Molecular dynamics techniques are used to investigate $\left\{10\overline{1}2\right\}$ and $\left\{11\overline{2}1\right\}$ twin thickening processes through spontaneous formation of twinning disconnections (TD) under applied shear deformation with a constant strain rate, based on an embedded atom method potential model of Zr. The observed stress responses and twin boundary structure changes verify that both twins thicken through TD formation. However, the thickening processes of $\left\{10\overline{1}2\right\}$ and $\left\{11\overline{2}1\right\}$ twins show significant differences. While $\left\{10\overline{1}2\right\}$ TD loops were observed to form and expand during the twin thickening process, the $\left\{11\overline{2}1\right\}$ twin boundary exhibits a more structurally disordered interface throughout the simulation, and no sharp boundary corresponding to a $\left\{11\overline{2}1\right\}$ TD ‘loop’ can be identified. Moreover, the $\left\{11\overline{2}1\right\}$ twin requires a much lower critical shear stress for twin thickening, approximately 10% of that for the $\left\{10\overline{1}2\right\}$ twin. The low critical shear stress of $\left\{11\overline{2}1\right\}$ twin thickening also indicates that these twins should grow easily once nucleated. To the extent that these results apply to hexagonal-close-packed (HCP) materials more generally, promotion of $\left\{11\overline{2}1\right\}$ twin nucleation may be an effective way to enhance twinning deformation and twin boundary-related work hardening in HCP materials, with the potential to manipulate the balance between strength and ductility.

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• Crystal structure
• Defects
• Microstructure
• Plastic deformation
• Twinning