A quantitative microplasticity-based approach to rationalize the poor strengthening response of polycrystalline Mg alloys

Abstract

This work aims to understand the inefficiency of nanoprecipitates to strengthen a weakly textured, polycrystalline Mg-Gd-Y-Zr alloy. An experimental micromechanical approach consisting on micropillar compression combined with analytical electron microscopy is put in place to analyze the effect of nanoprecipitation on soft and hard basal slip and twinning in individual grains with different orientations. This study shows that, in grains that are favorably oriented for basal slip (“soft” basal slip), aging leads to extreme localization due to the ability of basal dislocations to shear the nanoparticles, resulting overall in the softening of basal systems. Additionally, in grains in which the c-axis is almost perpendicular to the compression axis, prismatic slip dominates deformation in the solid solution state and nanoprecipitation favors twinning due to the concomitant lattice solute depletion. Finally, in grains oriented with their c-axis making an angle of about 5–7° with respect to the compression axis, which deform mainly by “hard” basal slip, precipitation leads to the strengthening of basal systems in the absence of obvious localization. This work reveals that the poor hardening response of the polycrystalline alloy is related to the capability of basal dislocations to shear the nanoparticles, in the absence of Orowan looping events, and to the associated basal slip localization.