INFLUENCE OF QUENCH AND HEATING RATES ON THE AGING 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 sho wn that Al3Zr dispersoids are efficient nuclei for precipitation of co arse, equilibrium eta precipitates during the quench. Dispersoids are heterogeneously distributed in bands, resulting in bands of quenched-i nduced precipitates after a slow quench. The ageing response after a s low quench is characterized by various levels of heterogeneities resul ting in a 'composite' material. On a microscopic scale, quench-induced eta precipitates are surrounded by a precipitate-free zone. On a meso scopic scale, the material is separated in unperturbed regions and reg ions of high density of coarse precipitates and low density of hardeni ng precipitates. Finally, a slow quench makes the material more sensit ive to the heating rate to the ageing temperature, due to slower GP zo ne precipitation during natural ageing because of annihilation of vaca ncies during quenching. (C) 1998 Elsevier Science S.A. All rights rese rved.