Revealing the nano-scale mechanisms of the limited non-basal plasticity in magnesium

Abstract ⟨c+a⟩ dislocations are considered as the primary defects limiting the ductility of magnesium. It is argued that in small magnesium crystals the glide of such dislocations can be activated to accommodate plastic strain at high flow stress levels that are rarely achieved in the bulk. Here, we report ⟨c+a⟩ dislocation-mediated small-scale plasticity of c-axis oriented submicrometer-sized magnesium pillars observed by in situ transmission electron microscopy compression tests. After mobile ⟨c+a⟩ dislocations accommodate the initial plasticity of pillars, interactions between dislocations develop local dislocation entanglements, thereby preventing consecutive dislocation glide and surface annihilation. Supported by atomistic simulations, this increased pinning and multiplication is attributed to the formation of basal I1 and I2 stacking faults, induced by interaction of glissile pyramidal II dislocations, which serve as anchor points for dislocation sources. The ensuing plastic strain is accommodated via dislocation avalanches caused by simultaneous activation of multiple dislocation sources at increasing stress levels, forming a dislocation forest of ⟨c+a⟩ dislocations in the confined volume. This observation distinctly differs to common small-scale plasticity associated with a dislocation starved or exhausted state and provides a new concept towards plasticity and work hardening in bulk Mg.