Continuous and discontinuous grain coarsening in a fine-grained particle-containing Al-Sc alloy

An Al-0.2 wt% Sc alloy was solution treated, deformed by equal channel angular pressing (ECAP) to an effective true strain of 9.2 then aged for 3 h at 350 °C to produce a fine-grained (0.8 μm diameter) microstructure containing a large fraction (∼0.7) of high angle grain boundaries (HAGBs). This ageing treatment also generated a relatively uniform dispersion of 5 nm diameter Al3Sc particles. Grain stability was investigated at temperatures up to 550 °C using SEM, EBSD and TEM. It was found that the fine-grain structure was remarkably stable at temperatures up to 500 °C with grain coarsening occurring gradually with no marked change in the grain size distribution, texture and grain boundary character. This homogeneous coarsening behaviour is usually termed continuous recrystallization. In this regime, both the fine-grained microstructure and Al3Sc particles exhibit third order coarsening kinetics with dR/dt ∝ dr/dt which indicates that grain coarsening is controlled by the rate of particle growth with the latter controlled by bulk diffusion of scandium in the Al matrix. During extended annealing at 500 °C and for short times at higher temperatures, there is a notable transition from continuous to discontinuous grain coarsening whereby a small number of grains grow rapidly to produce a coarse (>10 μm) grain size. An analytical mean field model of grain coarsening in particle-containing alloys was shown to adequately predict this transition in coarsening behaviour.