single phase solid solution, and subsequently heat treated

below the solvus temperature for that particular

as-quenched composition, intermediate metastable phases

may precipitate instead of the equilibrium phases

predicted by the equilibrium phase diagram. These phases

are Ni2Mo, a Pt2Mo superlattice; Ni3Mo, as a D022 phase

rather than the equilibrium orthorhombic phase; and/or

Ni4Mo, the previously described binary equilibrium phase

which can exist as a metastable phase at certain Ni-Mo

compositions.

2.2.1.1 Ni4Mo


 The Ni4Mo phase is a BCT structure derived from the

disordered FCC lattice. Its lattice parameters as a BCT

cell are a' = b' = 5.727 Angstroms, c' = 3.566 Angstroms.

The BCT unit cell can be derived from the FCC parent by

using the following transformation:

 XA' = 1/2 (311+A2); A2' = 1/2 (-A1+312); A3' = A3,


where Al, A2, and A3 are the lattice vectors for FCC and

Al', A2', and A3' are the vectors from the BCT unit cell

(Mishra, 1979). The c axis of the Dla undergoes a

contraction of 1.2% during the transformation from

disordered FCC to ordered Dla. Hence, A3' is only

approximately equal to A3. A convenient way to visualize

the Ni4Mo structure is to consider the ordering of Mo on