Influence of preliminary extrusion conditions on the superplastic properties of a magnesium alloy processed by ECAP

The microstructures and properties of a two-phase Mg-8% Li alloy were evaluated in three different conditions: after casting; after casting followed by extrusion at different temperatures and speeds; and after casting, extrusion and processing by ECAP for four passes at room temperature using a die with a channel angle of 135°. The results show extrusion introduces significant grain refinement and there is additional refinement in ECAP. In tensile testing, the elongations to failure increase with decreasing extrusion temperature, but are essentially independent of the extrusion speed. The ductilities are low in the cast condition, intermediate in the extruded condition and high after extrusion and ECAP. For the last condition, an exceptionally high elongation of ∼1780% was achieved at a testing temperature of 473 K. It is shown that it is advantageous to use a die with a channel angle of 135° because it permits pressing at room temperature where grain growth is limited.