Influence of quench and heating rates on the ageing response of an Al-Zn-Mg-(Zr) alloy

The influence of the quench rate on the microstructure and the ageing response of a Zr-containing Al-Zn-Mg alloy was investigated. It is shown that Al₃Zr dispersoïds are efficient nuclei for precipitation of coarse, equilibrium η precipitates during the quench. Dispersoïds are heterogeneously distributed in bands, resulting in bands of quenched-induced precipitates after a slow quench. The ageing response after a slow quench is characterized by various levels of heterogeneities resulting in a 'composite' material. On a microscopic scale, quench-induced η precipitates are surrounded by a precipitate-free zone. On a mesoscopic scale, the material is separated in unperturbed regions and regions of high density of coarse precipitates and low density of hardening precipitates. Finally, a slow quench makes the material more sensitive to the heating rate to the ageing temperature, due to slower GP zone precipitation during natural ageing because of annihilation of vacancies during quenching.