Exploring the hidden world of solute atoms, clusters and vacancies in aluminium alloys

Abstract

Precipitation hardening involves solutionising, quenching and annealing steps, the latter often at various temperatures. The phenomena observed in Al-Mg-Si alloys are very complicated and partially not well understood. During and after quenching, solute atoms diffuse through the lattice assisted by vacancies and form atom clusters that gradually grow. These act back onto vacancies, which complicates the situation. We apply positron annihilation techniques in addition to traditional hardness, resistivity and thermal measurements to clarify what happens in various stages of thermal treatment: The quenching process can be divided into a stage of vacancy loss and of precipitation. Very short artificial ageing treatments after heating at different rates show that there is a competition between vacancy losses and cluster formation as the temperature increases. The difference between natural ageing and artificial ageing can be defined based on the importance of excess vacancies. Based on such results the behaviour of “invisible” objects such as vacancies and small clusters can be better understood but some open question remain such as the kinetics of secondary ageing or the details of the negative effect of natural ageing on artificial ageing.