CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No.
611614,751, filed March 23, 2012, the contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
The present invention generally relates to compositions containing titanium
and aluminum and the processing thereof. More particularly, this invention relates to
methods of processing cast titanium aluminide intermetallic compositions that entail hot
isostatic pressing and heat treatment to close porosity and yield a desirable
microstructure.
Because weight and high temperature strength are primary considerations in
gas turbine engine design, there is a continuing effort to create relatively light weight
alloyslcompositions 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 titaniumbased
alloys has increased through the use of titanium intermetallic systems based on the
titanium aluminide compounds Ti3A1 (alpha-2 (a-2) alloys) and TiAl (gamma (y) alloys).
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. Additions of chromium and niobium are known to promote certain
properties of TiAl intermetallics, such as oxidation resistance, ductility, strength, etc. As
a nonlimiting example, U.S. Patent No. 4,879,092 to Huang discloses a titanium
aluminide intermetallic composition having an approximate formula of
Ti46-50A46-~0Cr~Nbozr ,n ominally about Ti-48A1-2Cr-2Nb. This alloy, referred to herein
as the 48-2-2 alloy, is considered to have a nominal temperature capability of up to about
1400°F (about 760°C), with useful but diminishing capabilities up to about 1500°F
(about 815°C). In gas turbine engines used in commercial aircraft, the 48-2-2 alloy is
well suited for low pressure turbine blade (LPTB) applications.
The production of components fiom TiAl intermetallics is complicated by
their relatively low ductility and the typical desire for these compositions to be
extrudable, forgeable, rollable andlor castable. Hot isostatic pressing (HIP) is commonly
performed to eliminate internal voids and microporosity in titanium aluminide
intermetallic castings. Because uncontrolled cooling rates typically performed following
HIP are not effective to generate a desired microstructure, responsiveness to post-HIP
heat treatments is another desirable characteristic in order to achieve microstructures and
mechanical properties needed for specific applications.
HIP cycles are typically separate fiom the heat treatment cycle in the
processing of castings. As an example, desired microstructures and mechanical
properties have been obtained in castings of the 48-2-2 alloy using a process represented
in FIG. 3. Following the production of the casting, a pre-HIP heat treatment is performed
' at a temperature within a range of about 1800 to about 2000°F (about 980 to about
1090°C) and 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 2 165°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. This sequence requires the use of at least two different
vessels and loading and unloading the casting three times from these vessels. In addition
to incurring additional cost and cycle time, this process has been associated with the loss
of aluminum from the casting surface, which leads to reduced environmental and/or
mechanical properties.
Unexpectedly, net-shape castings that have been produced, for example, by
spin casting from the 48-2-2 alloy to produce low pressure turbine blades have not
responded well to the heat treatment process described above, or to other processes
employed with conventional TiAl castings, such as gravity~casting and overstock casting.
In particular, the 48-2-2 alloy net-shape castings processed by net-shape casting methods
do not develop a desirable duplex microstructure containing equiaxed and lamellar
gamma TiAl morphologies that improve the ductility of the casting, particularly when the
volume fraction of the lamellar structure is about 10 to about 90 percent, particularly if
the volume fraction of the lamellar structure is about 20 to about 80 percent and ideally .
about 30 to about 70 percent. FIGS. 1 and 2 are photomicrographs showing desirable
duplex microstructures present in two conventional TiAl castings.
In view of the above, a method is needed that is capable of processing TiAl
intermetallics, including but not limited to net-shape geometries of the 48-2-2 alloy, to
yield a duplex microstructure containing equiaxed and lamellar morphologies. It would
be further desirable if such a method did not require a sequence in which a casting is not
required to be transferred between multiple different vessels.
BRIEF DESCFUPTION OF THE INVENTION
The present invention provides methods capable of processing compositions
containing titanium and aluminum, and especially titanium aluminide intermetallic
compositions (TiAl intermetallics) based on the TiAl (gamma) intermetallic compound,
to yield desirable microstructures. The methods have the further capability of being
performed in a single vessel, resulting in a less complicated process than conventional
methods used to produce compositions that require void closure (for example, by HIPing)
and heat treatment.
According to a first aspect of the invention, a method of processing a titanium
aluminide intermetallic composition includes hot isostatic pressing the composition at a
temperature of at least 1260°C (about 2300°F), cooling the composition to a temperature
of not less than 1120°C (about 2050°F), heat treating the composition at a temperature of
about 1150 to about 1200°C (about 2100 to about 2200°F), and then cooling the
composition to room temperature. Following the above procedure, the titanium
aluminide intermetallic composition exhibits a desirable duplex microstructure containing
equiaxed and lamellar morphologies of the gamma TiAl phase.
According to a second aspect of the invention, an alternative method of
processing a titanium aluminide intermetallic composition includes hot isostatic pressing
the titanium aluminide intermetallic composition, cooling the composition, heat treating
the composition at a temperature of at least 1260°C (about 2300°F) for about 2.5 to about
5 hours, cooling the composition to a temperature of not less than 1120°C (about
2050°F), holding the composition at a hold temperature of about 1150 to about 1200°C
(about 2100 to about 2200°F) for a duration of about two to about six hours, and then
cooling the composition to room temperature. Following this procedure, the titanium
aluminide intermetallic composition exhibits a desirable duplex microstructure containing
equiaxed and lamellar morphologies of the gamma TiAl phase.
A technical effect of the invention is the ability to produce desirable duplex
microstructures in TiAl intermetallics that may otherwise be difficult to obtain,
particularly if produced by net-shape casting methods such as spin casting and possibly
certain other casting techniques. Another technical effect is the ability to take advantage
of the energy available for phase equilibration during cool down from a HIP step to assist
in a subsequent heat treatment, which has been determined to eliminate the requirement
for conventional pre- and post-heat treatment cycles that may cause aluminum to be lost
from the casting surface as well as incur additional cost and cycle time. These
advantages have been particularly observed with net-shape castings produced by netshape
casting methods, such as spin casting, in the aforementioned 48-2-2 alloy, though
other TiAl intermetallic compositions also benefit from the processing methods provided
by the present invention.
Other aspects and advantages of this invention will be better appreciated from
the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 and 2 are photomicrographs showing the microstructures of two
castings formed of a TiAl intermetallic composition with a desirable duplex
microstructure.
FIG. 3 is a flow chart representing a method of processing castings formed of
TiAl intermetallic compositions in accordance with a prior art HIP and heat treatment
process.
FIGS. 4 and 5 are flow charts representing two methods of processing castings
formed of TiAl intermetallic compositions in accordance with embodiments of the
present invention.
FIGS. 6 and 7 are microphotographs showing the microstructures of two
castings formed of the same TiAl intermetallic composition, wherein the casting of FIG.
6 was processed in accordance with the prior art HIP and heat treatment process of FIG. 3
and the casting of FIG. 7 was processed in accordance with the HIP and heat treatment
process of FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 4 and 5 contain flow charts that represent two related methods by which
TiAl intermetallic compositions, including but not limited to the 48-2-2 alloy, can be
processed to yield a desirable duplex microstructure, with the additional benefit of
avoiding the disadvantages of the prior art process summarized in FIG. 3. In particular,
the methods of FIGS. 4 and 5 avoid the pre- and post-HIP vacuum heat treatments that
are believed to promote the loss of aluminum in TiAl intermetallic compositions. The
invention also takes advantage of the high gas pressures and protective (inert)
atmospheres used during HIP, the combination of which is believed to be capable of
reducing the loss of aluminum in a TiAl intermetallic composition. Furthermore, each of
the methods summarized in FIGS. 4 and 5 provide for interrupted cooling fiom a HIP
step (FIG. 4) or a temperature that is believed to take advantage of the non-equilibrium
phase distribution in TiAl intermetallic compositions following HIP (FIG. 5) to generate
(during a subsequent heat treatment) microstructures that are capable of providing
desirable mechanical properties, especially if the TiAl intermetallic composition is a cast
using a net-shape casting process, such as spin casting or other means.
As noted above, the processes summarized in FIGS. 4 and 5 are believed to be
particularly beneficial to the 48-2-2 alloy, whose composition is based on the gamma
(TiA1) intermetallic compound. Castings of the 48-2-2 alloy exhibit improved ductility
and other desirable properties if they contain a duplex microstructure containing equiaxed
and lamellar gamma phase morphologies. FIGS. 6 and 7 are representative of LPTB
castings produced from the 48-2-2 alloy. Both castings were produced by spin casting,
the casting in FIG. 6 was processed by a HIP and heat treatment procedure corresponding
to that represented in FIG. 3, and the casting in FIG. 7 was processed by a modified HIP
and heat treatment procedure corresponding to that represented in FIG. 4. The
microstructure of the heat treated casting shown in FIG. 6 possesses an excessive amount
of equiaxed gamma phase and an inadequate amount of the lamellar phase (less than 10%
volume fraction of the lamellar phase). Such a microstructure would yield a component
with insufficiently high temperature creep strength. The microstructure of the heat
treated casting shown in FIG. 7 has acceptable amounts of the equiaxed gamma phase
and the lamellar phase (about 20% volume fraction of the lamellar phase), the sole
exception being at the outermost surface of the casting where titanium levels are
depleted. However, the outermost surface can be removed by conventional techniques,
such as abrasive blasting or chemical milling, with the result that the entire remaining
casting would contain acceptable amounts of the equiaxed gamma phase and lamellar
phase.
While the invention has been shown to yield particularly advantageous results
with the 48-2-2 alloy, the invention is believed to be more generally applicable to
titanium aluminide intermetallic compositions, particularly TiAl (gamma) intermetallic
compositions modified with elements that are intended to promote various properties.
For example, the invention has also been shown to be effective with TiAl intermetallic
compositions that contain tantalum. Particular compositions that have been successfully
evaluated include TiAl compositions that contain chromium, niobium and/or tantalum,
for example, about 1.8 to about 2 atomic percent chromium, up to about 2 atomic percent
niobium, and up to about 4 atomic percent tantalum. Specific compositions that were
successfully evaluated contained, in atomic percent: about 47.3% aluminum, about 1.9%
chromium, about 1.9% niobium and the balance titanium and incidental impurities
(roughly corresponding to the 48-2-2 alloy); or about 47.3% aluminum, about 1.8%
chromium, about 0.85% niobium, about 1.7% tantalum and the balance titanium and
incidental impurities; or about 47.3% aluminum, about 2.0% chromium, about 4.0%
tantalum and the balance titanium and incidental impurities. More generally, the levels
of titanium and aluminum in these TiAl intermetallic compositions are selected to yield a
casting whose predominant constituent is the TiAl (gamma) intermetallic compound.
While the compositions evaluated all contained about 47.3 atomic percent aluminum and
about 46.7 to 48.9 atomic percent titanium, those skilled in the art will appreciate that
aluminum and titanium levels beyond these amounts can be used to yield a casting that is
entirely or predominantly the TiAl intermetallic compound, and such variations are
within the scope of the invention. Furthermore, those skilled in the art will recognize that
other alloy constituents could be included to modifL the properties of the TiAl
intermetallic compound, and such variations are also within the scope of the invention.
During investigations leading to the present invention, solidification modeling
was conducted that suggested that areas of low pressure turbine blade (LPTB) castings
formed by net-shape casting, including spin casting, solidified in less than a few seconds.
It was concluded that, compared to other casting methods andfor other types of castings,
such a rapid solidification rate may modify the route through the Ti-A1 phase diagram
that the alloy/composition takes during solidification and may lead to unexpected
responses to conventional heat treatments that are subsequently performed on the
castings. These unexpected results negatively impact the uniformity of the
microstructure of net-shape cast and heat treated components, such as the chemistry and
uniformity of the microstructure over the full chord and span in net-shape TiAl airfoils.
The process represented in FIG. 4 combines a HIP cycle with a heat treatment without
cooling to room temperature therebetween, which reestablishes phase equilbria that are
capable of developing a duplex microstructure that provides desirable mechanical
properties.
The process of FIG. 4 generally entails preparing a TiAl intermetallic
composition. A preferred but not limiting example entails spin casting an appropriate
melt containing the desired constituents of the TiAl intermetallic composition. The
composition (casting) is then loaded in a suitable HIP chamber and heated in a protective
atmosphere (for example, argon or another inert gas) to a temperature at which the
casting is to undergo HIPing. According to a preferred aspect of the invention, the HIP
temperature ( T H ~ Iis) a t least 2300°F (about 1260°C), more preferably at least 2350°F
(about 1290°C), and most preferably in a range of about 2375 to about 2425°F (about
1300 to about 1330°C). The pressure applied to the casting during the HIP cycle is
intended to eliminate internal voids and microporosity in the castings. For this purpose,
pressures of at least 15 ksi (about 1030 bar) are believed to be sufficient, with pressures
of about 18 ksi (about 1240 bar) and higher believed to be particularly preferred. The
duration of the HIP cycle may vary depending on the particular composition and pressure
used, but suitable results are believed to be obtained with HIP cycles having durations of
about 2.5 to about 5 hours, and particularly about 2.5 to about 3.5 hours.
Following the HIP cycle, the casting is cooled to a temperature of not less
than 2050°F (about 1 120°C), more preferably not less than 2100°F (about 1 150°C), and
most preferably about 2100 to about 2150°F (about 1150 to about 1175°C). The cooling
rate may vary, but rates of about 5 to about 20°F/minute (about 3 to about 11 "Clminute)
have been found to be acceptable. Without needing to be removed from the HIP
chamber, the casting then undergoes a heat treatment at a temperature of about 2100 to
about 2200°F (about 1 150 to about 1200°C), for example, about 2100 to about 21 50°F
(about 1150 to about 1175°C). The duration of this heat treatment may vary depending
on the particular composition and HIP treatment used, but suitable results are believe to
be obtained with heat treatment cycles having durations of about two to about six hours,
and especially about 4.5 to about 5.5 hours.
Following heat treatment, the casting can be cooled directly to room
temperature (about 20 to about 25°C) at any desired rate. At the result of this process, the
TiAl intermetallic casting preferably exhibits a duplex microstructure of the type seen in
FIG. 7. From the above, it should be evident that the casting is not required to be
removed from the HIP chamber during the steps identified in FIG. 4, and that the casting
can be continuously exposed to the inert atmosphere of the HIP chamber throughout the
process represented in FIG. 4.
The process set forth in FIG. 5 differs from that set forth in FIG. 4 by the
allowance of a full cool down (to room temperature) between the HIP cycle and the heat
treatment. The process of FIG. 5 additionally involves heating the casting to the THIP~
temperature prior to the heat treatment. This process is believed to allow more flexibility
in the temperature used for the HIP cycle, in that HIPing is not required to be performed
at the TH1pl temperature of FIG. 4, but instead can be at a temperature (designated as
THIPZth) at can be higher or lower than the temperatures within the ranges stated above for
THIP.I
In view of the above, the process set forth in FIG. 5 generally entails HIPing a
TiAl intermetallic composition (typically a casting) at a suitable temperature (THIp2),
which can be followed by cooling the casting to essentially any temperature (including
room temperature). Thereafter, the casting is heat treated at the THIPlte mperature (for
example, at least 2300°F (about 1260°C)) for a duration sufficient to ensure the entire
casting is at THIPl. The casting can then be cooled at a suitable rate (for example, about 5
to about 20°F/minute (about 3 to about 1 1°C/minute)) to a temperature of not less than
2050°F (about 1120°C), more preferably not less than 2100°F (about 1 150°C), and most
preferably about 2100 to about 21 50°F (about 1 150 to about 1 175°C). The casting can
then be subjected to the same heat treatment as described for the process of FIG. 4, after
which the casting can be cooled directly to room temperature (about 20 to about 25°C).
As the result of this process, the TiAl intermetallic casting preferably exhibits a duplex
microstructure of the type seen in FIG. 7. As with the process of FIG. 4, it should be
evident that the casting is not required to be removed from the HIP chamber for any step
of FIG. 5, and that the casting can be continuously exposed to the inert atmosphere of the
HIP chamber throughout the process represented in FIG. 5.
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.

1. A method of processing a titanium aluminide intermetallic composition
based on a TiAl intermetallic compound to yield a duplex microstructure containing
equiaxed and lamellar morphologies of the gamma TiAl phase, the method comprising:
hot isostatic pressing the titanium aluminide intermetallic composition at a
temperature of at least 1 260°C;
cooling the titanium aluminide intermetallic composition to a temperature of
not less than 1 120°C;
heat treating the titanium aluminide intermetallic composition at a temperature
of about 11 50 to about 1200°C; and then
cooling the titanium aluminide intermetallic composition to room temperature;
wherein the titanium aluminide intermetallic composition exhibits the duplex
microstructure following the step of cooling the titanium aluminide intermetallic
composition to room temperature.
2. The method according to claim 1, wherein the hot isostatic pressing step is
conducted at a pressure of at least 1030 bar.
3. The method according to claim 1, wherein the hot isostatic pressing step is
conducted at a pressure of at least 1240 bar.
4. The method according to claim 1, wherein the hot isostatic pressing step is
conducted at a temperature of at least 1290°C.
5. The method according to claim 1, wherein the hot isostatic pressing step is
conducted at a temperature of about 1300 to about 1330°C.
6. The method according to claim 1, wherein the hot isostatic pressing step is
conducted for a duration of about 2.5 to about 5 hours.
7. The method according to claim 1, wherein the titanium aluminide
intermetallic composition is cooled to a temperature of not less than 1 150°C during the
cooling step.
8. The method according to claim 1, wherein the titanium aluminide
intermetallic composition is cooled to a temperature of 1150 to about 1175°C during the
cooling step.
9. The method according to claim 1, wherein the heat treatment step is
performed at a temperature of about 11 50 to about 1175°C.
10. The method according to claim 1, wherein the heat treatment step is
performed for a duration of about two to about six hours.
11. The method according to claim 1, wherein the titanium aluminide
intermetallic composition consists of titanium and aluminum in amounts to yield the TiAl
intermetallic compound, one or more of chromium, niobium and tantalum, and incidental
impurities.
12. The method according to claim 1, wherein the titanium aluminide
intermetallic composition consists of, by atomic percent, about 1.8 to about 2%
chromium, up to about 2% niobium, up to about 4% tantalum, titanium and aluminum in
amounts to yield the TiAl intermetallic compound, and incidental impurities.
13. The method according to claim 12, wherein the titanium aluminide
intermetallic composition contains about 46.7 to 48.9 atomic percent titanium.
14. The method according to claim 12, wherein the titanium aluminide
intermetallic composition contains about 47.3 atomic percent aluminum.
15. The method according to claim 12, wherein the titanium aluminide
intermetallic composition contains, in atomic percent, about 1.9% chromium, about 1.9
atomic percent niobium, and no intentional amount of tantalum.
16. The method according to claim 12, wherein the titanium aluminide
intermetallic composition contains, in atomic percent, about 1.8% chromium, about 0.85
atomic percent niobium, and about 1.7% tantalum.
17. The method according to claim 12, wherein the titanium aluminide
intermetallic composition contains, in atomic percent, about 2% chromium, about 4%
tantalum, and no intentional amount of niobium.
18. A method of processing a titanium aluminide intermetallic composition
based on a TiAl intermetallic compound to yield a duplex microstructure containing
equiaxed and lamellar morphologies of the gamma TiAl phase, the method comprising:
hot isostatic pressing the titanium aluminide intermetallic composition;
cooling the titanium aluminide intermetallic composition;
heat treating the titanium aluminide intermetallic composition at a temperature
of at least 1260°C for about 2.5 to about 5 hours;
cooling the titanium aluminide intermetallic composition to a temperature of
not less than 1120°C;
holding the titanium aluminide intermetallic composition at a hold
temperature of about 1150 to about 1200°C for a duration of about two to about six
hours; and then
cooling the titanium aluminide intermetallic composition to room temperature;
wherein the titanium aluminide intermetallic composition exhibits the duplex
microstructure following the step of cooling the titanium aluminide intermetallic
composition to room temperature.
19. The method according to claim 18, wherein the titanium aluminide
intermetallic composition is cooled after the heat treating step to a temperature of not less
than 1150°C prior to the holding step, and the hold temperature is 1150 to about 1200°C.
20. The method according to claim 18, wherein the titanium aluminide
intermetallic composition consists of titanium and aluminum in amounts to yield the TiAl
intermetallic compound, one or more of chromium, niobium and tantalum, and incidental
impurities.