Search

We predict the existence of an isostructural first-order phase transition in high-pressure Al-rich NiAl alloys, between vacancy-rich and antisite-rich phases. A critical point terminates the region of phase coexistence. The driving force for this phase transition is the positive defect mixing enthalpy originating from elastic interaction between vacancies and antisites on the Ni sublattice. We highlight the similarity between the critical behavior of Al-rich alloys and that of binary liquid mixtures. Analogous isostructural phase transitions are expected in other compounds with constitutional defects.

Selective oxidation of the surface of an ordered alloy requires redistribution of the atomic species in the vicinity of the surface. This process can be understood in terms of the formation and movements of point defects in the compound. On the basis of ab initio density-functional calculation we found both the creation of exchange defects near the NiAl surface and segregation of Ni vacancies to the top layer to be extremely favorable in the presence of oxygen. Scenarios for the initial oxidation of NiAl are suggested which demonstrate the appearance of an additional energy barrier on the Ni-rich side compared to the Al-rich side. The expulsion of Ni from the oxide layer as it forms is the driving force for its stability.