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 Al containing alloy, the Pt2Mo

precipitates concurrently with D022. In the W containing

alloy, D022 is not stabilized. All three NixMo metastable

phases can co-exist in the Al bearing ternary, but only

Ni2Mo and Ni4Mo in the Ta and W containing one. The final

ordered state in all three cases is the equilibrium

orthorhombic Ni3Mo 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 Ni3Al


 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 L12 superlattice of the FCC matrix

corresponding to the approximate stoichiometric

composition Ni3Al. 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 Al, the gamma prime will first


I