First-principles study of stability of the bcc and ω phases of a low Al concentration Nb1 − xAlx alloy

The phase stability and site occupancy of bcc (body centered cubic) Nb5Al and slightly rearranged atomic structures have been examined by means of first-principles calculations. In order to use first-principles methods, a periodic cell is required and we used ordered Nb5Al compounds as a tractable example of a low Al concentration Nb1 − xAlx alloy (in this case, for about 17 at.% Al). The instability against an ω-structure atomic displacement was also studied, since this structure is detrimental to ductility. Mulliken population analysis was used to provide an understanding of the hybridization between the atoms and the electronic origin of the site occupancy and instability of the underlying bcc structures. By making calculations for several different configurations of the Nb–Al system we estimated the strengths of the Nb–Nb and Nb–Al bonds. It is shown that the stability of the underlying bcc phases is directly related to Nb–Nb and Nb–Al first-nearest-neighbor interactions. The firs