AAWSAP DIRD, Materials for Advanced Aerospace Platforms, January 2010

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in materials with small annual sales volume, so even licensing the new material to one
of these companies may not be commercially feasible .
This foregoing discussion highlights a commonly encountered inconsistency between
technical innovation and commercial progress. Until a production-scale source exists,
pricing of new materials is at best highly uncertain and potentially unstable over time.
This is in part due to the uncertainty surrounding demand and the associated volume of
material that will be required. Taken together, these factors act as a clear deterrent to
the adoption of new materials of literally all classes (polymers, metals, and ceramics).
For heavily loaded structures or structures that will experience temperatures higher
than about 200 °Celsius, Ti alloys are the preferred material class. Ti alloys are about
half as dense as steel or Ni-base alloys and possess a density-corrected strength and
stiffness competitive with that of other metallic materials. Numerous grades of Ti alloys
are in use today, and a strong domestic industrial base of suppliers exists for nearly all
these grades. Ti alloys for structural applications can be divided into three groups based
on their metallurgy: near a alloys, a+p alloys, and metastable p alloys (commonly called
p alloys for short). Both the aircraft and propulsion original equipment manufacturers
and several private engineering firms have extensive design experience with Ti alloys.
Although most of this experience is with Ti-6Al-4V (Ti-6-4 ), other alloys also are widely
used, particularly in jet engines and liquid-fueled rocket engines. The alloy most likely
to be used in a launch vehicle, Ti-6-4, has been available and in use for more than 40
years but is still highly competitive with newer grades in large part because of its
versatility. Potential applications of other alloys that have particularly attractive
characteristics, such as a higher temperature capability, are discussed in the Reusable
Reentry Vehicle and Propulsion Systems sections of this document.
The most commonly used structural grade of Ti alloy today, Ti-6-4, can be readily
fusion welded, formed both by forging and as a sheet product, and conventionally
machined, although each of these operations requires special precautions. In addition,
the feasibility of friction stir welding has been demonstrated. Ti-6-4 also can be
superplastically formed and diffusion bonded, enabling synthesis of innovatively shaped
components. For launch vehicles, the most likely application for Ti alloys is in the
structure that carries the reactions from the propulsion system to the vehicle itself.
These applications typically involve heavy sections to accommodate the large loads,
and the main property requirements are high strength, fatigue resistance, and fracture
toughness. Ti alloys have been used in both military and commercial aircralt. Heavily
loaded components in service today include the wing box of the B-1B bomber (Ti-6-4),
the landing gear beam in the B-747 (Ti-6-4), and the landing gear truck beam in the B­
777 (higher strength Ti-10V-2Fe-3AI [Ti-10-2-3]). The choice of Ti-10-2-3 for the
landing gear truck beam reflects the time-based maturation of the newer p-Ti alloys,
such as Ti-10-2-3. These alloys have the advantage of being "deep hardenable"­
compared with Ti-6-4, they can develop full strength in thicker sections during heat
treatment. For example, that the B-777's truck beam is up to 6 inches thick in some
locations factored significantly in the choice of Ti-10-2-3. Because Ti alloys are about
half as dense as steel, they are very competitive on a density-corrected basis. However,
mass is not the only driver for some applications; the volume of a component also must
be compatible with the space available for it. This factor also is a consideration in the
choice ofTi-10-2-3 for the B-777 truck beam. And it also applies to an application such
as landing gear, because it is retracted into the fuselage for aerodynamic reasons.
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