Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

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alloys' superior thermal fatigue resistance gives them great potential for use in TPS;
thus, it would be appropriate to revisit this class of alloy after addressing its past
problems.
Many of the Ni-base alloys are fusion weldable, but the higher strength grades are
prone to cracking without special precautions such as preheating the work piece. Most
alloys capable of being processed into sheet also can be processed to create a fine-
grained structure. The fine-grained material can be superplastically formed, which is
potentially of very important benefit to making light-gauge, load-bearing structures and
TPS components. Finally, although almost all Ni-base alloys contain Cr and Al, both of
which improve oxidation resistance by forming a stable, protective scale, this scale
begins to lose its effectiveness above about 1,000 °Celsius. A number of effective
environmental coatings have been developed by the gas turbine industry for Ni-base
alloys. The only concern or unknown for application in a reusable 550 vehicle is the
durability of these coatings under very high mass flow conditions, such as during
reentry.
Refractory Metal Alloys
Refractory metal alloys were discussed briefly earlier in connection with the DynaSoar
project. Subsequent to this, there has been limited systematic interest in designing
aerospace structures that incorporate refractory metal alloys. There has been interest
in using these alloys for high"".temperature gas-cooled nuclear reactors, where the
operating environment is benign and well controlled, but this has limited relevance to
the current discussion of space vehicles required to withstand reentry into the earth's
atmosphere. The attraction of refractory metals as a class is the high melting
temperature of Mo, Nb, and Ta. Although alloys of these metallic elements exhibit
"normal" temperature dependence of mechanical strength, O.STm of Nb or Mo is still a
higher temperature than Ni-base alloys can withstand. Of the three metals, Nb is by far
the most attractive because of its lower density, lower elastic modulus, and better (but
not good) oxidation resistance. The clear barrier to use of Nb alloys is their reactivity in
air at high temperatures. There are coatings for these alloys, but at 1,250 °Celsius, a
breach in the coating will cause immediate, catastrophic failure.
A reasonable design practice for critical structure, such as the TPS, is to disallow use of
any coated material if the material fails catastrophically when the coating is breached.
If this practice were the norm, then refractory metal alloys would not be usable in the
TPS or other critical hot structure in a reusable SSC vehicle.
Ceramic Matrix Composites
Ceramic matrix composites (CMCs) are attractive because they are much tougher than,
but retain the high-temperature capability of, monolithic c:eramics. CM Cs' toughness is
derived from the fibers used to reinforce the ceramic matrix. When a CMC is loaded in
tension and the stress in the ceramic matrix reaches a critical value, microcracks
develop. This stress level is known as the matrix microcrack stress. In a monolithic
ceramic such as silicon carbide (SIC) or silicon nitride (ShN4), microcracks would
propagate rapidly, causing immediate failure. For this reason, monolithic ceramics are
not suitable for tension-loaded applications. This characteristic is mitigated in CMCs by
reinforcing the ceramic matrix with high-strength ceramic fibers, typically Al2.O3 or SiC,
whose role is to bridge and arrest the cracks. Under continued loading, the fibers also
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