Secondary Precipitation in Aluminium Alloys & Its Role in Modern Heat Treatment

This paper reviews some of the practical outcomes of exploiting secondary precipitation during the heat treatment of aluminium alloys and discusses current understanding of this phenomenon. Recent studies have utilised the techniques of positron annihilation spectroscopy (PAS), 3D atom probe (3DAP) as well as the more traditional transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) to investigate early precipitation events that occur during secondary ageing at low temperatures (~20-65°C). This work has confirmed that clustering and GP zone formation can modify the nature and distribution of precipitates that form when ageing is subsequently resumed at more elevated temperatures. Prospects for achieving further improvements in heat treatment schedules and alloy compositions to take greater advantage of secondary precipitation are also considered.