Nonhysteretic superelasticity and strain hardening in a copper bicrystal with a ∑3{112} twin boundary

Superelasticity in nanoscale materials is of profound fundamental interest. Despite extensive research into low-dimensional systems such as nanowires and nanobeams, no compelling evidence exists for superelasticity in high-dimensional systems. Here, we demonstrate that a copper bicrystal with a ∑3{112} twin boundary (TB) during shear loading and unloading cycles exhibits nonhysteretic superelasticity. The upper bound of total recoverable shear strain associated with superelastic deformation is 24.8%. ∑3{112} TB under shear loading dissociates into two phase boundaries (PBs) with different characteristics, constituted by two phases with FCC and 9R structures. The transformation between these two phases, assisted by the reversible migration of tilt PB during loading and unloading cycles, accounts for the superelasticity. The driving force for the superelasticity is the potential energy difference per atom of the two phases. However, the irreversible migration of mixed tilt/twist PB with energy dissipation is responsible for strain hardening. This work, the first observation of nonhysteretic superelasticity in a copper bicrystal, may shed light on the design of bulk materials with superelasticity.