Precipitate-induced nonlinearities of diffusion along grain boundaries in Al-based alloys

Abstract

The formation of precipitates in metallic alloys is determined by a two-way chemomechanical coupling. While the dependence on solute diffusion is apparent, the opposite effect, namely the impact of nanoprecipitates on the diffusion, is highlighted in the present paper. Using severe plastic deformation and post-deformation annealing of an Al-based alloy, different microstructures and sizes of Al3Sc-based nanoprecipitates are produced. The enhanced diffusion along grain boundaries in the so-called C-type kinetic regime is used as a probe to ensure a well-defined distance of the diffusion path from the precipitates and to prohibit their evolution during measurements. Tracer measurements with the radioisotope Co57 reveal a remarkable nonmonotonic dependence of the diffusion rates on the annealing temperature. It has been fully explained by an ab initio informed phenomenological model that considers the elastic stress around the coherent and noncoherent precipitates.