AAWSAP DIRD, Materials for Advanced Aerospace Platforms, January 2010

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ensure that any outgassing that occurs in space would not liberate any of this toxic
material-at least not where it could be ingested by the crew.
Al Matrix Composites
Al matrix composites (AMCs) are typically made by mixing short fibers or even
particulate silicon carbide (SiC) with Al alloy powder and hot-pressing or mechanically
consolidating the mixture by extrusion or forging. AMCs have higher modulus and
strength but suffer ductility losses owing to the hard, nondeforming SiC second phase.
They also are much better in fatigue because the SiC particles or short fibers mitigate
the effects of planar slip in precipitation-hardening alloy matrices. Machining, fusion­
welding, or cold-forming AMCs is difficult. Mechanical fastening is possible, but drilling
fastener holes is difficult and expensive because the wear caused by the hard,
reinforcing phases shortens tool life.
Most of the AMCs produced to date have used relatively simple Al matrix alloys such as
6061 and 5083. This presumably is because of the conventional wisdom that AMCs are
not heat treatable and the perception that no benefit is gained by using more complex
alloy matrices. This is probably the case for a conventional solution treat-and-age
approach. Consequently, there has been little effort to optimize AMC systems. For the
right application where AMCs could provide a significant benefit, this could present an
opportunity. For example, using the high-temperature Al-Fe-Co-Ce alloy powder as the
matrix could prove interesting and might permit use of AMCs at up to 200 °Celsius-a
temperature at which Ti alloys do not provide any significant advantage, but one that is
too high for conventional Al alloys to be suitable.
In sum, while AMCs are not really a commercial materials system today, sufficient
research has been performed to establish proof of concept. If an adequate market for
AMCs were to emerge, the time and cost to make them commercially available could
prove acceptable.
Ti Alloys
Ti alloys also have been discussed earlier, but mainly in the context of heavy-section,
large-load-bearing applications for launch vehicles. Here, the potential of Ti alloys for
lighter gauge applications in the warm structure and the TPS is considered. Table 1
shows that at intermediate temperatures, Ti alloys are attractive in all aspects except
for specific stiffness. What this table does not capture is Ti alloys' propensity to react
with oxygen in the air to form an oxygen-stabilized a phase layer on the surface known
as a case. An example of a case is shown in Figure 3.
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