P. Kroeker; T. Vermeij; V. Devulapalli; J. Michler; X. Maeder
SSRN 6496421 (2026)
Deformation twinning is an important plasticity mechanism in hexagonal close-packed (hcp) metals. While the {10-12} twin is extensively studied, the interaction and secondary twinning of the {11-21} extension twin remains less well understood. This study investigates these mechanisms in single-crystal rhenium using in-situ transmission Kikuchi diffraction tensile testing along the [0001] axis. We observe two distinct secondary twinning phenomena. First, native secondary twins form abundantly within primary twins, independent of twin interaction sites. Their nucleation is rationalized by their high Schmid factor and the energetically favorable dissociation of basal dislocations at primary twin boundaries. Second, specific twin-twin interactions trigger a distinct secondary variant selection producing a crossing-like morphology. However, these configurations violate classical geometric criteria for twin crossings (Cahn, Acta Met., 1953). Instead, we demonstrate that this non-classical variant selection is governed by the local stress state and plausibly facilitated by the dissociation of non-basal dislocations. These findings establish a stress- and dislocation-based framework for predicting secondary twinning in {11-21} extension twins, offering new insights into deformation anisotropy of hcp metals.