Regensburg 2002 – scientific programme

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M: Metallphysik

M 16: Diffusion und Punktdefekte I

M 16.1: Fachvortrag

Wednesday, March 13, 2002, 15:30–15:45, H6

Diffusion Behavior in the B2 Compound NiAl: on the Validity of the Darken-Manning Relation — •Sergiy Divinski and Christian Herzig — Institut für Metallforschung, Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany

Recently we have measured Ni tracer diffusion in NiAl in extended temperature and composition intervals of the B2 phase NiAl and no minimum in the Ni diffusivity was found at the stoichiometric composition. Having performed atomistic calculations of different diffusion mechanisms in NiAl, these tracer data were shown to be well explained by the triple defect diffusion mechanism as the dominating process. Alternatively to the tracer diffusion data, the interdiffusion measurements in NiAl available in the literature demonstrate a deep minimum of the interdiffusion coefficient D at about Ni ₅₀ Al ₅₀ 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≅ 1 . To elucidate this problem a Monte-Carlo simulation of interdiffusion in NiAl was performed. The energy barriers were computed by molecular static calculations with empirical EAM potentials. It was shown that interdiffusion in NiAl indeed proceeds via the triple defect diffusion mechanism and that this mechanism corresponds to unusually small values of the vacancy wind factor around the stoichiometric composition, e.g. about 0.025 at T=1273 K. Applying the calculated S values the Darken-Manning relation was proven to be valid also in Ni-rich NiAl with the anti-site type of disorder.