First Principles Theory of Alloy Phase Stability: Ordering and Pre-Martensitic Phenomena in β-Phase NiAl

We review, briefly, the first principles KKR-CPA theory of the electronic structure and energetics of alloys in which compositional disorder plays a role. We also review the first principles theories of ordering and alloy phase stability that are built on the KKR-CPA description of the disordered state. We point to a number of underlying electronic driving mechanisms of ordering and clustering that have been uncovered. Specifically, we emphasize the important role that Fermi surface nesting plays in driving specific instabilities. Using a newly developed method for treating the effects of disorder in alloys that have complex lattices we present results for the ordering energies of beta phase Ni_cAl_(1-c) alloys. Finally, we show alloy “Fermi surfaces” in β-phase Ni_0.625Al_0.375 that support the notion that the pre-martensitic phenomena observed in this alloy are Fermi surface driven.