Microstructure evolution and mechanical...

Sept. 20, 2006

Microstructure evolution and mechanical properties in Ti-5Al-5V-5Mo-3Cr after solution heat treatment and ageing in the alpha-beta range

By: Roque Panza-Giosa

Supervisors: Dr. David Embury - Mc Master University Dr. Zhirui Wang - University of Toronto

Acknowledgements: Dr. Mike Mei - University of Toronto Mr. Neil Harris and Mr. Gerry Kouverianos - Goodrich

Sept. 20, 2006

Industrial interest in Beta Titanium alloys for Landing Gear applications

•Criteria for DesignStatic strength 300M - Ultra High Strength Steels (σu/den = 247MPa/gr/cm3)

Ti-10V-2Fe-3Al – Beta Ti most used (σu/den = 257MPa/gr/cm3)Fatigue strength

•Additional factorsIncrease component reliability

Corrosion resistance (environmental pressure)

Reduce maintenance costs

Ease of manufacturing

Despite their higher cost, the use Titanium alloys has increased steadily over the last 30 years.

Sept. 20, 2006

A380 Body Landing Gear

Titanium componentshighlighted by blue arrows (Ti-10V-2Fe-3Al)

Beta Titanium in Landing Gears

Sept. 20, 2006

A380 - BLG Bogie BeamTi-10V-2Fe-3Al

Overall size: 168 x 22 x 27 inch4250 x 570 x 670 mm

Weight: 3200 kg

In addition to mechanical properties, we must consider the sections sizes involved and the need to achieve uniform properties in sections up to 200mm.

1m

Sept. 20, 2006

Alloy DevelopmentWe need higher mechanical properties than Ti-10V-2Fe-3AlHigher hardenability - so that we can design larger Ti componentsAlloy must be easy to melt and forge, not prone to segregationMust offer manufacturing advantages

The current interest is in Ti-5Al-5V-5Mo-3Cr, metastable β alloy developed in Russia, a derivative of VT22 and later VT22-1 (Russian patent ~ 2001)

Ti-5-5-5-3 Ti-10-2-3UTS 1240MPa (180ksi) 1170MPa (170ksi)YS 1170MPa (170ksi) 1100MPa (160ksi)Total elong. 6%; Kic 33 MPa√m (30 ksi√in); E 115GPa (17x103Ksi)Solution Treated in the α-β + Air Cooled + Aged between 580-650˚CAir Cooled => Able to maintain properties for thick sections 200mm (8”)Beta Transus 850˚C (1560˚F)

0.30 maxZr0.18 max.O4.0 - 5.5Mo4.0 - 5.5V

0.10 max.C2.5 - 3.5Cr0.30 - 0.50Fe4.4 - 5.7Al

Weight %ElementWeight %ElementWeight %ElementWeight %Element

Sept. 20, 2006

Current SituationTi-5Al-5V-5Mo-3Cr is a new metastable β alloy; the microstructure-property relationship is not well known.

Problem StatementWe do not fully understand the microstructural changes their effect on mechanical properties. This prevents us from using this alloy in new landing gear designs.

ApproachCharacterize the microstructure after various ageing times.Compare properties at various ageing times.

Sept. 20, 2006

Metallurgy of Titanium and AlloysPhysical Properties

Atomic number 22 Atomic weight 47.9Density 4.51 g/cm3Allotropic

- α Hexagonal Close Packed up to 883ºC (Beta Transus)

- β Body Centered Cubic above 883ºC

Beta transus = lowest temperature above which the microstructure is 100% β

Sept. 20, 2006

Effect of Alloying ElementsAlloying elements will tend to stabilize either

the α or the β phasesα Stabilizers β Stabilizers

Aluminum NitrogenGallium Oxygen Germanium Carbon

Vanadium Molybdenum ChromiumIron Niobium CopperNickel Tantalum

Sept. 20, 2006

Strengthening Mechanisms in Ti5553

Depends on thermo-mechanical processing and heat treatment

Cooled from the α + β field, then aged to transform the retained β phase into α + β

In the β alloys – Strength depends on shape, scale, distribution and relative amounts of primary α and decomposition products of the βphase in the microstructure

Sept. 20, 2006

Heat treatment of Titanium Alloys

Beta solution heat treatment

Alpha-Beta solution heat treatment

Microstructure - primary alpha + retained beta

Tem

pera

ture

% β stabilizer

β

α

α + β

β transus

Pseudo-Binary Phase Diagram

●

●

●

Ageing in Alpha-Beta range Microstructure – resulting from transformation of retained beta

Sept. 20, 2006

Transformation of the retained β PhaseTe

mpe

ratu

re

% β stabilizer

β

α

α + β

β transus


●

●

ω + β β’+β

Ms

ω - hexagonal metastable phase which forms when α formation is difficult

β’ – solute lean metastablephase which forms when ωformation is suppressed, β’coherent with β. Also known as phase separation (β→ β + β’)

Possible reactionsβo → β + αβo → β + ω→ β + ω + α → β + αβo → β + β’ → β + β’ + α → β + α

Sept. 20, 2006

Tem

pera

ture

% β stabilizer

β

α

α + β

β transus


●

●

Step 1 – Forging in the Beta phase field ~ 20-30 % reduction

Manufacturing sequence for Ti5553

● Step 2 – Forging in the Alpha-Beta field ~ 30 % reduction

Step 3 – Beta anneal

Step 4 – Final forging in the Alpha-Beta field ~ 30-40 % reduction, followed by air cool

Step 5 – Alpha-Beta solution heat treatment and air cool

Step 6 – Ageing to transform retained Beta into Alpha+Beta

Sept. 20, 2006

Solution Heat Treatment50˚C below Beta Transus (790˚C; BT~840˚C for this heat)Soak time of 120 minutesAir cooled (aided by a fan)Results in ~16% primary alpha in retained beta matrix

Ageing600 ± 5˚C Ageing time - 5 min. - up to 48 hrs.

All heat treatments done in air, as in industrial practiceAll material from a single forging, all tensile specimens taken in the same orientation

Heat treatment of Ti-5Al-5V-5Mo-3Cr forging

Sept. 20, 2006

Characterization Work in this Research

Hardness changes upon ageing following solution heat treat in the α+β range and air cool

Microstructural changes upon ageing as determined byOpticalSEM TEM

Changes in mechanical properties (tensile and compression)

Sept. 20, 2006

Hardness Changes Upon Ageing

Sept. 20, 2006

Hardness change with ageing time

Rapid hardness increase within 30 minutes

Hardness versus Ageing Time (600C)

30

32

34

36

38

40

42

44

46

0 10 20 30 40 50 60

Ageing Time (minutes)

HR

C

Sept. 20, 2006

No change in hardness from 30 minutes to 48 hours

Hardness change with ageing timeHardness vs. Ageing Time (600C)

283032343638404244

0.01 0.1 1 10 100

Log Ageing Time (hrs)

Har

dnes

s H

RC

Sept. 20, 2006

Tensile and Compressive PropertiesAged at 600˚C

Sept. 20, 2006

Tensile Properties• Within 20 minutes tensile strength reaches ~1300 MPa, yield strength 1200 MPa, both remain relatively constant up to 48 hours.• With ageing, ductility decreases rapidly at first, then increases, reaching a peak at 6-8 hours and decreases gradually with further ageing.

145.51275.51303.12880 (48hrs)

125.51282.41310.02880 (48hrs)

135.51289.31316.91440 (24hrs)

135.51268.61296.21440 (24hrs)

2251220.41254.91080 (18hrs)

1551227.31268.61080 (18hrs)

1781227.31275.5780 (13hrs)

2791199.71241.1780 (13hrs)

2281248.01316.9360 (6hrs)

2281241.11303.1360 (6hrs)

2081227.31282.4180 (3hrs)

2061254.91296.2180 (3hrs)

1461234.21303.160

1161234.21310.060

1251234.21316.920

1451234.21303.120

4716855.0910.10

4516861.9889.40

% RA% ElongationYS (MPa)UTS (MPa)Ageing time (mins)

Sept. 20, 2006

Fracture Stress

Fracture stress shows little dependence on microstructure

Fracture Stress (MPa) and Reduction Area vs Ageing Time (hrs)

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

1400.0

1600.0

1800.0

Ageing Time (hrs)

MPa

0

10

20

30

40

5060

70

80

90

100

%R

A

Fracture Stress (MPa)Reduction of Area (%)

Fracture Stress (MPa) 1454 1478 1472 1534 1581 1506 1491 1417 1441

Reduction of Area (%) 46 13 12.5 20 22 22 18.5 13 13

0 0.33 1 3 6 13 18 24 48

Sept. 20, 2006

Fracture Stress -Tension and Compression

Fracture stress is the same in tension and compression; the sameaged at 500C and 600C

Comparision of Fracture Stress - Tension and Compression Aged at 500C and 600C

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

1400.0

1600.0

1800.0

1 3 6Ageing time (hrs)

Frac

ture

Stre

ss (M

pa)

Tension-Aged at 600C Tension-Aged at 500C Compression-Aged at 600C

Sept. 20, 2006

Fracture Mode Evaluation

Solution Heat Treated Aged 20 minutes Aged 60 minutes

Aged 360 minutes Aged 48 hoursAged 24 hours

Sept. 20, 2006


Aged 60 minutes

100% dimple rupture,very shallow dimples ~2-5 µm diameter

Sept. 20, 2006


Aged 48 hours

Dimple rupture. Dimple size does not change with ageing time.

Sept. 20, 2006

Compression PropertiesRod aged at 600˚C for 6 hours, compression loaded to failure

Fracture stress ~ 1568 MPaRod Fractured at 45 degrees, very few secondary cracks

Sept. 20, 2006

Microstructural Changes Upon Ageing

Optical Characterization

Sept. 20, 2006

Solution heat treated Aged 15 minutes Aged 60 minutes

Primary alpha (2-5 µm in diam.) Localized dark etching regions Complete transformation within in retained beta. Some grain reveal non-homogeneous retained beta regions.boundaries (~5 µm) are evident precipitation of alpha withinNo evidence of martensite. retained beta.

Etched in Kroll’s

Optical Characterization

Sept. 20, 2006


SEM Characterization

Sept. 20, 2006 Etched in Kroll’s

Solution heat treated Aged 15 minutes Aged 60 minutes• After solution heat treatment, microstructure consists of uniformly distributed primary alpha in retained beta matrix.• After ageing for 15 minutes at 600˚C, fine needles of alpha begin to precipitate, spacing is roughly 0.3µm, length is several microns - transformation is non-homogeneous.• After ageing for 60 minutes at 600˚C, laths of needle-shaped alpha are evident, some alpha precipitation is seen at low angle GB’s

α

β

Sept. 20, 2006

Aged 6 hrs Aged 24 hrs Aged 48 hrs

• Scale of microstructure remains the same from 60 minutes (no change in primary alpha volume fraction, or needle size) up to 48 hours

• Grain boundary alpha is seen to grow in thickness, to form a continuous film network

Sept. 20, 2006


TEM Characterization(early stages)

Sept. 20, 2006

SADF from β phase<110>β zone diffraction patterns, no spots for α or ω

• Microstructure consists of primary α in retained βmatrix• No α is evident within β or at grain boundaries

Solution heat treated

Sept. 20, 2006

• Alpha precipitation nucleated at GB’s and within grains.

• Precipitation is non-uniform and heterogeneous.

• The precipitates are parallel thin slivers

Aged 5 minutes

α

Sept. 20, 2006

Aged for 5 minutes• We see emerging patterns of contrast ~parallel with 0.1 – 0.2µm spacing• Grow lengthwise with increasing ageing time; nucleation appears to be at dislocations and stacking faults (this needs crystallographic confirmation)

Sept. 20, 2006

Aged 20 minutes Aged 6 hrs.

50nm

0.5um

With increased ageing α needles then coarsen to a thickness and spacing of 0.1-0.2 µm, 1-3 µm long forming laths.

SADP, BCC [011] zone;weak spots correspond to α, no ωpresent

Sept. 20, 2006

Aged 24 hrs Aged 48 hrsWith increasing ageing times, continuous, relatively thick (>0.3µm) alpha films can be seen at grain boundaries

Sept. 20, 2006

Rationalization of ResultsThis is not a complete study, but nevertheless a series of basicquestions emerge that will assist in developing a more complete rationalization

Phase transformation βo → β + αTransformation is heterogeneous and appears to nucleate at stacking faults, grain boundaries and dislocations.

Laths appear to grow by production of dislocations.

Need to explainRapid hardeningConstant flow stress at longer times

timeσy

Sept. 20, 2006

Discussion PointsWhy do the Hardness, Tensile Strength and Yield Strength increase with ageing?

Consider the following:

σy σi + k√rf + kyd-½ + k*√c + αGb√ρ

σy = yield strengthσi = resistance of lattice to dislocation movementk, k*, α = constants r, f = radius and volume fraction of precipitatesky = locking parameterd = grain sizec = solute atom concentrationG,b = shear modulus, Burgers vectorρ = dislocation density

PeierlsStress

Dispersion Strengthening

Boundary Strengthening

Strain Hardening

Solid Solution Strengthening

The only significant feature changing in the microstructure is the size and volume fraction of alpha precipitates

Sept. 20, 2006

Discussion PointsWhy does the Yield Strength remain constant after 20 minutes ageing?The strength increases with size and volume fraction of alpha precipitates until a critical density is reached. This critical density is the point at which a network of alpha precipitates begins to constrain dislocation motion within the regions not yet transformed. This critical point is reached at around 20 minutes ageing. Consequently, there is strain partitioning between transformed and un-transformed beta until transformation percolates the structure.

α

βα

αα

α

Sept. 20, 2006

Discussion PointsWhy does the Fracture Stress not change with microstructure?We have measured the nano-hardness of the primary alpha and found it to be softer than the retained beta and much softer than the transformed beta.The alpha films along GB’s have limited ability to deform due to their thin nature compared with the structure around them. Hence, these films are constrained from deformation, and fracture occurs at a stress higher than that of the weaker alpha film. In the case of the solution heat treated specimens, where there is no GB alpha film but weak primary alpha, fracture occurs at a higher stress due to fracture along GB’s; hence, fracture is intergranular and takes place at a similar stress. This is supported by the scale of the dimple morphology (~2-5µm) which is of the same order as the grain size.

α

Transformed βα

αα

GB - α Steel

Copper

Nano-hardnessPrimary alpha 2.4GPaRetained beta 3.3GPaTransformed beta 4.2GPa

Sept. 20, 2006

Modification of the Structure

Microstructure is a heterogeneous precipitation plus a mixture of alpha + beta, we could:

Refine the structure by rapid heat treatment or repeated thermal cycles as in steels.Cold work material prior to ageing to increase precipitation kinetics and change the distribution of laths.

Sept. 20, 2006

SummaryThe microstructure resulting from solution heat treatment consists of uniformly distributed globular primary alpha (~2-5 µm in diameter) in a matrix of retained beta; No ω phase present after solution heat treatment.

A rapid increase in hardness is observed within minutes of ageing; the hardness, tensile strength and yield strength reach a peak at approx. 20 minutes and remains constant up to 48 hrs ageing.

The ductility, measured by % total elongation and % reduction of area decrease rapidly with ageing time; remains low for the first few hours of ageing, increase gradually to a peak at ~3-13 hrs and decrease again with further ageing.

Following minutes of ageing, fine alpha needles begin to precipitate within the retained beta.

Sept. 20, 2006

SummaryNucleation is heterogeneous, at grain boundaries and within grains; nucleation is non-homogeneous, some grains transform ahead of others. Precipitation sequence appears to be: βo → β + α

After 20 minutes ageing, a network of alpha needles measuring 1-3 µm in length and with a spacing of 0.1-0.2 µm are established within the beta phase.

Transformation continues with increasing ageing time forming complex laths, but the scale of the needles remains constant from 20 minutes up to 48 hrs. It is the invariant scale of the microstructure that accounts for the constant yield strength.

Grain boundary alpha films are seen to grow and coarsen with increasing ageing time.

True-fracture stress is independent of changes in microstructure.

Sept. 20, 2006

Future WorkDetailed characterization of nucleation and growth process of α by TEM.

Explore the influence of other stress states to determine the shape of the

failure surface in stress space .

Fracture mode analysis of tensile and compression specimens by SEM.

Changes in nano-hardness at the primary α, transformed β and grain

boundary α and their effect on fracture process.

Effect of microstructure on fatigue life - industrial interest.

Ways to manipulate the microstructure to improve mechanical properties

Microstructure evolution and mechanical...

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