Initial clustering after quenching in AlZnMg alloys

Abstract

After solution treatment and quenching a fast increase in hardness takes place in Al-Zn-Mg alloys in the first few minutes of ageing at or near room temperature. The kinetics of this process have been studied on a number of different compostition alloys by microhardness measurements between 246 and 295 K and by resistivity measurements between 263 and 323 K. On the basis of the observed microhardness and resistivity kinetics the activation energy of the process was determined. According to this the ageing process starts with the diffusion of zinc atoms to magnesium atoms and Guinier-Preston zones develop from the so formed Mg-Zn clusters. The average Mg/Zn ratio in the clusters is 1/4. In the alloys investigated the microhardness after 15 min ageing at room temperature can be composed simply of the contributions to the hardness of pure aluminium, MgZn₄ clusters and of magnesium atoms remaining in solid solution.