Twin nucleation from a single <c+a> dislocation in hexagonal close-packed crystals

Twinning plays an important role in governing the balance between strength and ductility in hexagonal-close-packed (HCP) metals. Here, we report a combined experimental and theoretical study of twin nucleation from a single <c+a> dislocation in HCP crystals. Specifically, high-resolution transmission electron microscopy has been used to identify {112¯1}twin nuclei in HCP rhenium, providing evidence of their nucleation from a <c+a> dislocation. The favorability of this dislocation-based nucleation mechanism is rationalized by an anisotropic elasticity model of <c+a> dislocation dissociation, parametrized by density functional theory calculations, which suggests the conditions for disconnection nucleation and propagation, under which this {112¯1}twinning mechanism is expected to be effective. The analysis serves to advance our understanding of the origin of the unique predominance of {112¯1}twinning in rhenium, which correlates with the high strength and ductility featured by this metal. It also provides new insights into design strategies that may be effective in activating this twinning mode and enhancing the balance between strength and ductility in HCP alloys more broadly.