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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 N2M0, a Pt2Mo superlattice; N3M0, as a DO22 phase
rather than the equilibrium orthorhombic phase; and/or
N4M0, the previously described binary equilibrium phase
which can exist as a metastable phase at certain NiMo
compositions.
2.2.1.1 N4M0
The N4M0 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:
Al' = 1/2 (3A1+A2); A2' = 1/2 (-A1+32); A3' = 3,
where Al, A2, and S3 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 N4M0 structure is to consider the ordering of Mo on