The effect of large vacancy concentration on intrinsic and interdiffusion coefficients: A first-principle study of B2-NiAl

Most treatments of interdiffusion in metallic alloys implicitly or explicitly rely on the assumption of a dilute vacancy concentration. Important intermetallic compounds such as B2-NiAl, however, are characterized by large vacancy concentrations. Here we investigate how large vacancy concentrations affect intrinsic and interdiffusion coefficients. We use B2-NiAl as an example and calculate all parameters that enter into a phenomenological description of interdiffusion from first principles, including thermodynamic factors, the effect of a concentration dependent volume, intrinsic diffusion and interdiffusion coefficients. We find that in Al-rich alloys, characterized by high vacancy concentrations, the thermodynamic factor of Al is larger than that of Ni by several orders of magnitude, while in Ni-rich alloys, where the vacancy concentration is very low, they are identical. Similarly, the effects of changing cell volume on intrinsic diffusion coefficients are very different for Al and Ni for the alloy compositions in which the vacancy concentration is high. Comparison of calculated interdiffusion coefficients with experimentally measured values show acceptable agreement in Ni-rich alloys but poor agreement in Al-rich alloys.