36
originally, Clapp and Moss (1966, 1968a, 1968b). His
findings show that the transformation from the SRO ordered
state to the long range ordered one is as follows.
Initially the Pt2Mo phase forms in all the ternary
alloys. In the A1 containing alloy, the Pt2Mo
precipitates concurrently with DC>22* In the W containing
alloy, DO22 is not stabilized. All three NixMo metastable
phases can co-exist in the A1 bearing ternary, but only
N2M0 and N4M0 in the Ta and W containing one. The final
ordered state in all three cases is the equilibrium
orthorhombic N3M0 phase, which heterogeneously nucleates
at grain boundaries and subsequently consumes the body of
the grain. The presence of Ta greatly accelerates the
kinetics of the formation of this final, equilibrium
phase.
2.2.6 Ni-Al N3AI
Nickel-Aluminum alloys that would be candidates for
superalloy applications are generally two phase alloys
consisting of gamma phase, the disordered FCC matrix, and
gamma prime phase, an LI2 superlattice of the FCC matrix
corresponding to the approximate stoichiometric
composition N3AI. The crystal structure of this compound
is shown in Figure 2.12.
When small volume fractions of gamma prime are
precipitated from gamma solid solution, that is, when the
alloy is relatively lean in A1, the gamma prime will first