Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

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Therefore, instabilities that cause time-dependent reductions in toughness must be
understood before these materials can be considered candidates for future TPS
applications.
Titanium Aluminides
Titanium aluminides are intermetallic compounds that form between Ti and Al. There
are three such compounds-TbAI, TiAI and AhTi-but the one of principal interest is
TiAI, often referred to as gamma titanium aluminide or y-TiAI. This compound contains
about 35 weight% Al, but ternary and quaternary alloys based on TiAI typically contain
somewhat less than this amount. y-TiAI is interesting for intermediate-temperature
applications for several reasons. These include a lower density, at least a 125 °Celsius
higher temperature capability, and better surface stability (oxidation resistance)
compared with conventional Ti alloys. The surface scale that forms during oxidation of
y-TiAI is Al rich and is more protective than the TiO2 scale that forms on conventional Ti
alloys. Like most other intermetallic compounds, y-TiAI has very limited ductility at
temperatures up to about 600 °Celsius but is not brittle in the classical sense; that is, it
has the capability to deform plastically before fracturing. The ductility increases as
temperatures increase and is quite good at normal service temperatures.
y-TiAI alloys' limited ductility poses manufacturing problems for any wrought products.
These issues are manageable but reduce product yields, which In turn impacts cost.
Recent work in Austria has demonstrated that some y-TiAI alloys can be made into
sheet. This is particularly relevant to applications such as metallic TPS. Other work has
demonstrated that some y-TiAI alloys can be conventionally forged, albeit with care.
Should one or more of the efforts currently under way to produce low-cost, prealloyed
Ti powder prove successful, the availability of affordable y-TiAI alloy powder could be a
major cost breakthrough.
The development and maturation of y-TiAI alloys has taken more than 30 years. This is
in part due to the lack of actual production applications because of designer's concerns
regarding the limited low-temperature ductility of all the y-TiAI alloys. Today, finally,
there are several applications of alloys based on y-TiAI, the most significant of which are
two stages of low-pressure turbine blades in the engine General Electric is providing for
the Boeing 787 and the growth 747 (called the 747-8). These blades are cast to near
net shape and will operate at temperatures up to about 750 °Celsius. Another
application is a cast -,-TiAI turbocharger rotor Mitsubishi uses in a production auto
because it has half the mass of an equivalent Ni-base alloy component.
A potentially ideal, high-volume application of y-TiAI alloys is exhaust valves for autos.
If the low-cost powder mentioned earlier becomes a reality, this factor, coupled with
the current pressure to improve fuel consumption in American cars, could make this
prospect a reality. The point of this short discussion of non-space-related applications of
y-TiAI alloys is that realization of any or all of these applications will create a stronger
production base for this class of alloy, which in turn will make their introduction into
future space platforms easier and more cost effective.
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