Orientation-dependence of dislocation-induced incipient plasticity in Mg via nanoindentation technique

Abstract

Magnesium (Mg) and its alloys have gained significant attention in recent years due to their lightweight nature and potential applications in transportation, aerospace, and biomedical industries. However, one of the major limitations to their broader application is the pronounced poor ductility. To better understand the root causes of this limitation, it is essential to investigate the plasticity across different crystallographic orientations. This thesis investigates the onset of plastic deformation, with a particular focus on dislocation-mediated incipient plasticity in a coarse-grained Mg–2 wt.% Gd alloy using the nanoindentation technique. A nanoindentation protocol was designed to suppress the formation of deformation twins by applying low indentation loads (1000 µN), enabling the isolation and examination of dislocation nucleation events. A novel “grain signing–finding approach” was developed to accurately identify and repeatedly test specific grain orientations. This allowed for over 100 indents per orientation across 13 distinct grains, including basal, prismatic, and pyramidal orientations. High-resolution scanning electron microscopy (HRSEM) and scanning probe microscopy (SPM) revealed no evidence of twinning, supporting that deformation at the nano-scale was mediated exclusively by dislocations. Load-displacement curves showed three distinct modes of the first pop-in event—(i) ideal pop-in, (ii) deviation before pop-in, and (iii) excursion before pop-in—all of which demonstrated strong orientation dependence. Furthermore, two types of pop-in sequences were observed: single and successive pop-ins, with successive events occurring only in basal orientations. The basal grains exhibited a higher median resolved shear stress (~1.3 GPa) compared to prismatic and pyramidal grains (~1.0 GPa), consistent with theoretical predictions for dislocation nucleation. This suggests the involvement of pyramidal ⟨c+a⟩ dislocations in basal orientations, facilitating plastic deformation along the c-axis. The activation volume associated with the first pop-in ranged from 27 to 40 ų—involving about ∼2 atoms of magnesium—supporting a surface dislocation nucleation mechanism and ruling out twinning, which typically requires a much larger activation volume. Interestingly, the observation of successive pop-ins in basal orientations, along with higher stress levels, suggests the possibility of a cross-slip nucleation mechanism to accommodate ⟨c⟩-axis deformation.