Movement of alloying elements in Mg -8.5 wt-%Li and AZ91 alloys during tensile tests for superplasticity

Abstract

Magnesium - 8.5 wt-% lithium and AZ91 alloys, processed using rotary die equal channel angular pressing, exhibit superplasticity. Deformed specimens have been used to investigate the movement of alloying elements during superplastic deformation. Optical microscopy and electron probe microanalysis were used for the investigation of microstructure changes in Mg - 8.5 wt-%Li and AZ91 alloys. In a two phase Mg - 8.5 wt-%Li alloy, the average content of β phase (lithium richer than α) decreased during superplastic deformation. No β phase layer was found at all in the surface area of the deformed specimens. The higher the test temperature, the thicker the layer without β phase is. In addition, the higher the test temperature, the lower the average β phase content is. The results indicate that lithium is lost during superplastic deformation. At too high a temperature of 673 K, lithium oxidises were obtained in the tested alloy. At the optimum conditions of superplasticity, no oxidation was obtained in the specimen. In the AZ91 alloy, zinc was almost lost at the superplastic deformed area. Both Mg - 8.5 wt-%Li and AZ91 alloys show low activation energies derived from the stress - strain rate results, connecting to the microstructure changes during tensile tests. The mechanism of superplasticity of these alloys seems to be grain boundary sliding assisted by the movement of the relatively low activation energy elements during tensile tests.