid factor distribution for specimens

in Domain 1 4-27

4.2.6 Texture evolution in Domain 2 4-30

4.2.7 Summary 4-32

CHAPTER 5 HOT WOKABILITY ANALYSIS AND TEXTURE

CHARACTERISTICS OF TX32 MAGNESIUM ALLOY

WITH ALUMINUM ADDITIONS

5.1 Hot deformation behavior of cast TX32-0.4Al alloy

5.1.1 Initial as-cast material microstructure and texture 5-1

5.1.2 Compressive stress-strain behaviour under hot working

conditions 5-3

5.1.3 Processing map 5-4


5.1.5 Deformation mechanisms 5-8

5.1.6 Texture analysis in Domain 1 5-9

5.1.7 Texture randomization in Domain 2 5-12

5.1.8 Flow instabilities 5-15

5.1.9 Summary 5-17

5.2 Hot deformation and microstructural features of 1% Al containing

TX32 magnesium alloy

5.2.1 Initial as-cast material microstructure 5-18


5.2.3 Flow curves 5-21

5.2.4 Hot deformation behavior and processing map 5-22

5.2.5 Tensile tests and fractography 5-28

5.2.6 Flow instabilities 5-29


5.2.8 Summary 5-33

5.3 Effect of Al on hot deformation mechanisms and dynamic

recrystallization in TX32 magnesium alloy

5.3.1 Comparison of processing maps 5-34

5.3.2 Deformation mechanism in Domain 1 5-36

5.3.3 Deformation mechanism in Domain 2 of TX32 alloy and

Domain 3 of TX32-1Al alloy 5-39

x

5.3.4 Mechanism in Domain 2 of TX32-1Al alloy 5-41

5.3.5 Mechanism in Domain 4 of TX32-1Al alloy 5-43

5.3.6 Summary 5-45

CHAPTER 6 HOT WOKABILITY ANALYSIS AND TEXTURE

CHARACTERISTICS OF TX32-0.4Al ALLOY

WITH SILICON ADDITIONS

6.1 Initial material characterization of silicon containing TX32-0.4Al alloys

6.1.1 Microstructures 6-1

6.1.2 XRD analysis 6-3

6.1.3 SEM-EPMA analysis 6-4

6.1.4 Differential scanning calorimetry (DSC) analysis 6-5

6.1.5 Volume fraction of intermetallic particles 6-7

6.2 Hot deformation behavior of TX32-0.4Al magnesium alloy with

0.2% Si additions


6.2.2 Hot working behavior 6-9


6.2.4 Summary 6-13


0.4% Si additions



6.3.3 Processing map 6-16

6.3.4 Specimens after compression testing 6-17

6.3.5 Microstructures of deformed specimens 6-19


6.3.7 Flow instability 6-22

6.3.8 Comparison with TX32 and TX32-0.4Al alloys 6-23

6.3.9 Texture evolution 6-23

6.3.10 Summary 6-26


0.6% Si additions


6.4.2 Hot working behavior 6-27

6.4.3 Microstructures and textures 6-30


6.4.5 Summary 6-34


0.8% Si additions

xi



6.5.3 Processing map of TX32-0.4Al-0.8Si alloy 6-37

6.5.4 Microstructure and texture evolution 6-39


6.5.6 Summary 6-46

6.6 Effect of silicon on hot deformation mechanisms and dynamic

recrystallization in TX32-0.4Al alloy 6-48

CHAPTER 7 CONCLUSIONS 7-1

CHAPTER 8 SUGGESTIONS FOR FUTURE WORK 8-1

REFERENCES R-1

APPENDICES

Appendix A.1 Compressive flow stress data at various temperatures,

strain rates (έ) and strains (ε) (data corrected for adiabatic

temperature rise) for the TX32 cast alloy A-1



temperature rise) for the TX32 cast-homogenized alloy A-2



temperature rise) for the TX32-0.4Al alloy A-3



temperature rise) for the TX32-1Al alloy A-4



temperature rise) for the TX32-0.4Al-0.2Si alloy A-5










LIST OF PUBLICATIONS BASED ON THIS THESIS P-1

xii

LIST OF TABLES

Page

Table 2.1: Properties of pure Mg 2-1

Table 2.2: ASTM element code designation for the Mg alloys 2-6

Table 2.3: Characteristics of slip modes in Mg single crystals 2-9

Table 2.4: Activation energies and suggested rate controlling mechanisms

in the hot working of AZ31 alloy (under compression) 2-18

Table 4.1: Peak efficiency conditions and kinetic parameters in the Domains

1 and 2 of the processing maps for the AC and CH TX32 alloys 4-27

Table 6.1: Activation parameters for hot working of TX32, TX32-0.4Al,

and TX32-0.4Al-0.4Si alloys 6-25

xiii

LIST OF FIGURES

Page

Fig. 2.1: (a) The primitive hexagonal unit cell illustrating the axes

a1 = a2 ≠ c and corresponding angles α = β = 90○, γ = 120

○ and

(b) the hexagonal close-packed structure. The primitive hexagonal

unit cell is delineated by thick-solid lines. 2-2

Fig. 2.2: Mg - Al phase diagram (ASM Handbook). 2-3

Fig. 2.3: Directions of the Mg alloy development. 2-5

Fig. 2.4: Diagram of the Schmid factor. 2-6

Fig. 2.5: Schematic of the important planes and directions in the Mg lattice. 2-8

Fig. 2.6: Schematic of the interplanar spacings of (0001), (1012), (1011)

and (1010) planes in Mg. 2-8

Fig. 2.7: Stress vs strain curve in the single crystals of Mg and Mg alloys

compressed along (a) <1010> direction, with expansion

limited to <1 2 10>, and (b) <1 2 10> direction, with expansion

limited to <1010>. 2-10

Fig. 2.8: Influence of the deformation temperature on the CRSS of Mg single

crystals. 2-12

Fig. 2.9: CRSS for the prismatic slip vs. zinc concentration. 2-13

Fig. 2.10: CRSS for the prismatic slip vs. temperature at various

Al concentration. 2-13

Fig. 2.11: Raj map for aluminum showing limiting conditions for damage

nucleation. 2-19

Fig. 2.12: Processing map for the as-cast AZ31 alloy. 2-21

Fig. 2.13: Movement of dislocations to produce polygonization. 2-22

Fig. 2.14: TEM micrographs of a (a) square twist boundary and a

(b) hexagonal twist boundary in copper. 2-23

Fig. 2.15: Orientation of the basal plane (0001) in a hexagonal crystal.

The position of the (0001) pole on the unit sphere with regard to

an external reference frame is described by the two angles α and β. 2-26

Fig. 2.16: Binary phase diagrams of (a) Mg-Sn and (b) Mg-Ca. 2-27

Fig. 2.17: Influence of Sn/Ca ratio on the (a) corrosion rate and

(b) creep resistance of Mg-xSn-xCa alloys. 2-28

Fig. 2.18: Compression creep curves of TX32 alloy compared with AZ91

and AE42 at 80 MPa and (a) 135 and (b) 175 ○C. 2-29

xiv

Page

Fig. 3.1: Schematic flow chart of the experimental procedure. 3-1

Fig. 3.2: (a) A melting furnace, and (b) the cast billets of TX32 alloys. 3-2

Fig. 3.3: (a) Geometry of the specimen for compression testing and

(b) servo-hydraulic compression testing machine. 3-4

Fig. 3.4: Schematic of steps for compression test, microstructure and

texture studies. 3-7

Fig. 3.5: (a) Schematic of EBSD system, (b) a typical EBSD diffraction

pattern, and (c) components of EBSD system. 3-8

Fig. 4.1: (a) Optical micrograph, (b) SEM image with phases marked,

(c) XRD pattern, and (d) Pole figures of the TX32 magnesium

alloy in the as-cast (AC) condition. 4-2

Fig. 4.2: Variation in the ultimate compressive strength (UCS) of the

AC TX32 alloy with temperature (inset shows the

corresponding compressive stress-strain curves). 4-3

Fig. 4.3: True stress – true strain curves obtained for the TX32 alloy

under compression at different strain rates and at the test

temperatures of (a) 300 ○C and (b) 500

○C. 4-4

Fig. 4.4: Processing map for the Mg-3Sn-2Ca (TX32) alloy. The numbers

associated with the contours represent the power dissipation

efficiency in percent. 4-5

Fig. 4.5: Microstructures of the TX32 alloy deformed at 300 ○C/0.01 s

-1

(instability regime) showing flow localization (marked by arrows).

The compression axis is vertical. 4-6

Fig. 4.6: Microstructures of the TX32 alloy deformed at (a) 300 ○C/0.0003 s

-1

(Domain 1), and (b) 500 ○C/0.1 s

-1 (Domain 2).


Fig. 4.7: (a) Variation of the normalized flow stress values (at a strain of 0.5)

with strain rate at different test temperatures. (b) Arrhenius plot

showing the variation of the flow stress for the TX32 alloy

normalized with the shear modulus using the inverse of the

temperature (Kelvin) at different strain rates. 4-8

Fig. 4.8: Crystallographic textures obtained using EBSD for the conditions

from Domain 1. Contour lines referring to 1, 2, and 3 times random

and maximum intensities (“max”) are shown. The x-axis in the

pole figures is the compression axis. 4-10

xv

Page

Fig. 4.9: Schmid factor distribution of the grains for (a) basal (0001) <11 2 0>,

(b) prismatic (1100) <11 2 0> and (c) pyramidal (11 2 2) <11 2 3 >

slip systems for the samples deformed at 300 and 350 ○C/0.0003 s

-1

(Domain 1). 4-11

Fig. 4.10: Microstructure of the TX32 alloy deformed at 350 ○C/0.0003 s

-1

(Domain 1). 4-13

Fig. 4.11: Inverse pole figures relative to the compression direction (CD) for

Domains 1 and 2. 4-13

Fig. 4.12: Transmission electron micrographs of the specimen deformed at

300 ○C/0.0003 s

-1 (Domain 1) of the TX32 alloy showing

(a) dislocation array and (b) tilt boundaries 4-14

Fig. 4.13: Crystallographic textures obtained using EBSD for the conditions

from Domain 2. The X-axis in the pole figures is compression axis. 4-15

Fig. 4.14: Schmid factor distribution of the grains for basal (0001) <11 2 0>,

prismatic (1100) <11 2 0>, and pyramidal (11 2 2) <11 2 3 >

slip systems for the sample deformed at 500 ○C/0.1 s

-1

(condition near to peak efficiency in Domain 2). 4-16

Fig. 4.15: TEM images of the specimen deformed at 500 ○C/0.1 s

-1

(Domain 2) of the TX32 base alloy (a) showing dislocation tangles

and (b) revealing cross-slip. 4-17

Fig. 4.16: (a) Optical micrograph, (b) SEM image (phases marked),

(c) XRD pattern, and (d) pole figures of the TX32 magnesium

alloy in the cast-homogenized (CH) condition. 4-20

Fig. 4.17: Compressive true stress-true strain curves obtained for the TX32

(AC and CH) alloy at different strain rates and at test

temperatures of (a) 350 ○C

and (b) 500

○C. 4-21

Fig. 4.18: Processing map for the TX32 alloy in the CH condition 4-22

Fig. 4.19: Optical microstructures of the CH and AC TX32 alloys deformed

at very low strain rate. The compression axis is vertical. 4-23

Fig. 4.20: Optical microstructures of the CH and AC TX32 alloys deformed

under Domain 2 conditions. The compression axis is vertical. 4-24

Fig. 4.21: Optical microstructures of the CH TX32 alloy deformed at

(a) 300 ○C

/10 s

-1 and (b) 300

○C/1 s

-1 exhibiting an adiabatic shear

band and flow localization (marked by arrows) in the cracking and

instability regimes, respectively, in the processing map.


xvi

Page

Fig. 4.22: Pole figures corresponding to Domain 1 of the CH - TX32 alloy

(The x-axis is the compression axis). 4-28

Fig. 4.23: Schmid factor distribution of the grains for the (a) basal,

(b) prismatic, and (c) pyramidal slip systems of the specimens

deformed under Domain 1. 4-29

Fig. 4.24: Pole figures corresponding to Domain 2 of the CH - TX32 alloy

(The x-axis is the compression axis). 4-31

Fig. 4.25: Schmid factor distribution of the grains for the prismatic

(1100) <11 2 0> slip system for the specimens deformed at

Domain 2 conditions. 4-31

Fig. 5.1: (a) Optical micrograph in the as-cast condition, (b) SEM image

showing intermetallic particles, (c) DSC curve, and (d) pole figures

in the as-cast condition of the TX32-0.4Al magnesium alloy. 5-2

Fig. 5.2: True stress - true strain curves obtained for the TX32-0.4Al alloy

in compression at different strain rates and at the test temperature

of (a) 300 ○C and (b) 450

○C. 5-3

Fig. 5.3: Processing map for the TX32-0.4Al alloy. The numbers associated with

the contours represent efficiency of power dissipation in percent. 5-4

Fig. 5.4: Microstructures of the TX32–0.4Al alloy deformed at

(a) 300 ○C/0.0003 s

−1 and (b) 350

○C/ 0.001 s

−1 (Domain 1).


Fig. 5.5: Optical microstructures of the TX32–0.4Al alloy deformed at

(a) 400 ○C/0.1 s

−1, (b) 450

○C/0.1 s

−1, and (c) 500

○C/0.1 s

−1

(Domain 2). The compression axis is vertical. 5-6

Fig. 5.6: (a) Variation of flow stress with strain rate, and (b) Arrhenius plot

showing the variation of normalized flow stress with (1/T). 5-7

Fig. 5.7: Pole figures corresponding to conditions from Domain 1

(The x-axis in the pole figures is the compression axis). 5-9

Fig. 5.8: Schmid factor distributions for samples deformed at 300 ○C/

0.0003 s-1

and 350 ○C/0.001 s

-1 (Domain 1) conditions (a) basal

(0001) <11 2 0>, (b) prismatic (1100) <11 2 0>, and

(c) pyramidal (11 2 2) >< 3211 slip systems. 5-10

Fig. 5.9: Pole figures corresponding to conditions from Domain 2

(The x-axis in the pole figures is the compression axis). 5-12

xvii

Page

Fig. 5.10: EBSD Schmid factor distribution maps for the sample deformed to

450 ○C/ 0.1 s

-1 (a) for (0001) <11 2 0> slip system (b) for (11 2 2)

>< 3211 slip system. The red and green colors represent high to

moderate Schmid factors. 5-13

Fig. 5.11: Misorientation angle distribution (MOD) for the specimens

compressed at (a) 300 ○C/0.0003 s

-1 (Domain 1) and

(b) 500 ○C/0.1 s

-1 (Domain 2). 5-15

Fig. 5.12: Optical microstructures of TX32-0.4Al alloy deformed at

(a) 300 ○C/10 s

-1 exhibiting adiabatic shear band, and (b) 350

○C/1 s

-1

showing flow localization (marked by arrows) in the cracking and

instability regimes respectively in the map.


Fig. 5.13: (a) Optical micrograph, (b) SEM image showing intermetallic

phases in the as-cast condition of the TX32-1Al magnesium alloy. 5-18

Fig. 5.14: XRD pattern in as-cast condition for TX32-1Al alloy

(inset shows the pattern for TX32 base alloy). 5-19

Fig. 5.15: DSC curves in the as-cast condition for TX32, TX32-0.4Al,

and TX32-1Al alloys. 5-19

Fig. 5.16: Variation of ultimate compressive strength (UCS) for as-cast

TX32, TX32-0.4Al and TX32-1Al alloys with temperature. 5-20

Fig. 5.17: True stress - true plastic strain curves obtained for the

TX32-1Al alloy at (a) 450 ○C and (b) 0.01 s

-1. 5-21

Fig. 5.18: Processing map for the TX32-1Al alloy at a strain of 0.5.

The numbers associated with the contours represent the

efficiency of power dissipation in percent. 5-23

Fig. 5.19: Top view of the specimens deformed at different compression

temperatures and strain rates. The compressive direction

is perpendicular to the viewing plane. 5-24

Fig. 5.20: (a) Optical microstructure of the TX32-1Al alloy compressed at

300 ○C/0.0003 s

-1 (Domain 1). The compression axis is vertical.

(b) Variation of grain size and peak efficiency values with

temperature at 0.0003 s-1

. 5-25

Fig. 5.21: Optical microstructures of the TX32-1Al alloy deformed at

(a) 350 ○C/ 0.001 s

-1, (b) 350

○C/0.01 s

-1, (c) 400

○C/0.001 s

-1,

and (d) 400 ○C/0.01 s

-1 (Domain 2).


Fig. 5.22: Variation of grain size and peak efficiency values with temperature

at 0.1 s-1

. 5-26

xviii

Page

Fig. 5.23: Optical microstructures of the TX32-1Al alloy deformed at

(a) 450 ○C/0.1 s

-1 (Domain 3), (b) 500

○C/0.1 s

-1 (Domain 3),

(c) 450 ○C/0.0003 s

-1 (Domain 4), and (d) 500

○C/0.001 s

-1


Fig. 5.24: Grain boundary orientation distribution with respect to the

compression axis for the specimen deformed at 450 ○C/0.0003 s

-1

(Domain 4). 5-28

Fig. 5.25: Fracture surface of the TX32-1Al alloy specimen tested in uniaxial

tension at 475 ○C/ 0.0003 s

-1 (Domain 4). 5-29

Fig. 5.26: Fracture surfaces of the TX32-1Al alloy specimens tested in

uniaxial tension at the conditions of (a) 375 ○C/0.01 s

-1 (Domain 2)

and (b) 475 ○C/0.1 s

-1 (Domain 3). 5-30

Fig. 5.27: Microstructures of the TX32-1Al alloy samples deformed at 10 s-1

and the test temperatures (a) 300 ○C and (b) 500

○C which

correspond to the instability regions.


Fig. 5.28: (a) Variation of flow stress with strain rate, and (b) Arrhenius plot

showing the variation of normalized flow stress with (1/T). 5-32

Fig. 5.29: Processing maps for (a) TX32, (b) TX32-0.4Al, and (c) TX32-1Al

alloys. 5-35

Fig. 5.30: TEM microstructures of the specimen deformed at 300 ○C/0.0003 s

-1

(Domain 1) of the TX32-1Al alloy exhibiting (a) planar slip and

(b) polygonized tilt boundary. 5-37

Fig. 5.31: Micro-textures obtained using EBSD for peak efficiency

condition (300 ○C/0.0003 s

-1) in the Domains 1 of the

processing map of (a) TX32 base alloy, (b) TX32-0.4Al, and

(c) TX32-1Al alloys. The x-axis in the pole figures is the

compression axis. 5-38

Fig. 5.32: Schmid factor distribution of the grains for basal (0001) <11 2 0>

slip systems for the specimens deformed at 300 ○C/0.0003 s

-1

(Domain 1) for all the three alloys. 5-39

Fig. 5.33: TEM microstructures of the specimen deformed at 500 ○C/0.1 s

-1

(Domain 3) of the TX32-1Al alloy showing (a) dislocation angles

and (b) twist boundaries.. 5-40

Fig. 5.34: Micro-textures for the (a) TX32 and (b) TX32-1Al alloys,

obtained using EBSD for specimens deformed at 500 ○C/0.1 s

-1

conditions. The x-axis in the pole figures is the compression axis. 5-41

xix

Page

Fig. 5.35: TEM images of the specimen deformed at 400 ○C/0.01 s

-1

(Domain 2) of the TX32-1Al alloy (a) exhibits planar slip and

(b) shows tilt boundaries. 5-42

Fig. 5.36: Micro-textures obtained using EBSD for conditions

(a) 350 ○C/0.01 s

-1 and (b) 400

○C/0.01 s

-1 from the Domain 2 in

the processing map of TX32-1Al alloy. The x-axis in the pole

figures is the compression axis. 5-43

Fig. 5.37: Optical microstructure of the TX32-1Al alloy deformed at

450 ○C/0.001 s

-1 (Domain 4). The compression axis is vertical. 5-44

Fig. 5.38: TEM microstructures of the specimen deformed at

500 ○C/0.0003 s

-1 (Domain 4)

of the TX32-1Al alloy. 5-44

Fig. 5.39: Micro-textures for the specimen deformed at 500 ○C/0.0003 s

-1

(Domain 4) of the TX32-1Al alloy. The x-axis in the pole figures

is the compression axis. 5-44

Fig. 6.1: Optical microstructures in the as-cast condition of the

(a) TX32-0.4Al base alloy, (b) TX32-0.4Al-0.2Si,

(c) TX32-0.4Al-0.4Si, (d) TX32-0.4Al-0.6Si and

(e) TX32-0.4Al-0.8Si magnesium alloys. 6-2

Fig. 6.2: SEM micrographs of the (a) TX32-0.4Al base alloy,

(b) TX32-0.4Al-0.2Si, (c) TX32-0.4Al-0.4Si, (d) TX32-0.4Al-0.6Si

and (e) TX32-0.4Al-0.8Si magnesium alloys in as-cast conditions. 6-3

Fig. 6.3: XRD pattern of TX32-0.4Al-0.8Si alloy in as-cast condition

(inset shows the XRD plot for the TX32-0.4Al-0.4Si alloy). 6-4

Fig. 6.4: (a) Back-scattered electron image of TX32-0.4Al-0.4Si alloy in

as-cast condition with marked intermetallic particles,

(b) Sn, (c) Ca, and, (d) Si distribution maps. 6-5

Fig. 6.5: (a) Back-scattered electron image of TX32-0.4Al-0.8Si alloy in

as-cast condition, (b) Sn, (c) Ca, and, (d) Si distribution maps. 6-6

Fig. 6.6: DSC curves of the Si-containing TX32-0.4Al alloys in

as-cast conditions. 6-6

Fig. 6.7: Volume fraction of intermetallic particles present in the

Si-containing TX32-0.4Al alloys in as-cast conditions. 6-7

Fig. 6.8: True stress - true strain curves obtained for the TX32-0.4Al-0.2Si

alloy in compression at different strain rates and at test


○C. 6-8

Fig. 6.9: Processing map for the TX32-0.4Al-0.2Si alloy. The numbers

associated with the contours represent the efficiency of power

dissipation in percent. 6-10

xx

Page

Fig. 6.10: Microstructures of the TX32–0.4Al-0.2Si alloy deformed at

(a) 300 ○C

/ 0.0003 s

−1 (Domain 1), and (b) 450

○C / 0.1 s

−1


Fig. 6.11: (a) Variation of flow stress (at a strain of 0.5) with strain rate

at different test temperatures, and (b) Arrhenius plot showing the

variation in normalized flow stress with inverse temperature (kelvin)

at different strain rates of the TX32-0.4Al-0.2Si alloy. 6-12

Fig. 6.12: Variation in the ultimate compressive strength (UCS) of as-cast

TX32, TX32-0.4Al, and TX32-0.4Al-0.4Si alloys with temperature. 6-14


alloy in compression at test temperatures of (a) 300 ○C and

(b) 450 ○C. 6-16


associated with the contours represent power dissipation


Fig. 6.15: Top and side views of the deformed TX32-0.4Al-0.4Si alloy

specimens in different deformation conditions. The compressive

direction is perpendicular to the viewing plane. 6-18

Fig. 6.16: Microstructure of the TX32-0.4Al-0.4Si alloy deformed at

(a) 350 ○C/ 0.0003 s

−1 (Domain 1), (b) 450

○C/0.1 s

−1,

(c) 500 ○C/0.1 s

−1 (Domain 2) exhibiting the DRX of the as-cast

microstructure. 6-19

Fig. 6.17: Variation of grain size with temperature at 0.0003 s-1

and 0.1 s-1

. 6-20

Fig. 6.18: (a) Variation in flow stress (at a strain of 0.5) with strain rate

at different test temperatures and (b) Arrhenius plot showing the

variation in normalized flow stress with inverse temperature at

different strain rates of the TX32-0.4Al-0.4Si alloy. 6-21

Fig. 6.19: Microstructures of the TX32-0.4Al-0.4Si alloy deformed at

(a) 300 ○C/10 s

-1, and (b) 300

○C/0.1 s

-1 exhibiting (a) adiabatic

shear band and (b) flow localization (marked by arrows), in the

cracking and instability regimes respectively in the map.


Fig. 6.20: Crystallographic textures obtained using EBSD for conditions of:

(a) 350 ○C/0.0003 s

-1 and (b) 500

○C/0.1 s

-1 of TX32-0.4Al-0.4Si

alloy. The compression axis is horizontal. 6-24

Fig. 6.21: Crystallographic textures obtained using EBSD for conditions

of: (a) 300 ○C/ 0.0003 s

-1 and (b) 500

○C/0.1 s

-1 of

TX32-0.4Al alloy. The compression axis is horizontal. 6-25

xxi

Page


alloy in compression at different strain rates and at the test


○C. 6-28





associated with the contours represent power dissipation efficiency

in percent. 6-29

Fig. 6.25: SEM micrograph of the TX32-0.4Al-0.6Si alloy deformed at

300 ○C/ 0.0003 s

-1 (Domain 1) exhibiting void formation at

the hard particles. 6-30


450 ○C/0.0003 s

-1 conditions

(Domain 2).


Fig. 6.27: The micro-textures obtained using EBSD for the condition

450 ○C/0.0003 s

-1 (Domain 2) of the TX32-0.4Al-0.6Si alloy.

The compression axis is horizontal. 6-31

Fig. 6.28: (a) Microstructure (the compression axis is vertical) and

(b) micro-texture (the compression axis is horizontal) of the

TX32-0.4Al-0.6Si alloy deformed at 500 ○C/10 s

-1 (Domain 3). 6-32

Fig. 6.29: (a) Variation in flow stress (at a strain of 0.5) with strain rate at

different test temperatures, and (b) the Arrhenius plot showing the

variation in normalized flow stress with inverse temperature at

different strain rates of the TX32-0.4Al-0.6Si alloy. 6-33

Fig. 6.30: Variation in ultimate compressive strength with temperature for

as-cast TX32, TX32-0.4Al, and TX32-0.4Al alloys with Si

additions. 6-35


alloy in compression at different strain rates and at test temperatures

of (a) 400 ○C and (b) 500

○C. 6-36




Fig. 6.33: Top and side views of the deformed TX32-0.4Al-0.8Si alloy

specimens under different deformation conditions.

The compressive direction is perpendicular to the viewing plane. 6-38

xxii

Page


450 ○C/0.0003 s

-1 (Domain 1). The compression axis is vertical. 6-39


450 ○C/0.001 s

-1 (Domain 1) of the TX32-0.4Al-0.8Si alloy

(a) reveals planar slip and (b) shows tilt boundaries. 6-40

Fig. 6.36: (a) Micro-textures and (b) Schmid factor distribution of the grains

for the specimen deformed at 450 ○C/0.001 s

-1 (Domain 1) of the

TX32-0.4Al-0.8Si alloy. The x-axis in the pole figures

is the compression axis. 6-41


500 ○C/10 s

-1 conditions

(Domain 2).



500 ○C/10 s

-1 (Domain 2) of the TX32-0.4Al-0.8Si alloy

(a) reveals planar slip and (b) shows tilt boundaries. 6-42

Fig. 6.39: (a) Micro-textures and (b) Schmid factor distributions of the grains

for the specimen deformed at 500 ○C/10 s

-1 (Domain 2) of the

TX32-0.4Al-0.8Si alloy.

The x-axis in the pole figures is the compression axis. 6-43

Fig. 6.40: SEM microstructures of the TX32-0.4Al-0.8Si alloy deformed

at (a) 300 ○C/0.0003 s

-1 showing voids at hard particles, and

(b) 300 ○C/0.1 s

-1 showing crack formation due to joining of

voids at adjacent hard particles. 6-44

Fig. 6.41: (a) Variation in normalized flow stress values (at a strain of 0.5)

with strain rate at different test temperatures, (b) the Arrhenius

plot showing the variation in normalized flow stress values

with inverse of temperature at different strain rates for the

TX32-0.4Al-0.8Si alloy. 6-45

Fig. 6.42: Processing maps for the (a) TX32-0.4Al base alloy,

(b) TX32-0.4Al-0.2Si, (c) TX32-0.4Al-0.4Si, (d) TX32-0.4Al-0.6Si

and (e) TX32-0.4Al-0.8Si alloys. 6-48

Hot Deformation And Microstructural Characteristics of Al...

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