4
disordered state to the ordered state, typically a
transition through the short range ordered state. Very
elegant theories dealing with the nucleation of these
phases have been developed.
These metastable phases can be found in alloys of the
ternary Ni-Mo-Al system, and also in the Pratt and Whitney
quaternary alloys (Aigeltinger, Kersker, and Hren, 1979).
They are not present at the high temperatures normally
encountered in the turbine section of an engine, at
temperatures in the range of 1100 C, and therefore do not
contribute to high temperature strength in such Ni based
ternary and quaternary alloys. Nevertheless, certain
aspects of their microstructures suggest that they might
still contribute to alloy strength at lower than normal
turbine operating temperatures.
The metastable phases investigated here are of the
type NixMo, where x can be 2, 3 or 4. They are coherent
with the FCC matrix which is a Ni rich solution in the
above superalloys. They may be present at temperatures of
700 degrees C and lower for very long times (Martin,
1982), and may delay the precipitation of the equilibrium
phases that would be predicted from the equilibrium phase
diagram.
Though there are similarities in the precipitation of
these NixMo metastable phases among the binary, ternary,
and quaternary alloys described earlier, there are