Properties and hardening behavior of equal channel angular extrusion processed Mg-Al binary alloys

We examined the competition between precipitation and grain-size refinement during equal channel angular extrusion (ECAE) of two fully solutionized coarse-grained Mg-Al alloys which was accomplished by using a “step-down” temperature method, up to 4 passes using the BC route. The extrudates show various levels of microstructural heterogeneity, especially at lower processing temperatures. Grain size refinement from hundreds of micrometers to a few micrometers was achieved by dynamic recrystallization (DRX). This is in conjunction with the formation of precipitates with a bimodal size distribution. One type is a few micrometers and found at grain boundaries, while the other precipitate type is a few hundred nanometers and found inside large parent grain remnants (PGR). The non-uniformity of the microstructure indicates that the ECAE deformation process is highly inhomogeneous. However, the presence of both fine grains and PGRs renders the extrudate with a higher capacity for actual strain hardening rate (ASHR: dσ/dε) comparable to that of the as-received materials. Dynamic compressive test results show a significant strength increase attributed to the precipitates, which is in addition to the strength improvement from grain size refinement (Hall-Petch Effect). These features validate the utility of ECAE for high-strength Mg alloy engineering. The work herein also provides microstructural insights to the severe plastic deformation (SPD) processing of Mg-Al alloys as well as a wealth of experimental data for the development of numerical models.