ω-phase formation in NiAl and Ni₂Al alloys

The phase stability of bcc-like NiAl and Ni₂Al with respect to phase transformations into the ω phase is examined by first-principles, full-potential, linear muffin-tin orbital method (FPLMTO), band-structure calculations. In agreement with experiment we find that bcc-like NiAl is stable, whereas Ni₂Al is unstable. We have also performed some calculations in slightly disordered phases, and find that the disordering tends to inhibit the transformation. This supports the suggestion that a combined displacive/ordering transformation mechanism is involved in the phase transformations of the Ni₂Al system. Calculated lattice parameters and the degree of internal shuffling of atoms for the ordered Ni₂Al phase were found to be in excellent agreement with the experiment.