Germanium Tracer Diffusion in a Series of Symmetrical Near Σ=7, Θ=38.2° [111] Tilt Grain Boundaries of Aluminum

Abstract

Grain boundary (GB) diffusion was measured in a series of Al bicrystals with symmetrical near Σ=7, Θ=38.2° (12-3) [111] tilt GBs. The actual orientations of the bicrystals were precisely measured with the Kossel technique. The tilt angles Θ ranged from 34.15° to 41.41°. Grain boundary diffusion parallel to the tilt axis was measured in the type-B kinetic regime at five temperatures in the range 518 - 680 K. A characteristic non-monotonic behaviour of the resulting dependence of the GB diffusivity sδD GB on the tilt angle Θ was observed for all five investigated diffusion temperatures. At the ideal Σ=7, Θ=38.2° (12-3) [111] tilt GB the GB diffusion is minimal; with increasing deviation from the CSL (coincidence site lattice) GB the diffusivity increases and reveals a broad maximum at about Θ=38.2°±3°. The opposite dependencies are observed for the Arrhenius parameters (sδD GB ) 0 (pre-exponential factor) and Q GB (effective activation enthalpy of GB diffusion), which were calculated from the temperature dependence of sδD GB . At the ideal Σ=7 CSL GB the Arrhenius parameters are maximal. Due to the rather small GB segregation of Ge in Al, the orientation dependence of sδD GB , (sδD GB ) 0 and Q GB qualitatively reflects the orientation dependence of D GB , D GB0 and H GB . The orientation dependence of the GB diffusion supports the CSL GB diffusion model.