Effect of various SPD techniques on structure and superplastic deformation of two phase MgLiAl alloy

Abstract

MgLiAl alloy containing 9 wt% Li and 1.5% Al composed of hexagonal alpha and bcc beta phases was cast under protecting atmosphere and hot extruded. Various methods of severe plastic deformation were applied to study their effect on structure and grain refinement. Rods were subjected to 1-3 passes of Twist Channel Angular Pressing TCAP (with helical component), cyclic compression to total strain epsilon = 5 using MAXStrain Gleeble equipment, both performed at temperature interval 160-200 A degrees C and, as third SPD method, KOBO type extrusion at RT. The TCAP pass resulted in grain refinement of alpha phase from 30 mu m down to about 2 mu m and that of beta phase from 12 to 5 mu m. Maxstrain cycling 10 x up to epsilon = 5 led to much finer grain size of 300 nm. KOBO method performed at RT caused average grain size refinement of alpha and beta phases down to about 1 mu m. Hardness of alloy decreased slightly with increasing number of TCAP passes due to increase of small void density. It was higher after MAXStrain cycling and after KOBO extrusion. TEM studies after TCAP passes showed higher dislocation density in the beta region than in the alpha phase. Crystallographic relationship (001) alpha|| (110) beta indicated parallel positioning of slip planes of both phases. Electron diffraction technique confirmed increase of grain misorientation with number of TCAP passes. Stress/strain curves recorded at temperature 200 A degrees C showed superplastic forming after 1st and 3rd TCAP passes with better superplastic properties due to higher elongation with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.29 after 3rd TCAP pass for strain rate range 10(-5) to 5 x 10(-3) s(-1). Deformation by MAXStrain cycling caused much more effective grain refinement with fine microtwins in alpha phase. Superplastic deformation was also observed in alloy deformed by KOBO method, however the value of m = 0.21 was obtained at lower temperature of deformation equal to 160 A degrees C and deformation rate in the range 10(-5) to 5 x 10(-3). Tensile samples deformed superplastically showed grain growth and void formation caused by grain boundary slip. Summarizing, all methods applied resulted in sufficient grain refinement to obtain the effect of superplastic deformation for alloys of two phase alpha + beta structure.