Grain boundary plasticity at intermediate temperatures in fine-grained Mg-Mn ternary alloys

The effect of micro-alloying with different elements (X) on grain boundary sliding behavior at intermediate temperatures (up to 473 K) is examined by two types of testing methods, i.e., damping and tensile tests, using fine-grained Mg-0.3 at%Mn-0.1 at%X alloys. The micro-alloying of X was selected to be Al, Li, Sn, Y and Zn, with focusing on common elements. In the damping tests, the micro-alloying element affects the damping capacity. Mg-Mn-Al, Mg-Mn-Li and Mg-Mn-Sn alloys have a high value of tanδ not only at room temperature but also at intermediate temperatures; however, the Mg-Mn-Y alloy does not indicate such a characteristic. It is interesting to notice that the tensile response to the micro-alloying elements shows a similar trend to that obtained in the damping tests. This resulted from the different magnitude of grain boundary sliding contribution to deformation. Observations of tensile deformed surface features indicates a partial contribution, approximately 30%, of this deformation mechanism in the alloys having good damping capacity and ductility. This is the major reason that experimental results clearly deviate from the values calculated by constitutive equation for general superplastic alloys. Nevertheless, the unique grain boundary sliding that occurs at intermediate temperatures is well consistent with previously reported results for tensile tests on fine-grained Mg and its alloys conducted at ambient temperature.