Interactions between precipitation and recrystallisation in an Al-Mg-Si alloy

Work has been carried out to study a broad range of possible behaviours in a simple model, high-purity, ternary Al-Mg-Si alloy, The starting conditions investigated, before deformation, included a supersaturated solid solution, a very coarse beta (Mg2Si) particle distribution, and a range of preaged conditions with a combination of beta' rods and beta platelets, obtained by isothermal treatments at 300 degreesC, On annealing at the same temperature of 300 degreesC, after cold rolling to a 70% reduction, the recrystallisation times were found to vary from 10 min to 1600 h, depending on the initial material condition. The fastest recrystallisation time was observed with the coarse beta particles, where nucleation of recrystallisation took place rapidly by particle stimulated nucleation. In the solution treated sample, recrystallisation took the intermediate time of 10 h and the recrystallisation rate was found to be dominated by the transformation of the metastable beta' phase, to the equilibrium beta phase at the migrating grain boundaries. By far the longest recrystallisation time of 1600 h occurred for a preaging time of 1 h, This remarkable stability was attributed to the recrystallisation kinetics being controlled by discontinuous coarsening of a very fine distribution of stable beta precipitates at the recrystallisation front. These precipitates formed early in the annealing treatment, by the transformation of the metastable beta' rods, broken up, and made partially incoherent by the deformation process.