Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

UNCLASSIFIED/ /FOR 9EEJCI41 I !SE ON! y
inception. In fact, Ni-base alloys are a prime enabler of the modern gas turbine engine.
As a result, this class of materials has a well-established industrial base for the
production of literally all product forms. There is a second class of Ni-base alloys that
has excellent resistance to attack in aggressive environments, such as those
encountered in petroleum recovery, but these alloys are not of interest for high-
temperature applications. To avoid any confusion, the term Ni-base alloys is used in
this document to mean high-temperature Ni-base alloys.
Ni-base alloys have a high elevated temperature strength because they typically
contain Al and Ti, which enable them to be precipitation strengthened. The alloys with
the highest temperature capability are strengthened by precipitates of Ni3AI (called y't
which have a positive temperature coefficient of strength up to aboL!t 1,000 °Celsius.
This unusual characteristic accounts for Ni-base alloys' elevated temperature capability.
In combination with the ordinary temperature dependence of the matrix strength, the
overall temperature dependence is quite low compared with that of any other class of
structural alloys. Ni-base alloys also contain alloying additions of refractory elements
such as tungsten (W), tantalum (Ta), and niobium (Nb). These additions also improve
the elevated temperature strength because they diffuse slowly and strengthen the
matrix. Because Ni-base alloys are used at elevated temperatures, they contain Cr
additions in significant concentrations to improve their oxidation resistance. The
tailoring of the properties of Ni-base alloys has been studied extensively and is quite
well understood empirically, albeit less so in a fundamental sense. Current-generation
Ni-base alloys contain as many as 10 alloying additions, and interelement interactions
make gaining a fundamental understanding of alloying effects a major challenge.
Production of Ni-base alloys has evolved over several decades. Both ingot metallurgy
and powder metallurgy methods are used to produce a full range of mill products. New
alloys have periodically been introduced in response to specialized needs. Sometimes
these alloy development efforts have been so focused on the \\specialized need" that
retention of other generally accepted Ni-base alloy properties has been overlooked. An
example of this is the development of early \'low coefficient of thermal expansion" (a)
alloys (the so-called low-a alloys). An example of an application for which low-a alloys
are attractive is structures in which there are two or more concentric rings connected
by radial struts. In the presence of a radial temperature gradient, these alloys tend to
crack during thermal cycling situations owing to "thermal fatigue." In principle, the
availability of an \\Invar type" alloy with structural characteristics reduces the thermal
strains and improves the thermal fatigue life. Invar is so named because it uses the
magnetic characteristics of Ni to offset normal thermal expansion, resulting in much
lower a values. It was recognized that this concept could also be adapted to Ni-base
structural alloys. The first-generation low-a alloy, Inco 903, had about half the
coefficient of thermal expansion compared with ordinary Ni-base alloys up to the Curie
temperature. This remarkable alteration of a physical property was achieved by
removing the Cr from the alloy to intensify the magnetic effects of magnetostriction. An
unfortunate, unintended consequence of this composition alteration was that the
oxidation resistance was seriously degraded. In particular, Inca 903 had a propensity
for intergranular cracking in air under stress, rendering it of limited utility as a
structural alloy. In fact, the cracking was so severe it was given a name: stress-
assisted grain boundary oxidation (SAGBO). A second-generation low-a alloy, Inco 909,
contained Si to help improve the oxidation resistance, but this alloy has not gained
widespread acceptance, possibly because Inco 903 problems were so severe. Low-a
17
UNCLASSIFIED/; FE5'\ 8PPlll1Ak W&& SHlb¥