omega-phase formation in NiAl and Ni2Al alloys

The phase stability of bcc-like NiAl and Ni2Al with respect to phase transf ormations into the omega phase is examined by first-principles, full-potent ial, linear muffin-tin orbital method (FPLMTO), band-structure calculations . In agreement with experiment we find that bcc-like NiAl is stable, wherea s Ni2Al is unstable. We have also performed some calculations in slightly d isordered phases, and find that the disordering tends to inhibit the transf ormation. This supports the suggestion that a combined displacive/ordering transformation mechanism is involved in the phase transformations of the Ni 2Al system. Calculated lattice parameters and the degree of internal shuffl ing of atoms for the ordered Ni2Al phase were found to be in excellent agre ement with the experiment.