Superplastic deformation of two phase MgLiAl alloy after TCAP pressing

Abstract

Magnesium based alloy containing 9 wt. % Li, 1,5 wt. % Al, composed of alpha + beta (hcp + bcc) phases was cast under argon atmosphere and extruded at 350 degrees C as bars of cross section 12x12 mm. Twist Channel Angular Pressing TCAP deformation was applied at a temperature of 160 degrees C using BC rotation of samples in the following passes. TCAP tool consisted of helical part in horizontal area of the channel with an angle of lead gamma = 30 degrees to simulate back pressure and thus to increase the extrusion force. The initial grain size of the hexagonal. phase estimated at 30 mu m decreased in the following passes down to 6 mu m and that of bcc. phase decreased after TCAP from initial 12 mu m, down to 5 mu m. TEM studies after TCAP passes showed higher dislocation density in the. region than in the alpha phase. Crystallographic relationship (001) alpha || (110) beta indicates parallel positioning of slip planes of both phases due to activation of slip. Electron diffraction indicates increase of grain misorientation with a number of TCAP passes. Stress/strain curves measured at temperature of 200 degrees C show superplastic forming after 1st and 3rd TCAP passes with better superplastic properties with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.31 after 1 TCAP pass using flow stress values from step strain increase curves. The sample after 3 TCAP passes showed increase of m value up to 0.47 for the strain rate range 10(-5)-5 x 10(-4) s(-1). Obtained values are slightly lower than those reported in the literature, however increase of a number of TCAP passes has a positive effect on superplastic properties due to fine grains and increase of their misorientation.