Modification of microstructure and properties of extruded Mg-Li-Al Alloys of alpha and alpha plus beta phase composition using ECAP processing

Abstract

Two magnesium based alloys containing 4.5 wt% Li and 1.5 wt% Al (alloy 1) and 9 wt% Li and 1.5 wt% Al alloy 2) were cast under argon atmosphere and hot extruded at 350. degrees C. Microstructure of alloy 1 consisted of hexagonal a phase of average grain size 20 mu m and small aluminum rich precipitates being the most probably AlLi2Mg phase. Alloy 2 in the extruded form consisted of lamellas of alpha + beta phases of thickness 5-20 mu m and length above 100 mu m. Significant grain refinement down to about 2 mu m was observed in one-phase hexagonal (hcp) alloy 1 after one pass of ECAP processing with helical component. Two-phase (hcp + bcc) alloy 2 showed higher non-homogeneity after the first equal channel angular pressing pass due to easier deformation of softer bcc phase, while both, alpha and beta phases exhibited low angle grain boundaries. The hardness and the yield strength of the alloys were higher for alloy 1 (68 HV and 205 MPa, respectively) than those of alloy 2 (61 HV and 175 MPa). Subsequent equal channel angular pressing passes were performed at lower extrusion stress. The hardness of both alloys did not change significantly after subsequent equal channel angular pressing passes and revealed tendency to decrease. Two-phase alloy showed superplastic properties already after one equal channel angular pressing pass at 160 degrees C with grain growth after superplastic tensile testing. Single phase hcp alloy did not show such properties after 1 pass, but after a few equal channel angular pressing passes it could be superplastically formed.