Experimental and computational studies of competitive precipitation behavior observed in an Al-Mg-Si alloy with high dislocation density and ultrafine-grained microstructures

The competitive precipitation behavior observed in microstructures with high dislocation density and ultrafine grains has been studied experimentally and computationally for cold-rolled and severe plastic deformed Al-Mg-Si alloys. The agehardenability at 443 K was reduced by the two deformation processes due to the accelerated formation of larger precipitates on dislocation lines and grain boundaries, in place of the transgranular precipitation in the matrix. The developed numerical model based on a classical heterogeneous nucleation theory clarified the dislocation density and grain size dependences of volume fraction of precipitates nucleated at different sites, in good agreement with experimental results. From such dependencies, it becomes possible that three strengthening mechanisms of precipitation hardening, strain hardening and strengthening by grain size reduction are optimally exploited by controlling microstructural parameters such as dislocation density and grain size.