Interactions between deformation, recrystallisation and phase transformations in an Al-Mg-Si alloy

The effect of the initial heat treatment on the recrystallisation of a balanced Al-Mg-Si alloy has been investigated. Samples were prepared in the solution treated, aged and slowly cooled conditions. The materials were then cold rolled to a 70% reduction and isothermally annealed at 300 degrees C while their microstructural evolution was followed with optical and electron microscopy. The initial heat treatment was found to dramatically influence both the recrystallisation kinetics and the recrystallised microstructure, recrystallisation times ranging from 10 minutes to 10 weeks. In the solution treated sample, precipitation onto the deformed microstructure was found to greatly retard recrystallisation compared to the slowly cooled alloy, where all the solute content had precipitated out to form coarse stable spherical beta particles. It was found that the rate of recrystallisation for the solution treated and water quenched material was governed by the transformation of the metastable beta' needles to the coarser beta equilibrium phase at the migrating grain boundary front. Surprisingly recrystallisation was by far the slowest in alloys which had been pre-aged for 1 hour prior to deformation, taking one thousand times longer than in the furnace cooled sample, where the kinetics were found to be governed by the discontinuous coarsening of finely spaced stable beta precipitates which formed during the early stages of annealing.