Atomistic Study of Twin Nucleation and Growth in Hexagonal Close Packed Metals

Twinning is an essential plastic deformation mechanism in hexagonal-close-packed (HCP) metals, where the number of operative dislocation slip systems is limited. Twinning plays an important role in governing the balance between strength and ductility of the materials. The {10-12} and {11-21} twinning are two commonly observed tension twins, and will be the focus of this dissertation. The nucleation and growth are the two stages of the twin formation process, understanding the mechanisms behind which facilitates the study of the mechanical properties of the HCP metals and gives guidance to the alloy design strategies./nGrain boundaries and dislocation pile-ups have always been thought of as the most possible twin nucleation sites, but there is a lack of experimental evidence and theoretical understanding of the twin nucleation mechanism from a single lattice dislocation. In this dissertation, a combined experimental and theoretical study of twin nucleation from a single dislocation in HCP crystals is presented. Specifically, high-resolution transmission electron microscopy has been used to identify {11-21} twin nuclei in HCP rhenium, and provided evidence of their nucleation from a dislocation. An anisotropic elasticity model of dislocation dissociation, parametrized by density functional theory calculations, provides conditions for twinning disconnection nucleation and propagation, necessary for the {11-21} twinning mechanism to be operative. The analysis serves to advance our understanding of the origin of the unique predominance of {11-21} twinning in rhenium, which correlates with the high strength and ductility of this metal. The model 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./nThis dissertation also explores twin thickening. Twin thickening requires that the twin grows normal to the K1 plane. Molecular dynamics simulations are used to investigate {10-12} and {11-21} twin thickening processes through spontaneous twinning disconnection (TD) formation under applied shear deformation with a constant strain rate, based on an embedded-atom-method potential model of Zr. The stress responses and twin boundary (TB) structure changes verify that both twins thicken through TD formation. However, the thickening process of {10-12} and {11-21} twins differ in significant ways. While {10-12} TD loops were observed to form and expand during the twin thickening process, the {11-21} TB exhibits a more structurally disordered interface throughout the simulation and no sharp boundary corresponding a {11-21} TD ‘loop’ can be recognized, i.e., the TD for this TB is structurally rough. Moreover, the {11-21} twin requires a much lower critical shear stress for twin thickening. The low critical shear stress of {11-21} twin thickening also indicates that these twins should grow easily once nucleated. To the extent that these results apply to HCP materials more generally, the promotion of {11-21} twin nucleation may be an effective way to enhance twinning deformation and twin boundary related work hardening in HCP materials, with the potential to enhance the strength and ductility combination.

Advisor: Chrzan, Daryl C.
Asta, Mark D.