Superior ductility Mg-Mn extrusion alloys at room temperature obtained by controlling Mn content

Abstract

The effect of Mn content (0.4, 0.8, 1.2 wt. %) on microstructure and mechanical properties of as-extruded dilute binary Mg-Mn alloys was studied. With the increase of Mn content from 0.4 wt. % to 1.2 wt. %, the volume fraction of dynamically recrystallized (DRXed) grains decreases from 89% to 40%, while the DRXed grain size decreases from 3.2 μm to 1.9 μm. The segregation of Mn atoms and nano α-Mn dynamical precipitates are observed at the grain boundaries of the extruded Mg-0.8Mn alloys and the amount of dynamically precipitates is increased with increasing Mn content. Tensile tests show that the Mn content significantly affects mechanical properties at room temperature (RT). The Mg-0.8 wt. % Mn alloy shows the excellent ductility of 81 % at room temperature in a strain rate of 1 · 10−3 s−1, the ductility decreases if the Mn content is lower or higher than 0.8 wt. %. The yield strength (YS) and ultimate tensile strength (UTS) increase with increasing Mn content. The large value of strain rate sensitivity exponent (m-value) and small activation volume (V*) of the as-extruded Mg-0.8Mn alloy suggest that grain boundary sliding (GBS) plays an important role in tensile plastic deformation. High volume fraction of the un-DRXed region leads to the reduced contribution of GBS to plastic deformation, decreases ductility at RT. The Atomic force microscopy (AFM) analysis indicates that the contribution of GBS to the total plastic deformation is 2.7 %, 55.1 % and 12.8 % for Mg-0.4Mn, Mg-0.8Mn and Mg-1.2Mn alloys, respectively.