Grain boundary width, energy and self-diffusion in nickel: Effect of material purity

The effect of material purity on grain boundary (GB) width and energy is investigated by GB self-diffusion measurements in Ni. The effect of matrix purity is examined for the first time for the low temperature C regime of GB diffusion. Grain boundary self-diffusion is measured in Ni of 99.99 wt.% purity in both B and C kinetic regimes, and the diffusional GB width δ was determined as δ = 0.6 nm. This result is in good agreement with previous measurements in 99.999 wt.% Ni. Thorough analysis of all available results on GB width in different coarse-grained and nanograined materials suggests that the diffusional GB width is independent of temperature, material purity and material nature and can be taken as 0.5 nm in undeformed materials. Grain boundary self-diffusion strongly depends on the amount of residual impurities, and the effect is marginal for less pure materials, Dgb99.999≫Dgb99.99≈Dgb99.6 at low temperatures in Ni of the indicated purity levels. The effective dependence of the GB energy and the self-diffusion coefficient on grain size in a material of a given purity level is evaluated within the framework of a newly developed model.