Dislocation penetration in basal-to-prismatic slip transfer in Mg: A fracture mechanics criterion

Abstract

Understanding the transition mechanism from microscopic to macroscopic yielding is crucial for developing high-strength metallic materials. Basal-to-prismatic slip transfer at grain boundaries (GBs) of Mg alloys is known to trigger macroscopic yielding. Here, the penetration behavior of pileup dislocations at a simple basal–prismatic (BP) boundary was analyzed using molecular dynamics simulations. Edge dislocation penetration creates a step in the BP boundary at the intersection with the slip plane, widened via subsequent penetration of dislocations. Many GBs, including the BP boundary, have a regular GB dislocation network that corresponds to a coincidence site lattice. Dislocation penetrations must gradually destroy the initial GB dislocation network, requiring increasing driving force for subsequent dislocation penetrations; thus, slip transfer at the boundary occurs sporadically and stably via the penetration of individual lattice dislocations. As the width of the step created by dislocation penetration increases, a GB dislocation network can be formed in the step, which can stabilize the energy of the step with specific widths. When the energy of step decreases with increasing step width, a smaller driving force is required for the subsequent penetration of dislocations, and thus slip transfer occurs unstably via the penetration of several sequential lattice dislocations. Once the step has a low energy at a specific width, it strongly obstructs dislocation penetration. This formation and disruption of the GB dislocation network cause intermittent penetration of dislocations. Finally, dislocation penetration at the BP boundary is discussed based on a fracture mechanics approach, which well-reproduces the transition between penetration behaviors and the macroscopic yield stress.