Microstructural evolution during recovery and recrystallization of a nanocrystalline Al-Mg alloy prepared by cryogenic ball milling

Abstract

The microstructural evolution during thermal annealing of a cryogenically ball milled Al-7.6 at% Mg alloy with a grain size of ~25 nm was examined using differential scanning calorimetry, x-ray diffraction, and transmission electron microscopy. Recovery occurs during annealing from 100 to 230 °C resulting in strain relaxation and grain coarsening, and recrystallization proceeds at higher temperatures up to about 370 °C with further grain growth. The stored enthalpy release during recovery was estimated to be ~450 J/mol, which is considerably higher than that in materials processed by other known cold-working methods. Only a fraction of the measured enthalpy was found to arise from the enthalpy releases due to grain coarsening and the reduction of high dislocation density. Both recovery and recrystallization give rise to non-uniform, bimodal grain-size distributions, which may result from heterogeneous nanostructures in the as-milled state. The detailed microscopic observations strongly support that grain coalescence is a feasible mechanism for grain coarsening during the recovery. In addition, the activation energy for recovery was calculated to be ~120 kJ/mol, indicating the process is diffusion-controlled (Mg in Al), whereas the activation energy for recrystallization was considerably higher, ~190 kJ/mol.