A systematic first-principles investigation was made of atomic hop mechanisms in B2–NiAl and a low barrier collective hop was discovered that could mediate Al diffusion through the anti-structural bridge mechanism. An alternative hop sequence was also found for the migration of a triple defect which differed from that proposed previously. To shed light on the dominant hop mechanisms that mediated diffusion in B2–NiAl, point defects and defect clusters in B2–NiAl at high temperature were studied by combining a cluster expansion with Monte Carlo simulations. Going beyond the mean-field approximation, it was found that the inclusion of interactions among the various point defects was crucial in predicting the concentration of defect complexes, such as the triple defects of B2–NiAl. Interactions among point defects also introduce an important degree of short-range order between Al antisite defects and Ni antisite defects. An increasing probability between pairs of Al antisite atoms and Ni vacancies was found as the alloy concentration of B2–NiAl became both Al rich and Ni rich, suggesting that the anti-structural bridge mechanism should play an important role in facilitating Al transport.

First-Principles Investigation of Migration Barriers and Point Defect Complexes in B2–NiAl. Q.Xu, A.Van der Ven: Intermetallics, 2009, 17[5], 319-29