Dynamic precipitation and recrystallization in Mg-9wt.%Al during equal-channel angular extrusion: A comparative study to conventional aging

Precipitation of fine intermetallic particles during conventional thermal aging can significantly enhance the mechanical properties of Al alloys. However, this method offers only limited strengthening in Mg alloys as thermal aging usually leads to intermetallic particles that are too coarse in size and too sparse in spacing. Dynamic precipitation during low-temperature deformation processing offers a chance to rectify this issue. Meanwhile, mechanical processing often drives precipitation and recrystallization concurrently, and a careful analysis of their interaction and their interdependent thermodynamic driving forces is needed. Herein, we investigate dynamic precipitation and recrystallization in a coarse-grained, fully solutionized Mg-9wt.%Al alloy following low-temperature Equal Channel Angular Extrusion (ECAE) using electron microscopy, theoretical calculations, and mechanical property evaluations. Through comparisons with conventionally aged samples, we find that dynamic precipitation during extrusion produces continuous, nanoscale Mg17Al12 particles with a high number density and a low aspect ratio due to dislocation-assisted nucleation, as well as some fragmentation via shearing during ECAE. The assisted nucleation is analyzed theoretically using classical nucleation theory in conjunction with deriving and accounting for Al solute clouds around the dislocations. Dynamic recrystallization is seen early in the extrusion process, but only to a limited extent along some grain boundaries. The volume fraction of the recrystallized region that contains submicron Mg and Mg17Al12 grains grows as the number of passes increases. Using thermodynamic analysis, we estimate the driving forces for the recrystallization process, and we identify the chemical energy of the Mg matrix as the major driving force for recrystallization. Our results offer key insights into the evolution of microstructure during dynamic precipitation and recrystallization, and thus provide guidance for the design of improved microstructures in Mg alloys.