Grain boundary diffusion in severely deformed Al-based alloy

Precipitation phenomena affect the kinetics of diffusion in the surrounding matrix. The present study is focused on the effect of Al₃Sc precipitates on grain boundary diffusion in an Al-based AA5420 alloy. Different states were prepared by combination of ECAP processing and heat treatments and the radioisotope ⁵⁷Co was employed as a sensitive probe of a given grain boundary state. The results suggest that Co diffusion at room temperature is intimately coupled to the precipitation behaviour of Al₃Sc at elevated temperatures. In fact, the Co diffusion coefficients are first decreasing with increasing precipitate size and approach a minimum at their average size of about 20 nm, which is probably related to maximal elastic stresses around the precipitates. As the precipitates grow further, the diffusion coefficients begin to increase due to a loss of coherence accompanied by a change in the chemical composition and a reduction of stresses around the precipitates. The effects of deformation-induced dislocations and deformation-modified states of grain boundaries are evaluated.

Financial support of DFG within SPP 1713, project DI 1419/7-1, is gratefully acknowledged.