Ni Tracer Self-Diffusion, Interdiffusion and Diffusion Mechanisms in NiAl

摘要

Recently Ni tracer self-diffusion was extensively measured in extended temperature and composition intervals of the B2 phase NiAl (Frank, Divinski, Sodervall, and Herzig, 2001). While the Ni tracer diffusivity D_(Ni)~* increases notably on the Ni-rich side of the composition in NiAl with increasing Ni content, D_(Ni)~* is practically not changed with increasing Al content on the Al-rich side, in spite of the large amount of structural Ni vacancies. When performing atomistic calculations of different diffusion mechanisms in NiAl, these tracer data were well explained by the dominant contribution via the triple defect diffusion mechanism. Different from the tracer diffusion data, the interdiffusion data D-tilde in NiAl available in the literature demonstrate a deep minimum at about the Ni_(50)Al_(50) composition with a substantial increase of D-tilde on both sides of the stoichiometric composition. Recalling the available thermodynamic data, it was shown that the whole set of diffusion data is inconsistent with the Darken-Manning equation applying the generally accepted value of the vacancy wind factor S approx= 1. To resolve the problem a Monte-Carlo simulation of the interdiffusion in the Ni_(50)Al_(50)/Ni_(58)Al_(42) single-phase couple was performed. The energy barriers were computed by molecular static calculations with empirical EAM potentials. The current study was confined to the Ni-rich side of compositions. It was definitively shown that interdiffusion in NiAl can proceed via the triple defect diffusion mechanism. It turned out that the vacancy wind factor S strongly depends on the composition. At the stoichiometric composition S adopts a very small value, e.g. S approx= 0.02 at T = 1273 K. Applying the calculated S values the Darken-Manning relation was proven to be valid in Ni-rich NiAl with the anti-structure type of disorder.