The influence of Sc on thermal stability of a nanocrystalline Al-Mg alloy processed by cryogenic ball milling

Abstract

The microstructural evolution during annealing of a cryogenically ball-milled Al-7.5Mg-0.3Sc (in wt pct) was examined using differential scanning calorimetry and transmission electron microscopy (TEM). The as-milled alloy was a supersaturated fcc solid solution with an average grain size of ∼ 25 nm and heterogeneous grain morphologies and size distributions. Calorimetric measurements at a constant heating rate of 32 K/min indicated two exothermic events in association with recovery from 100 ° C to 240 ° C and recrystallization from 300 ° C to 450 ° C. Prior to recrystallization, the precipitation of Al3SC may occur at low annealing temperatures producing a nonuniform dispersion of approximately spherical particles with diameters of 4 to 5 nm. Recrystallization gave rise to heterogeneous microstructures with bimodal grain size distributions, which may result from the heterogeneity of microstructure in the as-milled state. The heterogeneous microstructures of the recrystallized Al-Mg-Sc alloy were similar to those observed in the recrystallized Sc-free Al-Mg alloy.