Stabilizing a severely deformed Al–7Mg alloy with a multimodal grain structure via Mg solute segregation

Abstract

Single-phase Al–Mg alloys processed by severe plastic deformation (SPD) usually suffer from unsatisfactory thermal stability at moderate to high temperatures with recrystallization occurring and obvious grain coarsening. In the present work, an Al–7Mg alloy prepared by equal-channel angular pressing (ECAP) possessed markedly enhanced thermal stability upon annealing at moderate to high temperatures (200–275 ℃), compared with those ultrafine-grained dilute Al–Mg alloys with a uniform microstructure. The enhanced thermal stability is due primarily to the multimodal grain structure consisting of nano-, ultrafine- and micron-sized grains, strong segregation and/or clusters of Mg solute along grain boundaries (GBs), and Al3Mg2 precipitates formed during annealing. First, extensive recovery predominates over recrystallization and consumes most of the stored energy in the ECAPed Al–7Mg alloy annealed at ≤ 275 ℃, leading to the recrystallization and growth of nano/ultrafine grains being retarded or postponed. Moreover, Mg solute segregation and/or clusters along GBs of nano/ultrafine grains could further suppress grain growth via diminishing GB energy and dragging GBs efficiently. In addition, Al3Mg2 precipitates formed with increasing annealing time could inhibit grain growth by pinning GBs. The present multimodal-grained Al–7Mg alloy with enhanced thermal stability is believed to be particularly attractive in potential engineering applications at moderate to high temperatures.