Evolution of Ultra-Fine Grains in AZ31 and AZ61 Mg Alloys during Multi Directional Forging and Their Properties

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AZ31 and AZ61 Mg alloys were multi directionally forged (MDFed) during decreasing temperature from 623 K to 423 K to cumulative strain of ΣΔε=4.8 by Δε=0.8 pass strain at a strain rate of 3×10−3 s−1. In both Mg alloys, the average grain size gradually decreased with increasing cumulative strain. After straining to ΣΔε=3.2, i.e., after 4 passes of MDF, ultra fine grained (UFG) microstructures with average grain size of 1 μm were uniformly evolved. By prolonged straining, the grains became further finer. The AZ61 Mg alloy MDFed to ΣΔε=4.0 showed quite high hardness over 1.2 GPa, while that of the AZ31 Mg alloy was 850 MPa at ΣΔε=4.8. The differences of UFG evolution and mechanical behaviors during MDF of AZ 31 and AZ61 Mg alloys are precisely investigated.