Superplastic deformation of two phase MgLiAl alloy after TCAP pressing

Magnesium based alloy containing 9 wt. % Li, 1,5 wt. % Al, composed of a + b (hcp + bcc) phases was cast under argon atmosphere and extruded at 350 o C. Up to 3 passes of Twist Channel Angular Pressing TCAP deformation was applied at 160 o C. TCAP tool consisted of helical part in horizontal area of the channel with angle of lead γ = 30° to simulate back pressure. The initial grain size of hexagonal a phase estimated at 30 mm decreased in following passes down to 6 mm and that of bcc b phase decreased after TCAP from initial 12 mm down to 5 mm. TEM studies after TCAP passes showed higher dislocation density in the b region than in the a phase. Crystallographic relationship (001) a || (110) b indicated parallel positioning of slip planes of both phases. Electron diffraction confirmed increase of grain misorientation with number of TCAP passes. Stress/strain curves measured at temperature 200 o C showed superplastic forming after 1 st and 3 rd TCAP pass. Values of strain rate sensitivity coefficient m were calculated at 0.31 after 1 TCAP pass and increased after 3 rd TCAP pass up to 0.47 for the strain rate range 10 -5 – 5 10 -4 . Increase of number of TCAP passes had positive effect on superplastic properties due to finer grains and increase of their misorientation;