Ultrafine Grain Evolution in Mg alloys, AZ31, AZ61, AZ91 by Multi Directional Forging

摘要

AZ31, AZ61 and AZ91 Mg alloys having different initial grain sizes were multi directionally forged (MDFed) under decreasing temperature condition from 673 K to 473 K to a cumulative strain ΣΔε of 3.2.A pass strain of Δε=0.8 and a strain rate of 3 x 10-3 s-1 were employed.In all the alloys, the average grain size decreased rapidly with cumulative strain during MDF.Twinning and kinking dominantly contributed to grain fragmentation. After straining to ΣΔε=3.2 (i.e., after 4 passes of MDF), equi-axed ultrafine grains (UFGs) with an average size around 1 μm were uniformly evolved. The room-temperature hardness was gradually raisedwith decreasing grain size.The increment in hardness looked more significant with increasing content of Al in the alloys, even while the obtained grain size was coarser. This would be induced by coarser and higher density of Mg17Al12precipitates in addition to high higher Al content. The mechanisms of UFG evolutionduring MDF and the mechanical properties of UFGed alloys are analyzed and discussed.