In-situ quantitative investigations of differential mobilities of edge and screw pyramidal dislocations in magnesium

Abstract

Lightweight magnesium, promising for energy-efficient structural applications, faces limitations in plasticity tied to the behavior of pyramidal dislocations. Despite extensive studies, key aspects such as the differences in the mobilities of edge and screw components and the perplexing rectilinear and step-like dislocation morphologies, remain unresolved. Using in-situ transmission electron microscopy, the critical stress for activation of the edge and near-screw dislocations is quantitatively measured. We reveal that near-screw dislocations glide smoothly at low stresses that are close to that required for basal slip, whereas edge dislocations exhibit jerky motion and low mobility requiring activation stresses two orders of magnitude higher. The mobility of edge dislocations can be enhanced by forming near-screw steps that glide laterally, well rationalizing the commonly observed step-like dislocation configurations. As temperature increases, the difference in the mobilities between edge and near-screw dislocations narrows. Atomistic simulations indicate that the low mobility of edge dislocations is due to an irrational core structure where the theoretical half atomic plane is irrational and shifts to basal and prismatic planes, leading to the rectilinear morphology along the basal plane and necessitating a climb-like motion. This study addresses long-standing questions about pyramidal dislocations and provides new insight into the plasticity of magnesium.