The temperature dependence of grain-boundary diffusion of ⁶⁵Zn in polycrystalline aluminium

Abstract

It is shown that the penetration plots of the grain-boundary diffusion of ⁶⁵Zn in polycrystalline Al samples are curved at fixed temperatures and that this upward curvature changes with the annealing time. The penetration profiles can be interpreted in terms of sub-boundary diffusion characterized by the parameter P ₁ (= D ₁ K ₁δ, where D ₁ is the boundary diffusion coefficient, K is the segregation factor and δ is the grain-boundary width) as well as by a distribution of diffusion parameters for high-angle boundaries. The range of variation of the diffusion parameters for general large-angle grain boundaries can be characterized by the parameters P ₁, P ₃ and P ₄. The temperature dependence of P ₁, P ₂ and P ₃ has also been determined. It is concluded that if we approximate the penetration function by one effective slope in the B-kinetic regime of grain-boundary diffusion then the weight of different grain boundaries (with different grain-boundary diffusion coefficients) can vary greatly in the effective diffusion parameter at different temperatures. This can lead to upward curvature of the Arrhenius function. Since the perturbations caused by Zn atoms in Al are very weak (K_i ≈ 1), our results reflect a general property of tracer grain-boundary self-diffusion in polycrystalline metals as well.