Grain Boundary Grooving by Surface Diffusion in Nickel Bicrystals

Surface self-diffusion was investigated via thermal grooving experiments on nickel bicrystals in combination with numerical simulations of the groove profiles. The objective of this work was the clarification of the dominant mechanism of grain boundary grooving in nickel at T = 750 ∘ C and the determination of the respective diffusion coefficients with high accuracy. The literature values of surface self-diffusion coefficients in nickel exhibit high scatter most likely associated with adsorption of impurities, which makes a comparison between different sets of data impractical and drawing conclusions about surface diffusion anisotropy impossible. In this work, thermal grooving experiments were performed on nickel bicrystals with different grain boundary and surface orientations at T = 750 ∘ C in forming gas. The groove profiles were measured by means of atomic force microscopy. Surface diffusion was clearly identified as the dominant mechanism of grain boundary grooving. Asymmetrical grain boundary grooving by surface diffusion was modelled numerically. From the comparison of simulated and measured groove profiles, the surface diffusion coefficients were determined with high accuracy and for the first time the anisotropy of surface diffusion was quantitatively measured. The surface self-diffusion in nickel is highly anisotropic, varying between D S = 0.3 × 10 − 21 m 3 / s and D S = 12.0 × 10 − 21 m 3 / s, whereby the surface diffusion near { 100 } surfaces is an order of magnitude slower than near { 110 } and { 111 } surfaces. The present work demonstrates that thermal grooving is an excellent tool to study the surface diffusion and to determine the respective diffusion coefficients with high accuracy from the comparison of simulated and measured groove profiles.