The influence of temperature and storage time at RT on nucleation of the β″ phase in a 6082 Al–Mg–Si alloy

The age hardening process of a 6082 Al–Mg–Si alloy by isothermal ageing at 100, 125 and 150 °C for 0 to 40 days has been investigated by both conventional and high-resolution transmission electron microscopy. Changes in sizes and densities of the pre-β″ and β″ phases during ageing can explain the evolution of hardness, in particular a plateau in the curve of the hardness versus ageing time. Fully coherent pre-β″ particles (GP-I zones) are formed from atomic clusters that were created upon annealing or at natural ageing. Room temperature (RT) storage reduces the number of pre-β″ particles by a factor of 5. Ageing at 125 and 150 °C causes the phase pre-β″ to transform to the phase β″. An ageing temperature of 100 °C was found to be too low to initiate the transformation of pre-β″ to β″.