Enhancing ambient temperature grain boundary plasticity by grain refinement in bulk magnesium

Abstract

Room temperature ductility of magnesium is known to be dramatically enhanced by partial contribution of grain boundary sliding. However, the effect of grain size on grain boundary plasticity as well as accommodation process for this unique room temperature grain boundary sliding are unclear. In this study, tensile tests at ambient temperatures and detailed deformed microstructural observations are performed to investigate these points using extruded magnesium of average grain sizes between 1 μm and ∼50 μm. The results of tensile tests exhibit that elongation-to-failure in tension and deformation mechanism vary with grain sizes. Fine-grained magnesium (in orders of several micrometer) shows the elongation-to-failure in tension of ∼100% with activation energy of 80 kJ/mol, which is close to grain boundary diffusion of magnesium. This high ductility results from grain boundary sliding accommodated with dislocation slip by diffusion process. Accordingly, the ductility and activation energy reduces with grain size coarsening. Deformed microstructural observations using EBSD and TEM display the activation of non-basal dislocations at grain boundaries. Specific regions in geometrically necessary dislocation map of grain interior consisting of meso- and coarse-structures (>∼10 μm) show a high value, which suggests the presence of strains. Moreover, the formation of sub-grain boundaries is confirmed in grain interior of coarse-grained structure. This is attributed to dominant occurrence of intragranular deformation mode, associated with insufficient diffusivity, such as grain boundary diffusion.