Grain Growth in a Nanocrystalline Al-Sc Alloy

Abstract

A sub-micron grained microstructure in an Al-0.2 mass% Sc alloy was produced by high strain deformation using Equal Channel Angular Pressing (ECAP). The alloy was solution treated prior to deformation, deformed by ECAP then aged at low temperature to produce a sub-micron grained microstructure with a large fraction of high angle grain boundaries (HAGB) decorated with fine Al₃Sc particles. General grain stability and particle/grain boundary interactions were studied using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), focussed ion beam (FIB) microscopy and transmission electron microscopy (TEM). The fine-grained microstructure was found to be highly stable during annealing at temperatures up to 500°C due to Zener pinning from stable Al₃Sc particles. The volume fraction, f, and average radius, r, of particles and their rate of coarsening were found to have a strong influence on grain growth. It was found that the limiting grain size, R_c, in the Al-Sc alloy may reasonably be predicted by the relation: R_c = 0.17r/f. This relation is known to be applicable for coarse-grained alloys (>1 μm) and indicates its validity for predicting the limiting grain size in sub-micron, particle-containing alloys.