Mechanical properties of heat treated Ti-5Al-5V-5Mo-3Cr, an … · 2018-04-14 · September 22, 2008 Goodrich Proprietary Page 2 A new βTitanium alloy for aircraft Landing Gears

Mechanical properties of heat treated Ti-5Al-5V-5Mo-3Cr, an attempt to define critical properties of various microstructural features The properties and microstructure of Ti-5Al-5V-5Mo-3Cr were characterized under various stress states after the following heat treatments: 1) beta anneal and air cool; 2) beta anneal + solution heat treatment in the alpha-beta range; 3) beta anneal + solution heat treatment and ageing in the alpha-beta range. For each condition, the damage mechanisms and final fracture modes were evaluated and rationalized on the basis of microstructural features. The true fracture stresses for the various conditions are compared. Beta annealed material exhibits intense localized slip deformation leading to early crack formation and fracture. This mechanism is explained in relation to the presence of fine metastable phase precipitates resulting from the air cool step. Grain size dependence of the yield stress is described in terms of the Hall-Petch relationship. Authors: Roque Panza-Giosa – Goodrich Landing Gear

Dr. David Embury - McMaster University Dr. Zhirui Wang - University of Toronto Dr. Xian Wang - McMaster University

September 22, 2008 Titanium Conference Page 1

Mechanical properties of heat treated Ti-5Al-5V-5Mo-3Cr - An attempt to define critical properties of various

microstructural features

Roque Panza-Giosa

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

Other contributors: Dr. Xian Wang - McMaster University

Goodrich Proprietary Page 2September 22, 2008

A new β Titanium alloy for aircraft Landing Gears

• Advantages of Ti-5Al-5V-5Mo-3CrHigher mechanical properties than Ti-10V-2Fe-3Al

Higher hardenability – up to 150 mm

Easy to melt, not prone to segregation

Can be machined in the final heat treated condition

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

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

Chemical composition of Ti-5Al-5V-5Mo-3Cr, metastable β alloy


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

Overall size: 167.5 x 22.4 x 27.2 inch4255.0 x 568.1 x 689.8 mm

Weight: 3210 kg

In addition to mechanical properties, we must consider the sections sizes involved and the need to achieve uniform properties in sections up to 8” (200mm).

1m


Microstructure of Ti-5553After β annealing, microstructure consists of re-crystallized β grains with a very fine dispersion of ω phase

After α + β solution heat treatment (starting from α + β forged), the microstructure is primary globular α and retained β matrix, no ω phase

Upon ageing – develop laths of fine α precipitating from the retained β


Transformation of the retained β PhaseTe

mpe

ratu

re

% β stabilizer

β

α

α + β

β transus

Pseudo-Binary Phase Diagram

●

●

ω + β β’+β

Ms

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

β’ – solute lean metastable phase 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



Models to predict yield and fracture

Used to predict yield and fracture under complex stress states: Tresca, von-Mises, Mohr’s.


- We have a complex microstructure highly dependent on heat treatment cycle chosen

- Strive to understand the microstructural changes and their effect on mechanical properties

Objectives of Current Work

Characterize the microstructure after solution heat treatment in the β, α-β and after ageingObtain mechanical properties for tension, compression for each heat treat cycleRationalize the mechanical properties on the basis of microstructural features by characterizing the deformation, damage and fracture modes


Summary of Current WorkHeat Treatments Studied

α-β solution heat treatment at 790ºCα-β solution heat treatment + ageing at 600ºCα-β solution heat treatment + ageing at 500ºCβ annealing followed air coolingβ annealing followed by α-β solution heat treatment at 790ºCβ annealing followed by direct at 600ºC

MicrostructureOptical, SEM and TEM

Mechanical PropertiesTension Compression Shear

Fractographic AnalysisOptical and SEM


Microstructure after Various Heat

Treatments


Etched in Kroll’s

Microstructure after β annealing at 900ºC for 1.25hr.

Ti-5553 microstructure consists of equiaxed β; average grain size is ~200 µm

As-forged Ti-5553 microstructure.


Etched in Kroll’s

Microstructure after solution heat treatment at 790ºC + AC

• After solution heat treatment, microstructure consists of uniformly distributed primary α in retained β matrix, primary α appears semi-coherent• Primary alpha (2-5 µm in diam.) ƒ = 16%. Sub-grain boundaries (~5 µm) are evident


a) Solution heat treated b) Aged 15 minutes c) Aged 60 minutes

b) Localized dark etching regions reveal non-homogeneous precipitation of α within retained βc) Complete transformation within retained β regions

Microstructural changes with ageing at 600ºC


Mechanical Properties


Comparison of Mechanical Properties, Fracture Stress after Various Heat Treatments and Test Methods

Properties of Ti-5553 after β-annealing.

942.6834.3Compressionβ annealed at 900ºC

937.5782.6Tensionβ annealed at 900ºC

Fracture Stress (MPa)Yield Stress (MPa)TestHeat Treat Condition

1581.11244.5Tensionα-β solution treated + Aged 600ºC

1511.71289.3Tension

1454.7858.4Tensionα-β solution treated (790ºC 120 min)

α-β solution treated + Aged 500ºC

1582.21265.2Compressionα-β solution treated + Aged 600ºC

Fracture Stress (MPa)Yield Stress (MPa)TestHeat Treat Condition

Properties of Ti-5553 after α-β- ST and Ageing


True stress-strain curve comparison

True Stress-Strain Curves

0

200

400

600

800

1000

1200

1400

1600

0.000 0.100 0.200 0.300 0.400 0.500 0.600True Strain

True

Stre

ss (M

Pa)

Alpha-Beta ST+Aged 0 hrs - Tension Alpha-Beta ST+Aged 6 hrs - TensionAlpha-Beta ST+Aged 6 hrs - CompressionBeta Annealed - TensionBeta Annealed - Compression


• The α-β solution heat treated and the aged condition have roughly the same fracture stress

• The β annealed condition has much lower fracture stress and ductility

• For each heat treat condition, the fracture stress in tension and compression are the same

Above observations suggest that a shear event controls fracture process

Mechanical property observations


Mode of Fracture forβ – Solution Annealed


Morphology of Tensile Fracture

SEM Image of fractured tensile specimen.Note the faceted appearance and parallel ridges.


Optical image of fractured tensile specimen; slip bands and transgranular cracks throughout deformed region

Cross Section of Tensile Fracture


Damage Process During Plastic Deformation

Localized slip bands, leading to transgranular cracks within grains


Fracture stress is a function of the grain size

β annealed condition follows Hall-Petch relationship; the friction stress is of the order of 780MPa.

σ = σi + K d-1/2

True-Fracture Stress vs Beta Grain Size

σf =777 + 71.7 d-1/2

750

850

950

0.75 1.25 1.75 2.25

Beta Grain Size, d^-0.5 (mm-0.5)

True

-Fra

ctur

e St

ress

(MPa

)


Damage Process Leading to Fracture During Plastic Deformation

SEM Image of polished three point bend specimen. Note the steps and cracks due to localized slip


BF and DF images (foil prepared by ion milling) showing ω phaseω particles are of the order of ~20 nm


TEM, BF and DF images (foil prepared by ion milling)

Localized slip mechanism: 1) ω particles are cut by dislocations2) this plane (band) now offers less

resistance than adjacent planes3) further slip is localized to this plane/band


Mode of Fracture for α-β Solution Heat Treatment


Fracture Morphology of Tensile Specimens - Solution Treated Cond.

Transgranular fracture consisting of uniform dimples 5 µm.


Mode of Fracture for α-β Solution Heat Treatment

and Aged at 600ºC


Transgranular fracture consisting of uniform dimples 5 µm.

Fracture Morphology of Tensile Specimens- Solution Treated + Aged


Tensile and Compressive Crack PropagationSEM images of secondary crack tip from compression (left) and tensile tests (bottom) – showing crack path through both GB and primary α/transformed-β interface.

Fracture occurs by shear at 45º

Nickel layer


Fracture Mode ObservationsFor many alloys the fracture stress in compression is significantly greater than that in tension. In Ti-5553, the fracture stress is the same in tension and in compression. This suggests the material fails when a critical shear stress isreached, hence the biaxial fracture stress envelope is an extension of the yield envelope.

σ1

σ2

σ1

σ2


Thank you Questions?


Fig.1 [110]β zone axis pattern taken from Ti-5Al-5V-5Mo-3Cr (Left) and Ti-6Al-4V (Middle) alloys respectively and key to both patterns (Right) showing the coexistence of ω and/or α phase (foil prepared by ion milling)A comparison of selected electron diffraction (SAD) pattern in [110]β zone axis obtained from our sample with that

characterized by Z. Fan and A. P. Miodownik in Ti-6Al-4V alloy (Journal of Materials Science, 29 (1994) 6403-12) is shown in Fig.1. It can be seen that in β grain there are coexistence of both ω and α phases. Dark field (DF) image gives very fine ωand/or α particle as shown in Fig.2. Fig.3 corresponds to micrographs at high magnifications.


Fig.7 [001]β zone axis pattern taken from Ti-5Al-5V-5Mo-3Cr (Left and middle) and Ti-14Mol-6Al (Right) alloys respectively (foil prepared by ion milling)

Fig 7 which is similar to that of solutionzied and aged Ti-14Mo-6Al alloy (C. G. Rhodoes and J. C. Williams, Metallurgical Transaction A, 6 (1975) 2103-14). In the pattern the precipitate reflections are arced, or curved, rather than streaked in a particular crystallographic direction. This type of alpha phase is referred as Type 2 alpha phase by C. G. Rhodoes and J. C. Williams. Fig.8 is BF and DF image of this type of alpha phase in Ti-5Al-5V-5Mo-3Cr alloy.

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Mechanical properties of heat treated Ti-5Al-5V-5Mo-3Cr, an … · 2018-04-14 · September 22, 2008 Goodrich Proprietary Page 2 A new βTitanium alloy for aircraft Landing Gears