Plastic deformation mechanisms of Mg-5.2Li alloy single crystals along different orientations studied by micropillar compression

Abstract

The effect of 5.2 wt.% Li in solid solution on the critical resolved shear stress (CRSS) values of 〈a〉 basal, 〈a〉 prismatic, <c + a> pyramidal slips and extension twinning in Mg has been explored through compression tests on single crystal micropillars with different orientations. The addition of Li hardens 〈a〉 basal slip (CRSS: 30.2 MPa) but significantly softens 〈a〉 prismatic slip (29.0 MPa). This result contradicts the conventional wisdom, which asserts that 〈a〉 basal slip consistently exhibits the lowest CRSS among the plastic deformation mechanisms in Mg alloys. The Li addition also hardens <c + a> pyramidal slip (168.1 MPa) but significantly reduces the CRSS ratio between <c + a> pyramidal slip and 〈a〉 basal slip from 12.1 to 5.6. Moreover, the CRSSs for the nucleation and growth of extension twins are increased by Li addition (98.6 MPa and 71.8 MPa), whilst their ratios to 〈a〉 basal slip were reduced (3.3 and 2.4). Finally, evidence of non-basal slip and cross-slip of dislocations between basal and non-basal planes was found. These findings demonstrate that all non-basal slip systems and extension twinning are likely to dominate plastic deformation in Mg-Li alloys, which is seldom reported in Mg and its alloys. As a result, plastic anisotropy is greatly reduced by the presence of the soft 〈a〉 prismatic slip as well as by easier activation of <c + a> pyramidal slip, extension twins and cross-slip, leading to the high ductility and improved formability of Mg-Li alloys.