Advances in Gammalloy Materials–Processes–Application Technology: Successes, Dilemmas, and Future
Advances in Gammalloy Materials-Processes-Application Technology: Successes, Dilemmas, and Future
For the last several years, gamma titanium aluminide (c-TiAl)-based alloys, called ''gammalloys,'' in specific alloy-microstructure forms began to be implemented in civil aero-engines as cast or wrought low-pressure turbine (LPT) blades and in select ground vehicle engines as cast turbocharger rotors and wrought exhaust valves. Their operation temperatures are approximately up to 750 C for LPT blades and around 1000 C for turbocharger rotors. This article critically assesses current engineering gammalloys and their limitations and introduces eight strengthening pathways that can be adopted immediately for the development of advanced, higher temperature gammalloys. Intelligent integration of the pathways into the emerging applicationspecific research and development processes is emphasized as the key to the advancement of the gammalloy technology to the next higher engineering performance levels.
The finally realized implementations of current
engineering gammalloy materials (cast 4822, cast
45XD and wrought TNM) as aero-engine
low-pressure turbine blades (LPTB) for intermediate service
temperatures (up to 750 C) began to establish the
foundations of their respective materials–processes–
manufacturing technologies.1–6 The first commercial
flights of gammalloy LPTBs took place in March,
2012 when Japan Airlines received purchased
Boeing 787 Dreamliners powered by GEnx-1B engines
equipped with alloy 4822 LPTBs on the last two LPT
stages. That was 2 years after the first flight of B787
with GEnx engines, approximately 8 years after
their successful implementation in GEnx engines,
17 years after the first certification for CFM CF6
engines [Guy Norris, Power House, Flight
International, 13 June 2006], approximately 24 years after
the development of alloy 4822 at GE and nearly
40 years after the first gammalloy exploration
program initiated at AF Materials Laboratory and
conducted by P&W.7,8
Cast gammalloys (Ti-46/47Al-3/6Nb base) began
to be used for automotive engine turbocharger
wheels first at 850 C (1999) and then at gradually
increased gas temperatures up to 1000 C (2011):9,10
This enhancement has been possible because of less
(Published online February 1, 2018)
stringent requirements in property balances,
however with slow adoptions due to the reliability and
cost issues. The first commercial application of
gammalloy exhaust valves eventually took place
with a near-alpha extruded and head-forged form of
alloy 02B (Ti-44Al-6Nb-0.3W-0.1B)11 in 2013 for
race vehicle (F-1 and Moto GP) engines12 where the
operating temperatures are over 800 C.
Despite the exciting developments, their ongoing
application expansion activities13,14 and new
applications4,6 have shown that their respective
temperature capabilities for aero-engine applications have
remained unchanged ever since. The future of
gammalloy technology will depend on whether or
not or how we can develop or design gammalloy
yielding greater service temperature (750–900 C)
capability for specific aero-engine applications and
greater producibility–reliability for ground vehicle
engine component applications.
This article first summarizes the exploration
history of gamma TiAl alloys and reclassifies them
by renaming them gammalloys, redefines their
microstructures, assesses current engineering
gammalloys and their limitations in service
temperature, describes specific pathways to achieving
greater temperature capabilities and then
introduces the emerging ‘‘application-specific research
and development (AsRD)’’ processes, with a recent
realized example, for the development of higher
The first major gammalloy development programs
performed at PW investigated many alloy
compositions, including Ti-45Al-5Nb, and in the end (1981)
recommended Ti-48Al-based alloys, such as
Ti-48Al2W, Ti-48Al-2Nb-1W and Ti-48Al-1V-0.1C.7,8 Based
on the Ti-48Al-based compositions that undergo
peritectic solidification, many conventional
gammalloy (CG) compositions were explored and evaluated
for over a decade starting in the late in 1980s. These
alloys can be grouped in area CG of a quasi-ternary
isothermal (900 C) phase field section (Ti-Al-Nbeq)
containing c, a2 and B2 (Fig. 1). The phase field was
constructed by converting the electron probe
microanalysis (EPMA) data of the equilibrium
compositions of constituent phases to Nbeq using the
betaforming equivalency (BFE) determined for the first
time for gammalloys utilizing available ternary
phase diagrams.15 The BFE relation, Nb = Cr/
3 = Mn/2 = V/2 = Mo/6 = W/8, was estimated based
on Ti-44Al-X-based gammalloy compositions15 and
has been proven valid for a wide range of
The phase field section in Fig. 1 redefines
gammalloys as conventional gammalloys (CG) and beta
solidified gammalloys (BSG) located across the
arrowed dividing line near the gamma phase field.
CG compositions exist within the range
)B-(00.8)(C, Si) and BSG compositions within Ti-(
The typical CG alloys include: 4822
(Ti-48Al-2Nb2Cr),1 XD alloys (Ti-45Al-2Nb-2Mn-0.8v/oTiB2),3 K5
(Ti-46.2Al-3Nb-2Cr-xB-yC/Si)16 and TNB
(Ti-45Al5/8Nb-0.2B-0.2C).17 These gammalloys generate
numerous microstructures that sensitively depend
on alloy composition, solidification path, processing
route, post-processing heat treatment cycle and
aging/stabilization condition. The microstructures
were first identified and classified into near-gamma,
duplex, nearly lamellar (NL) and fully lamellar
(FL),18 and XD lamellar (XDL) is a complex form of
FL. The first four are produced in cast as well as
wrought forms,15,18 while XDL is in cast forms,3 and
their microstructure–property relationships are too
complex to quantify. However, the relationships
could be described for well-controlled duplex and
BSG alloys are Ti-(
)Al based and contain
significantly low (generally below 75%) amounts of
gamma phase compared to those (> 85%) in CG
alloys, as indicated by alloy 1 (Ti-42A-5Mn)20 and
alloy 2 (TNM: Ti-43Al-4Nb-1Mo-0.1B),5,6 which are
approximately located in Fig. 1. Unlike CG alloys, a
Fig. 1. A quasi-ternary isothermal (900 C) phase field section
(Ti-AlNbeq), consisting of c, a2 and b/B2 phases, shows two regions
divided by an arrowed gray line that contain conventional gammalloy
(CG) compositions and beta solidified gammalloy (BSG)
compositions, respectively. CG alloys are characterized by peritectic
solidification, and the BSG region includes special types of areas (A, B, C)
that yield c-rich (> 85%) alloys, called beta-gamma gammalloys
(BG) (Gamteck DKI Base, Y-W. Kim, 15.0310).
small boron addition, (0.04–0.2) at.% B, helps the
BSG compositions solidify to fine-grained (< 50 lm)
lamellar structures, however with significant
amounts of grain boundary gamma and/or b/
B25,20–23 and varying randomness depending on the
cooling rate.24 Because of these and the apparent
inability of producing coarser-grained isotropic FL
microstructures and fair formability, efforts had been
made to control characteristic wrought-processed
nearly lamellar structures. This led to the
development of TNM-WNL, a BSG alloy wrought material
with a specific NL (WNL) structure, which has
managed to make the last stage LPT application in
geared turbofan engines.6
BSG alloys have not demonstrated the formation
of fine-grained (40–150 lm), fully lamellar (FGFL)
microstructures that are c rich (> 85%) in any
material forms. This dilemma was resolved by a
special form of BSG alloys, called beta gammalloys
(BG), that exist very close to the gamma phase field
in three types, A, B and C, as mapped in Fig. 1.15
Type B and A BG alloys lie within the c-rich and
blean phase distributions, (85–92)c-(0–2.5)b/B2-(15–
7)a2 (vol.%). A type B BG alloy, 09C
(Ti-43.8Al-4Nb2Cr-xB-yC), has demonstrated FGFL (30–100 lm)
microstructures in a wrought-processed material
form that yields remarkable RT tensile properties
(YS and ductility) and high-temperature strength
retention.15 Importantly, this alloy demonstrates
similar characters in a cast/annealed material
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High-Nb BSG alloys based on Ti-(44/45)Al have
attracted significant attention, and typical examples
include Ti-45Al-8.5Nb-0.2W-0.2B-0.0Y25 and Ti-44/
45Al-6/4Nb-2Mo-Y/B alloys.26 These alloys contain
more Al and Nb + Mo than TNM alloy, indicating
greater environmental resistance and improved
property balance, and enhanced operation
temperatures if they exhibit the BG type phase
distributions. Property measurements on a wide range of
processing–microstructure combinations showed
these potentials and the possibility of producing
FGFL material (FGFL).26,27 This was manifested by
a type A BG alloy 02B (Ti-44Al-6Nb-0.3W-0.1B) for
successful high-temperature racing valve
Table I lists and summarizes gammalloys, their
types and composition ranges, phase and
microstructure distributions, and related details, as discussed
ASSESSING CURRENT ENGINEERING GAM
Table II compares the current engineering
gammalloys, Ti-4822, 45XD and TNM, and their
processing-material details designed for their respective
LPTB applications, and their pertinent engineering
microstructures are shown in Fig. 2. The poor (or low)
general plasticity is a continued concern that has
often discouraged continued research or component
development activities. This, however, has not
deterred (rightly so) the application-specific
component development activities at major aero-engine
OEMs, as the current alloy materials in controlled
microstructure forms demonstrate good resistance to
fracture and fatigue as well as reasonable threshold
intensities (DKth). Despite the low ductility (2–0.6%),
therefore, their LPT blades have already been put in
service extensively in advanced aero-turbine engines
for more than 5 years.
The limiting factors for their low service
temperatures (< 750 C) that can be drawn from Table II
and Fig. 2 include: (
) none is FL with anisotropic
lath structures; (
) casting Duplex/NL consists of
degenerated lath structures (Ti-4822); (
) XDL with
random lath bundles and boundaries and low
anisotropy (45XD); (
) NL with low gamma volume
(< 72%), grain boundary gamma and B2 particles
and high a2 volume (TNM). Collectively, significant
increases in RT strength levels (4822 to XD to TNM)
would not raise the operation temperature levels
visibly, as the increases in RT strength lower the
high temperature strength retention more rapidly.
The method to raise the service temperature is to
improve both the strength and its high temperature
strength retention capability together, which is
directly related to the above shortcomings. However,
the (obvious) shortcomings have not been assessed or
even noticed, largely because of our tendency to take
the existing situation for granted. This lack of the
integral physical-mechanistic understanding has
lreyadn ()raLXD iednFL
iean lle ra
-rg lam -egg
lrga2482 :45XXDD :lraCGH
Ti-4822: The phase distribution range of Ti-4822 (spec) was estimated based on the value (95.5c + 4a2 + 0.5B2) measured on the Ti4822
(actual) casting duplex.23 45XD: The phase distribution was estimated based on the composition and the BSE imaging in Fig. 2b.23 TNM:
The phase distribution was estimated23 based on the experimental value,6 72c + 23a2 + 5B2, measured on a composition of
often led us to vague R&D directions or targets,
thereby guiding the materials community to delve
into added investigation of the known data,
knowledge and information (DKI) or unnecessary or
redundant details. Most of the additionally generated
research and development outcome over the last
decade therefore has not helped resolve the
PATHWAYS TO ADVANCED GAMMALLOYS
The above shortcomings of the current
gammalloys can be alleviated or removed by realizing the
pathways to achieving greater high-temperature
(HT) capability that will increase both the RT
strength level and/or HT strength retention. The
needed steps and abilities required for this process
) hands-on experience in fundamental as
well as applied RD, (
) mechanistic as well as
physical understanding of the existing DKI, (
valuation and selection of DKI for specific targets,
) processing experience and knowledge, (
knowledge of industrial infrastructure, and (
manufacturing and application information.
The most important requirement, DKI, has been
documented over the last 30 years in a number of
publications since 1989.19,30–41 The critical areas
composition-processing-microstructureproperty relations, involving phase distribution,
microstructure evolution and control, inverse
relations, solid solution and compositions of gamma
phase, precipitation hardening,
deformation-fracture, creep and fatigue; (
) processing details such
as ingot-casting, investment casting, forging,
extrusion, rolling, directional processing and heat
treatment cycles; (
) alloying and phase distribution
effects on oxidation, surface protection,41 joining,
machining, wear and erosion.
Since alloy materials having higher temperature
capability tend to lower the ductility, the best way
to compensate for the degradation is minimizing it
and/or toughening the newly strengthened matrix.
This can be accomplished by generating a
microstructure and material form based on fine lamellar grains
consisting of the highest possible anisotropic laths
with a desired c-rich (> 85%) phase distribution,
which will help retain the plasticity and enhance the
resistance to crack initiation/growth and creep
deformation. The fullest high temperature performance in
balance will be reached when the alloy material is
produced into uniform FGFL15 or refined FL
(RFL)15,16 forms with the highest deformation
anisotropy in the lamellar grains and optimized grain
boundary morphologies. Further improved
temperature capabilities can be most effectively achieved in
lamellar structures when small additions of C or
C + Si are segregated from decomposing a2 laths in
the forms of stable incoherent precipitates (Ti2AlC,
Ti5Si3) along the lath interfaces.33 The used-to-be
coherent and easy slip planes are disturbed with
incoherent precipitates, residual alpha-2, small
amounts of B2 particles and some openings,
effectively pinning and blocking dislocation movements.
Another or additional improvement in RT as well
as high temperature performance can be made
when the material is processed directionally into
an aligned lamellar grain structure employing
controlled alpha-field hot working, such as
extrusion,12,15 forging15 or rolling. Recent advances in the
directional solidification (DS) processes for small
ingots have shown some possibilities for producing
highly textured DS42–44 or single-crystalline like
PST45 material forms. Producing such forms with
highly anisotropic creep-resistant laths is a
daunting task even for simple shapes, not to mention
Based on the above discussions, eight (
pathways to achieving greater temperature
capabilities are identified as listed below:
P1—Microstructure optimization (4822-NL formed
upon cooling, 45-Refined XDL, TNM-NFL).
P2—Higher order (than 4822), gamma-rich
conventional gammalloys (HCG) in NL forms.
P3—NFL microstructure generation in
conventional gammalloys in cast or wrought material
P4—FGFL microstructure generation in BG
alloys in cast as well as wrought material forms.15
P5—RFL microstructure generation in HCG alloy
wrought processed material forms.15
P6—Incoherent particle strengthening of the
HCG alloy cast as well as wrought FL (NFL,
FGFL, RFL) material forms.33
P7—Directional processing—alpha forging,15
alpha extrusion11,12 and possibly alpha rolling.
realization requires long-term efforts.
Most of the pathways have proven to be valid
experimentally or using DKI-based concepts,23 and
each can be validated/realized with minimum
validation experiments, except for P8, on a few
pathway-specific-alloy-processing combinations. All
alloys for P2 through P7 are required to have phase
distributions within the c-predominant (85–92)c-(0–
2.5)b/B2-(16–7)a2 (%), which are rich in c and
deficient in beta.
Two examples are a BG alloy 9C
(Ti-43.7Al-4Nb2Cr-0.2B-0.2C) for P415 and an HCG alloy K3
(Ti45.5Al-2Cr-2.5Nb-0.3W-X) for P5.15 The 9C
composition was used to yield FGFL material forms
through wrought processing (alloy 9C) and also
casting (alloy CN). Their average FL grain sizes in
these alloy forms were approximately 55 lm and
60 lm, respectively. Alloy K3 was controlled to have
two different wrought RFL grain sizes, K3FL1
(110 lm), K3FL2 (220 lm) and K3FL3 (700 lm).
Figure 3 compares 9C (a) and K3FL2 (b)
microstructures in BSE imaging conditions.
Figure 4 shows tensile yield strengths (a) and
specific YS values (b) as a function of temperature
for three current gammalloy materials (4822-CDP/
NP, 45XDL, TNM-WNL) and two advanced
gammalloy materials (9C-FGFL, CN-FGFL, K3FL1
and K3FL2). For TNM, the data from a
cast + heat treated material46 were used for
convenience. For comparison, the yield strength
variation of a high-Nb BSG gammalloy with
modulated microstructures47 is also plotted along
with Ni-base alloy 718. The YS levels of current
gammalloys vary significantly, however, to reach
similar low values around or below 200 MPa at
1000 C. For example, TNM shows a very low
retention from 950 MPa at RT to a value below
200 MPa at 1000 C. Even this low retention46 is
better than those of the stronger mod Hi-Nb alloy
material47 and the forged + heat treated 718.
Remarkable HT strength retentions are achieved
in advanced gammalloys, especially an HCG alloy
K3 in RFL forms (110 lm and 220 lm) that
exhibit a high YS = 730 MPa at RT and
420 MPa at 1000 C, showing a 58% retention.
This high HT strength retention is expected to
yield accordingly enhanced creep resistance at
aProperty ranges in wrought superalloys, PD (phase distribution); CG (conventional, peritectic solidified gammalloys): 4822 and XD; BSG
(beta solidified gammalloys): BG (beta gammalloys); TNM; ACG (BG and advanced CG gammalloys); YS (yield strength); TS (tensile
strength); K1c (crack initiation toughness); DKth (stress intensity for fatigue crack initiation); BDTT (brittle-ductile transition
temperature); L-Creep (long-term creep resistance); L-Oxidation (long-term oxidation resistance).
ADVANCES IN GAMMALLOY TECHNOLOGY
AND APPLICATION-SPECIFIC RD PROCESS
Figure 5 graphically summarizes the previously
discussed advances in the gammalloy materials–
processes–application technology in terms of
operation temperature capability. The lateral expansion
of current aero-engine LPTB gammalloys is shown
to be steady but remains in the same temperature
range (< 750 C). A short-lived development effort
for beta solidified alloys with two-phase (c + b)
distributions20,48 is added in the schematic.
Automotive engine turbocharger wheels began to be used
in diesel engines at 850 C and later on in gasoline
engines at much higher temperatures up to 1000 C
or higher. At these high temperatures, most
gammalloy materials need to be properly coated.41
Higher temperature (> 740 C) service
capabilities for future or higher-performance aero-engine
rotational applications require advanced
gammalloys in specific processing-phase-microstructure
combination forms that can be developed and
produced through specific development pathways
integrated with value-added DKI, as discussed already.
The enhanced service temperatures are expected to
range from 740 C to over 900 C depending on the
selected pathway. The pathways can be selected and
executed independently or in parallel, or two or
three may be combined into an integrated form.
The application-specific research and
development (AsRD) process that has been proposed and
exercised11,12,15 is target-oriented and starts with
the knowledge of applications requiring specific
properties. The next step is to identify a relevant
microstructure type and form that can generate
such properties, which leads to the selection or
prediction of the most suitable pathway consisting
of the alloy and processing route. This top-to-bottom
step needs detailed valuation of DKI and its
integration into the process. Accelerated validation
experiments and tests are to be followed to
determine and optimize the
alloy-processing-microstructure-property combination that can be used to
produce the material suitable for the application.
Example The wrought gammalloy exhaust valves
produced for select race vehicles by Dell West
Engineering are the first gammalloy component
developed employing the AsRD process.11,12 The required
properties were high tensile and high-cycle-fatigue
(HCF) strengths and their high retention and
oxidation resistance at HT. Pathway P7,
alpha-extrusion,15 was selected for producing textured FL
microstructures suitable for the valve forms, and an
innovative multi-rods extrusion method was
formulated and employed for the cost effectiveness.12 The
alloy that would potentially satisfy the specific
property requirements through the specified
extrusion-forging process was determined and selected to
be an Hi-Nb BG alloy 2B (Ti-44Al-6Nb-0.3W-X) that
had been developed earlier for improved
processibility and HT uses.11 Accelerated validation
experiments were conducted following the formulated alloy
material-processing-heat treatment path that would
generate desired microstructures and properties.12
The experiments included ingot size optimization,
acceptable composition range determination,
material and component evaluation, engine tests and
scaling-up, while a component-specific isothermal
head forging process was developed and refined in
parallel. The whole process, including the
establishment of a manufacturing vendor chain, took 2 years
until the thus-produced valves were accepted and
implemented in race vehicle (Formula 1 and Moto
Grand Prix) engines.12
Table III lists important properties of current
engineering gammalloys (CG and BSG) and
advanced gammalloys, BG and advanced CG
(ACG), and typical superalloys. Some values for
advanced gammalloys were estimated based on a
few data points. Nonetheless, they are useful as
future references as advanced gammalloy
development processes proceed.
Finally, we have three gammalloys implemented in
aero-engine turbines. They are one (4822) among many
conventional (peritectic) alloy compositions that have
been explored over the last 3 decades, one XDL alloy
(45XD) and one beta solidified alloy TNM. Their
operation temperature limitation (750 C) has
remained unchanged for nearly 3 decades to many
years. This slow progress is largely due to our tendency
to neglect the vast DKI pileup and instead chase the
new ones, which are often vaguely targeted, redundant
or subjective. As demonstrated in the specified
pathways, exciting opportunities exist for moving the
gammalloy materials-processes forward to achieving
greater temperature capabilities. This opportunity can
be realized through the application-specific R&D
process, which is a powerful, accelerated methodology to
select and utilize the application- or component-specific
pathways to targeted advanced alloy materials and
processes technology development.
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