Evaluation of diffusion mechanisms in NiAl by embedded-atom and first-principles calculations

The energetics of Ni vacancy jumps in the intermetallic compound NiAl are studied by combining embedded-atom and first-principles calculations. The embedded-atom potential used in this work is fit to both experimental and first-principles data and provides an accurate description of point defect energies and vacancy jump barriers in NiAl. Some of the embedded-atom results reported here, are independently verified by plane-wave pseudopotential calculations. The results suggest that the atomic configuration produced by a nearest-neighbor jump of a Ni vacancy is mechanically unstable. Because of this instability, the vacancy implements two sequential nearest-neighbor jumps as one collective, two-atom transition. Such collective jumps initiate and complete six-jump vacancy cycles of a Ni vacancy, which are shown to occur by either four or three vacancy jumps. Next-nearest-neighbor vacancy jumps are shown to have diffusion rates comparable to experimental ones at the stoichiometric composition, suggesting that this is an important diffusion mechanism in NiAl.