Orientation-dependence of incipient plasticity in a coarse-grained Mg revealed by nanoindentation pop-ins

• We employed nanoindentation across a broad spectrum of crystallographic orientations in a Mg-2wt% Gd alloy to elucidate the influence of orientation on the onset of plasticity, i.e., pop-in events. • We demonstrated orientation-dependent pop-in events in a Mg alloy, providing new insights into how various slip systems—including basal 〈a〉, prismatic 〈a〉, and pyramidal 〈c + a〉—activate to induce incipient plasticity within different grains. • We reported a successive pop-in phenomenon, for the first time in Mg, in grains near (0001) orientations, which we attribute to the formation of basal-pyramidal dislocation locks. The nanoindentation pop-in behaviors of 13 grains with diverse crystallographic orientations were analysed using a coarse-grained Mg-2 wt.% Gd alloy. Within nanoscale stressed volumes within all grains, the converted shear stresses for the first pop-in, calculated using the indentation Schmid factor, ranged from 1 to 1.3 GPa, consistent with theoretical predictions for dislocation nucleation in Mg. The estimated activation volume of the first pop-in was approximately 27–40 ų (involving about ∼2 atoms), aligning with reported atomistic simulations of the surface dislocation semi-loop nucleation. While indented near the ⟨c⟩-axis, grains exhibit higher first pop-in loads and successive pop-ins, implying the possibility of a cross-slip nucleation mechanism to accommodate ⟨c⟩-axis deformation.