Twin nucleation mechanisms at a crack tip in an hcp material: Molecular simulation

Twinning is an important deformation mode in hexagonal close-packed (hcp) materials that can strongly affect fracture toughness. In order to clarify the early stages of twin nucleation, lattice statics simulations of zirconium crystals containing mixed-mode basal cracks with [over(1, ̄) 2 over(1, ̄) 0] and [1 over(1, ̄) 0 0] front orientations were carried out using an embedded-atom method potential. The simulations show that crack tip twin nucleation is a two-stage process: (I) initial plastic deformation occurs within a thin layer ahead of the crack, possibly involving basal slip, crack tip blunting by the formation of Frank partials or an hcp-face-centered cubic (fcc) transformation produced by Shockley partials emitted from the crack tip and (II) a twin forms in the surrounding hcp matrix. In this second stage, either a {1 1 over(2, ̄) 1} twin is nucleated homogeneously or a {1 0 over(1, ̄) 1} twin is nucleated heterogeneously by Shockley partials that nucleate inside the fcc region and penetrate the hcp matrix.