TITANIUM ALUMINIDE INTERMETALLIC COMPOSITIONS
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No.
61/61 5,253, filed March 24, 2012, the contents of which are incorporated herein by
reference.
The present invention generally relates to compositions containing titanium
and aluminum and the processing thereof. More particularly, this invention relates to
titanium aluminide intermetallic compositions (TiAI intermetallics) based on the TiAl
(gamma) intermetallic compound, with controlled additions of carbon to enhance creep
resistance while maintaining acceptable room temperature ductility.
Because weight and high temperature strength are primary considerations in
gas turbine engine design, there is a continuing effort to create relatively light weight
compositions that have high strength at elevated temperatures. Titanium-based alloy
systems are well known in the art as having mechanical properties that are suitable for
relatively high temperature applications. High temperature capabilities of titanium-based
alloys have increased through the use of titanium intermetallic systems based on the
titanium aluminide compounds Ti3AI (alpha-2 (01-2)) and TiAl (gamma (y)). These
titanium aluminide intermetallic compounds (or, for convenience, TiAl intermetallics) are
generally characterized as being relatively light weight, yet are known to be capable of
exhibiting high strength, creep strength and fatigue resistance at elevated temperatures.
However, the production of components from TiAl intermetallics by extrusion, forging,
rolling and casting is often complicated by their relatively low ductility.
As taught in U.S. Patent No. 4,879,092 to Huang, additions of chromium and
niobium promote certain properties of gamma TiAl intermetallics, such as oxidation
resistance, ductility, strength, etc. Huang discloses a particular titanium aluminide
intermetallic composition having an approximate formula of Ti46-50A146-50Cr2Nobr2 ,
nominally about Ti-48AI-2Cr-2Nb. This alloy, referred to herein as the 48-2-2 alloy, is
considered to exhibit desirable environmental resistance, room temperature ductility and
damage tolerance that enable its use in gas turbine applications, for example, in the low
pressure turbine sections of gas turbine engines and particularly as the material for low
pressure turbine blades (LPTB).
Additions of carbon have been proposed for TiAl intermetallics to promote
certain properties. For example, U.S. Patent No. 3,203,794 to Jaffee et al. discloses that
carbon can be included in amounts of up to 1 atomic percent (10,000 ppm) in a gamma
TiAl alloy that contains 34 to 46 atomic percent aluminum. Another example is U.S.
Patent No. 4,294,615 to Blackburn et al., which discloses the inclusion of carbon in
amounts of 0.05 to 0.25 atomic percent (500 to 2500 ppm) in a gamma TiAl alloy that
contains 48 to 50 atomic percent aluminum and 0.1 to 3 atomic percent vanadium. U.S.
Patent No. 4,661,3 16 to Hashimoto et al. discloses a gamma TiAl alloy that contains 30
to 36 weight percent aluminum and 0.1 to 5 weight percent manganese, and may further
include in carbon amounts of 0.02 to 0.12 weight percent in the alloy. However, Jaffee et
al., Blackburn et al., and Hashimoto et al. generally disclose that carbon additions tend to
reduce ductility. On the other hand, U.S. Patent No. 4,916,028 to Huang discloses that
carbon additions of 0.05 to 0.3 atomic percent (500 to 3000 ppm) can improve ductility in
rapidly solidified and extruded components produced from a gamma TiAl alloy that is
based on the 48-2-2 alloy and contains 46 to 50 atomic percent aluminum, 1 to 3 atomic
percent chromium, and 1 to 5 atomic percent niobium. Notably, Blackburn et al. taught
that carbon concentrations in the range of 0.05 to 0.25 atom % (0.02 to 0.12% weight),
and preferred in the amount of 0.1 to 0.2 atom % (0.05% to 0.1% weight), have
advantages in Ti-AI-V alloys of improving high temperature properties, but with some
reduction of room temperature ductility. Blackburn et al. did not teach the use of carbon
at levels below 500 ppm in chromium and niobium containing alloys. Accordingly there
is a need to increase creep performance and maintain a minimum level of ductility and
fatigue crack growth resistance in niobium- and chromium-containing TiAl alloys.
The 48-2-2 alloy has a nominal temperature capability of up to about 1400°F
(about 760°C), with useful but diminishing capabilities up to about 1500°F (about
8 15°C). However, more expansive use of this alloy within the low pressure turbine and
elsewhere could be possible if improved creep resistance could be achieved at
temperatures exceeding 1500°F (about 8 15"C), for example, to temperatures of about
1600°F (about 870°C). Accordingly, there is a desire to expand the creep capability of
the 48-2-2 alloy, though without sacrificing the environmental resistance, room
temperature ductility and damage tolerance of this alloy system. An acceptable level of
creep resistance for LPTB applications, a nominal ductility of I%, and a minimum
ductility of 0.5% are believed to be desired if not necessary in order to provide adequate
design margin as well as the ability to cast and machine components with complex shapes
from the alloy. Notably, while improved creep resistance has been demonstrated in
gamma TiAl intermetallic compositions through additions of high levels of refractory
elements such as niobium and with carbon contents of typically 1000 ppm or more, with
the exception of U.S. Patent No. 4,916,028, carbon additions at these levels have been
associated with reductions in ductility, often resulting in a nominal ductility of 0.1% or
less.
BRIEF DESCRIPTION OF THE INVENTION
The present invention provides gamma titanium aluminide intermetallic
compositions (gamma TiAl intermetallics) based on the TiAl (gamma) intermetallic
compound. The gamma TiAl intermetallics contain chromium and niobium, as well as
controlled amounts of carbon that achieve a desirable balance in room temperature
mechanical properties and high temperature creep capabilities at temperatures
approaching and possibly exceeding 1600°F (about 870°C).
The TiAl intermetallic compositions are based on the aforementioned 48-2-2
alloy and contain 46 to 50 atomic percent aluminum, 1 to 3 atomic percent chromium,
and 1 to 5 atomic percent niobium, but they further contain carbon that, when included in
very controlled amounts of about 160 to 500 ppm (about 0.016 to 0.05 atomic percent), is
capable of promoting the creep resistance properties of the composition without
unacceptably decreasing its room temperature ductility.
Other aspects and advantages of this invention will be better appreciated from
the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow chart representing a method of processing castings formed of
TiAl intermetallic compositions of this invention.
FIG. 2 contains four graphs that plot fatigue creep resistance, room
temperature and high temperature elongation, and crack growth threshold (AKth) of four
experimental gamma titanium aluminide intermetallic compositions containing varying
amounts of carbon between 160 and 500 ppm.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a gamma TiAl intermetallic composition that
contains controlled amounts of chromium, niobium, and carbon to achieve a desirable
balance of room temperature mechanical properties and high temperature creep
capabilities that render the composition suitable for use in high temperature applications,
including but not limited to the low pressure turbine section of a gas turbine engine.
Mechanistically, carbon is known to increase the strength of TiAl
intermetallic compositions by serving as an interstitial strengthening agent. According to
the present invention, very controlled carbon additions are capable of promoting creep
resistance properties without unacceptably decreasing room temperature ductility of
gamma TiAl intermetallic compositions that contain 46 to 50 atomic percent aluminum, 1
to 3 atomic percent chromium, I to 5 atomic percent niobium. This advantageous
balance of properties can be particularly achieved if the carbon level is about 160 to 500
ppm (about 0.016 to 0.05 atomic percent), more particularly about 160 to 470 ppm (about
0.016 to 0.047 atomic percent). The carbon additions can be introduced when preparing
a primary or secondary melt, using virgin or revedrecycled materials of the gamma TiAl
intermetallic composition.
During investigations leading to the present invention, it was determined that,
in gamma TiAl intermetallic compositions containing 1 to 3 atomic percent chromium
and 1 to 5 atomic percent niobium, an inverse linear relationship exists between carbon
content and room temperature ductility within a narrow carbon content range of 160 to
500 ppm. Concomitantly, the creep resistance of such compositions was observed to
improve as the carbon content was increased over this range. On the basis of these
relationships, it was further determined that controlled additions of carbon can result in
improved creep resistance while maintaining adequate ductility to enable the design and
manufacturing of components from such compositions, for example, when cast and
processed to produce low pressure turbine blades of gas turbine engines.
During the investigations, alloys containing four different levels of carbon
were prepared: 160,270,420 and 500 ppm. The compositions were produced by melting
ingots of the aforementioned 48-2-2 alloy in an induction skull melter, adding the
controlled amounts of carbon to the melt, and then recasting the melt. Aside from their
carbon contents, the nominal chemistries of the TiAl intermetallic compositions were, in
atomic percent, about 48% aluminum, about 2% chromium, about 1.9% niobium, and the
balance titanium and incidental impurities. Each composition was heat treated, hot
isostatically pressed (HIPed), and tested for mechanical properties. The results of these
tests are plotted in graphs in FIG. 2. As seen in the creep plot, creep resistance was
observed to improve with carbon content, but room temperature and 1400°F (about
760°C) elongation decreased with carbon content. The crack growth threshold (Ktt,) at
800°F (about 425°C) was acceptable at all of the tested carbon levels. The latter property
is an important consideration for the gamma TiAl intermetallic composition of this
invention, since it is a primary parameter of concern for long-term reliability of LPT
blades and other components that are similarly subject to conditions that might promote
crack propagation.
Overall, the results of the investigation indicated that carbon contents within
the ranges tested should provide a high temperature capability exceeding 1500°F (about
815"C), and likely about 1600°F (about 870°C) or more. Because a minimum room
temperature ductility of 0.5% was determined to be a requirement for LPTB applications,
the results from the investigated range further indicated that a preferred maximum carbon
content for the gamma TiAl intermetallic composition of this invention is 470 ppm. In
particular, the specimen containing a carbon level of 500 ppm was concluded to exhibit
insufficient room temperature ductility to enable a gamma TiAl intermetallic composition
based on the 48-2-2 alloy to be readily processable as an LPT blade. Because a nominal
room temperature ductility of 1 .O% was identified as desired for LPTB applications, the
results of the investigation indicated that the tested carbon level of 270 ppm (0.027
atomic percent) provided a particularly desirable balance of properties. From this, it is
believed that a nominal carbon content of about 300 ppm (0.03 atomic percent) was
likely to provide an optimal balance between creep strength and room temperature
ductility.
Gamma TiAl intermetallic compositions of this invention can be processed
according to a procedure represented in FIG. 1. As a nonlimiting example, following the
production of a casting of the gamma TiAl intermetallic composition, a pre-HIP heat
treatment can be performed at a temperature within a range of about 1800 to about
2000°F (about 980 to about 1090°C) for a duration of about five to twelve hours.
Thereafter, the casting is cooled and transferred to a HIP chamber and then subjected to a
high pressure HIP step (for example, 25 ksi (about 1720 bar) or more) at about 2165°F
for a duration of about three hours. The HIPed casting is then cooled, removed from the
HIP chamber, and then subjected to a post-HIP solution treatment at a temperature of
about 2200°F for a duration of about two hours. While such a process is believed to be
acceptable, a more preferable process is believed to be disclosed in U.S. Patent
Application Serial No. 6 116 14,75 1 filed March 23,20 12, whose contents are incorporated
herein by reference. The preferred process is particularly adapted to yield castings
formed of gamma titanium aluminide intermetallic compositions that exhibit a desirable
duplex microstructure containing equiaxed and lamellar morphologies that promote the
ductility of the casting.
While the invention has been described in terms of particular embodiments, it
is apparent that other forms could be adopted by one skilled in the art. Therefore, the
scope of the invention is to be limited only by the following claims.

We Claim:
1. A titanium aluminide intermetallic composition based on a gamma
TiAl intermetallic compound, tine titanium aluminide intermetallic composition
consisting of titanium and aluminum in amounts to yield the gamma TiAl
intermetallic compound, chromium, niobium, carbon in an amount of 160 to 470
ppm, and incidental impurities.
2. The titanium aluminide intermetallic composition according to claim
1, wherein the titanium aluminide intermetallic composition contains about 46 to
50% aluminum.
3. The titanium aluminide intermetallic composition according to claim
1, wherein the titanium aluminide intermetallic composition contains about 160 to
420 ppm carbon.
4. The titanium aluminide intermetallic composition according to claim
1, wherein the titanium aluminide intermetallic composition contains about 270 to
420 ppm carbon.
5. The titanium aluminide intermetallic composition according to claim
1, wherein the titanium aluminide intermetallic composition contains about 300
ppm carbon.
6. The titanium aluminide intermetallic composition according to claim
1, wherein the titanium aluminide intermetallic composition is in the form of a
casting and has a duplex microstructure containing equiaxed and lamellar
morphologies.
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7. The titanium aluminide intermetallic composition according to claim
1, wlierein the titanium aluminide intermetallic composition exhibits a minimum
room temperature ductility of not lower than 0.5%.
8. The titanium aluminide intermetallic composition according to claim
1, wherein the titanium aluminide intermetallic composition exhibits a room
temperature ductility of at least 1%.
9. The titanium aluminide intermetallic composition according to claim
1, wherein the titanium aluminide intermetallic composition consists of, by atomic
percent, 1 to 3% chromium, 1 to 5% niobium, 160 to 470 ppm carbon, titanium
and aluminum in amounts to yield the gamma TiAl intermetallic compound, and
incidental impurities.
10. The titanium aluminide intermetallic composition according to
claim 9, wherein the titanium aluminide intermetallic composition contains about
46 to 50% aluminum.
11. The titanium aluminide intermetallic composition according to
claim 9, wherein the titanium aluminide intermetallic composition contains about
160 to 420 ppm carbon.
12. The titanium aluminide intermetallic composition according to
claim 9, wherein the titanium aluminide intermetallic composition contains about
270 to 420 ppm carbon.
13. The titanium aluminide intermetallic composition according to
claim 9, wherein the titanium aluminide intermetallic composition contains about
300 ppm carbon.
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14. The titanium aluminide intermetallic composition according to
claim 9, wherein the titanium aluminide intermetallic composition is in the form of
a casting and has a duplex microstructure containing equiaxed and lamellar
morphologies.
15. The titanium aluminide intermetallic composition according to
claim 9, wherein the titanium aluminide intermetallic composition exhibits a
minimum room temperature ductility of not lower than 0.5%.
16. The titanium aluminide intermetallic composition according to
claim 9, wherein the titanium aluminide intermetallic composition exhibits a room
temperature ductility of at least