Processing of - MTU Aero Engines

Aeromat Presentation Jun 9, 2009
Processing of (α/β) and (α2/γ) Titanium Alloys to
Tailor Microstructure for Performance
Gerhard Welsch
J. Hausmann. K, Baumann, S. Lenser, K. Kelm, L. Chernova; K. Weber; S. Reh
W. Smarsly
Case Western Reserve University
Deutsches Zentrum für Luft- und Raumfahrt
MTU Aero Engines
Objective
1. Develop unique titanium alloys
composition
microstructure(s)
properties for high performance applications
2. Exceed properties of standard (mill-processed) alloys
in cast or wrought (plate, sheet, rod, wire) products
3. Cost-effectiveness
Material selection
Primary and secondary processing methods
Reproducibility
Reliability

Rationale for this study:
Ti-6w%Al-4w%V, or in atom percent Ti-10.5Al-3.5V, is the most widely used and
perhaps best investigated high-strength, two-phase (α+β) solid-solution titanium
alloy. It represents the vast majority of structural titanium alloys used for
demanding applications below 400C, e.g., in aircraft structures and engines.
Ti-Al-X, with approximate composition (in atom%) of ~44Al and ~6X, is an
intermetallic (α2+γ) two-phase alloy with X = element selected from the group V,
Nb, Ta, Cr, Mo, W. It is lighter, stiffer, and potentially stronger than the (α+β)
titanium alloys. Despite its huge potential, e.g. for use in turbines or machinery for
efficient energy conversion, problems with processing and consistency of
microstructure and ductility have prevented the alloy from a breakthrough on the
industrial stage.

‘Specific strength’ (normalized to density) of alloys used in aircraft (gas turbine)
engines
Based on ‘specific strength’ only the use-preferences of the alloys are:
a) Low-temperature range (below~200C): Comparable specific strength of highstrength
titanium solid-solution and intermetallic Ti-Al-X alloys
b) Between 200 to ~800 C: Superiority of intermetallic Ti-Al-X alloy
c) Above 800 C: Nickel superalloys are superior.
Increases in ‘specific strength’ are possible for (α+β) and for (α2+γ) alloys

Below:
References for room-temperature-strength increases (Yield and UTS) by grain refinement.. The example
is for ultrafine-grained Ti-6Al-4V alloys:
Tensile strength of industry-standard fine-grained (~1.2μm) and of deformation-refined ultrafinegrain-size
Ti-6Al-4V alloy with average alpha grain dimensions of 0.25 and 0.45μm.
Conclusion: Strength of (α/β) Ti alloys can be increased beyond industry standard by grain
refinement.
Grain size was refined by repeated deformation-processing between 550 and 800C
[O.A. Kaibyshev and F.Z. Utyachev: Superplasticity, Structure Refinement and Treatment of
Hard-to-Deform Alloys [in Russian], Nauka, Moscow (2002)]
Evaluation: Hall-Petch relation at room temperature
σTS = σo + k d -1/2 , σo = 605MPa k = 430 MPa (μm) 1/2

Purpose of study:
• Investigate exemplar α/β (Ti-6Al-4V) and α2/γ (Ti-Al-X) alloys as representatives
of each alloy family.
• Investigate, in particular, processing methods, resulting microstructures and
properties of these alloys.
For each alloy type the achievable mechanical properties (stiffness, strength,
ductility) and thus their utility are controlled by microstructure. It includes global
and localized composition, crystal properties of phases and crystal defects, also
grain- and phase-boundaries, and size and distribution of grains or phases. It also
includes the state of understanding of how these features, ranging from nm to mm,
contribute to the strength and ductility or to failure of the alloys.
We want to examine what microstructures can be formed, how they are formed,
and what has been achieved by industrial and/or experimental methods. Based on
review of literature and own experimental contributions recommendations are
offered for the design and processing of α/β (Ti-6Al-4V) and α2/γ (Ti-Al-X) alloy
types.

Purpose of processing
shape the microstructure
enable performance such as strength, stiffness, reliability
incorporate microstructure and performance in a shaped part
Processing goal: Refinement of composition and microstructure
1) Alloy homogeneity and uniformity
2) phases and phase-mixtures in starting and processed conditions (α/β), (α2/γ)
3) Parameters of microstructure:
4) Texture
5) Residual stress
grain size of massive phases
grain size of plate- and rod shaped phases
size of lamella colonies
orientation-relations between phases
misfit and/or stress at phase- and grain boundaries
6) Plastic forgiveness - mobile dislocations & twinning
A rationale for each and selected examples will be provided below.

Selection of alloy elements for titanium
In Ti-6Al-4V alloy Al and O promote the formation of α-phase and partition
toward the hexagonal titanium phase. Oxygen hardens and potentially embrittles
the alloy. Up to 0.2w% (~0.6 a%) oxygen can be used in the Ti6Al-4V alloy.
Vanadium promotes the formation of β-phase and partitions toward the BCC
titanium phase.
In Ti-Al-X alloys Al and O promote the formation of α2-phase and partition toward
the hexagonal (Ti3Al)-based phase. Oxygen can be tolerated in much smaller
concentration than in the Ti6Al-4V alloy, e.g., less than 0.2a%.
“Blue-tinted elements’ serve as stabilizers for a ductile beta phase, also for
modification of lattice parameters, e.g. the c/a ratio in the intermetallic γ phase, and
as modifiers for phase transformation, e.g., by shifting the (β→β+α)transformation
temperature of Ti-6Al-4V alloy or the α→γ transformation and
α→(α2+γ) eutectoid temperatures of the intermetallic TiAl-X alloys.

Lattices of various titanium phases
Ti-6-4
During usual synthesis (liquid solidification cooldown) the sequence of
phase formation can be gleaned from the phase diagram.
Beta = parent phase. It exists over a wide temperature range (from solidus to It has
six densest-packed (110) planes. Each contains two
dense-packed atom directions.
Upon phase transformation at or below the transus temperature (~1000C) the
newly forming alpha phase aligns a dense-packed [1 1 -2 0] direction along one
of the dense-packed [1 1 1]-parent directions. The Burgers’s orientation relation
between the product phase emanating from the parent beta phase is

The transformation β β+ α typically begins at a nucleation site, e.g. a beta grain
boundary. Prior beta grain boundaries are often decorated with alpha phase from
early nucleation events.

Shape Microstructures in (α/β) and in (α2/γ) titanium alloys – nm to mm hierarchy
Performance through
high (specific) strength
high (specific) stiffness
reliability
These are enabled by:
Degrees of refinement in composition and microstructure
1 Alloy cleanliness
Chemical homogeneity and uniformity
2 Grain-size
Colony size
Lamella width
3 Volume fraction and distribution of phases
Phase continuity (parent phase versus transformation-generated phase)
4 Grain boundaries
Phase boundaries without orientation relation
Phase boundaries with (Burgers, Blackburn, …) orientation relation
5 Texture – geometrical and crystallographic
6 Anisotropy and residual stress
Elastic anisotropy
Plastic anisotropy
7 Crystal defects comprising . . . including (mobile) dislocations

Phases/Alloys Ti Al X= Group V/VI
elements
Composition (atom %) V, Nb, Ta; Cr,
Mo W
(α+β) Ti-6-4 bal. 10.5[Al] - 0.6 [O] 3.5[V]
(α2+γ) Ti-Al-X bal. ~44[Al] - 0.2 [O] 6 [Nb]
…
Analogies
In a typical finish-processed conditionTi-6-4 consists of a mixture of two phases:
1) cubic (BCC) β-phase: Volume 100 % above α/β transus (980 to 1010 C)
2) hexagonal (HCP) alpha phase is stable below α/β transus temperature and
is usually in equilibrium with V-enriched
β-phase
When α+β is formed by transformation from β-parent phase the α and β phases are
orientation-related in a way that provides a best match of dense-packed atom
planes and atom directions, namely the Burgers orientation relation:
Plane matching and Direction matching
(110)β // (0001)α and one of two β directions // one of three α directions

Processing strategies for exemplar (α/β)-Ti-6Al-4V and (α2/γ/β)-Ti-Al
alloys
Chemical homogeneity over long range and short range
segregation
Casting: Time of solidification t [Chvorinov, 1940]
Cooling rate ~ t -1
Example: Ti-47Al button:
Dendrite size = f (cooling rate) n

Homogenization: diffusion time in single-phase field tD ~ a (X 2 /D)
where X = dendrite/segregation spacing, and D =effective diffusivity
Grain Coarsening upon solution treatment:
When mostly single phase (need not be homogeneous)
Recrystallization and grain growth kinetics different (faster) than
element homogenization.
Grain Refinement: through short-time (preferably isothermal)
deformation-processing at solution-treatment temperature or lower
temperature.
Example: Hot extrusion (some T-increase) and rapid cooling

Examples of segregation in Cast structure + extrusion forming

After homogenization treatment (grain growth but not fully
homogeneous)
Clemens et al, Intermetallics 16 (2008) 827-833

Illustration of phase transformations using
quasi-binary phase diagrams for two alloys: Ti-10Al- 3.5V and Ti-45Al-X (at %)

Transformation of (Ti-6-4 wt%) during cooling from β solid solution (parent
phase)
Three approaches:
1. slow cooling: β → (α+β)Widmannstätten colonies
2. quenching and re-heating: β → α’ → (α+β)fine-lamella
colonies
3. deformation-processing + recrystallization of (α+β)fine-lamella colonies
→ duplex or equiaxed
microstructure
Transformation of TiAl-X alloy during cooling from α solid solution
α → (α+γgrains) → γgrains + (α2/γ)lam. colonies
Solid sol. Nucleation at α- grain b’s “ nucleation at α- grain boundaries
Likely orientation-related “ at γ/α grain interfaces
to an α- grain (111)//(0001). “ oriention relation between α2/γ
1 or 2 or 3 orientations can join lamellae
and lead to formation of γ/γ grain b’s

Illustration of microstructure evolution during
transformation:
α → (α+γgrains) → γgrains + (α2/γ)lam. colonies
FIG. 6-12

Typical microstructures:
in Ti-6-4 alloy: β→(α + β) lamella colonies (Widmannstätten-type)
Ti-6-4alloy: Duplex microstructure formed by sub-transus deformation processing
of (α/β) lamella colonies: → Duplex α-grains + (α + β) lamella colonies
Important: - Homogeneity of (β) parent phase - especially prior β grain boundaries
- Small grain size of (β) parent phase
- Refined transformed microstructure (undercooling → rapid
nucleation)
Choose cooling rate or holt-time at temperature to achieve
- small colony or domain size
- Fine lamella widths

Examples of microstructures in T-Al-X alloys
a) Nearly fully lamellar: (γ + α2) lamella colonies in a Ti-45Al-5Nb alloy [S. Gebhard, K. Baumann]
b) Duplex microstructure of γ-grains + (γ + α2) lamella colonies (duplex) [S. Lenser]
Example micrographs show grains of primary γ-phase preferentially at grain boundaries of
parent α-phase. Variation in volume fraction of γ-grains due to variation in Al-concentration.

This (enlarged) figure shows the nucleated gamma grains at prior alpha grain boundaries, and
alpha-2/gamma lamella colonies in the prior alpha grains. It also contains an indication that the
alloy was not fully homogeneous, e.g., the higher volume fraction of gamma grains (top portion
of figure) indicates a higher Al-concentration than in the bottom portion of the Figure.
A non-uniform distribution of the phases (gamma being elastically stiffer, alpha-2 being more
resistant against plastic deformation) will lead to nonuniform elastic/plastic response during a
tensile test.
An example of this is shown below.

Tensile tested sample to onset of fracture

Alloy type (at%) (α+β): Ti - 10Al - 3.5 V (γ+α2): Ti ~ 45Al ~ 6
GroupV,VI
(wt%) (Ti-6Al-4V)
Tensile stress-strain curves
Overview of tensile stress-strain curves of exemplar alloys
For each the potential exists of significant further improvement, eg by better
homogenization and further grain refinement

Tensile test (S. Lenser, DLR): Ti-44Al-6Nb alloy - cast, extruded, solution-treated + hot forged ,
air-cooled. Tensile sample from one of the principal directions of the ingot.
Youngs’ modulus =
Yield Strength
Strain hardening: two regimes a) dσ/dε ≈ 0, b) dσ/dε > 0 indicate … plasticity-inducing
mechanisms compare with TEM results

Examples of micromechanisms of plastic deformation
a) Slip mechanisms in phases and phase aggregates of Ti-6-4 alloy [Welsch and Bunk, Metall.
Trans. 1979]

) Slip mechanisms in phases and phase aggregates of (γ + α2) or (β + γ
+ α2) alloys (from cited literature)
[Oehring and Appel 2003]

Slip directions Slip Planes
⅓ (0001), {1 -1 0 0}, {1 -1 0 1}
. . .
Weak Links and strengthening mechanisms
4 Grain boundaries
Phase boundaries without orientation relation
Phase boundaries with orientation relations (Burgers, Blackburn, …)
5 Texture – geometrical
crystallographic texture
6 Anisotropy and residual stress
Elastic anisotropy
Plastic anisotropy
7 Crystal defects comprising . . . including (mobile) dislocations

Illustration of various phase arrangements in microstructures of Ti-6Al-4V and
TiAl-X alloys
To be expanded
. . .
Show the potential weak links
-------------------------------------------------------
Conclusions:
Further property improvements can be expected from each alloy type
(α+β) titanium alloys and
γ-rich titanium aluminide alloys
by refinement of chemical homogeneity, phase distribution and grainand/or
colony size:
Processing on large scale
better liquid uniformity
but less control of chemical homogeneity
and less control of microstructure (uniformity and grain size)
Processing on smaller scales
better temperature control

END
easier to form uniform and refined microstructures