In order to clarify the early stages of twin nucleation, lattice statics simulations of Zr crystals containing mixed-mode basal cracks with [12▪0] and [11▪0] front orientations were carried out using an embedded-atom method potential. The simulations showed that crack tip twin nucleation was a 2-stage process: (i) initial plastic deformation occurred within a thin layer ahead of the crack, possibly involving basal slip, crack tip blunting by the formation of Frank partials or an hexagonal close-packed to face-centered cubic transformation produced by Shockley partials emitted from the crack tip and (II) a twin formed in the surrounding hexagonal close-packed matrix. In this second stage, either a {11▪1} twin was nucleated homogeneously or a {10▪1} twin was nucleated heterogeneously by Shockley partials that nucleated within the face-centered cubic region and penetrated the hexagonal close-packed matrix.

Twin Nucleation Mechanisms at a Crack Tip in an HCP Material - Molecular Simulation. L.Kucherov, E.B.Tadmor: Acta Materialia, 2007, 55[6], 2065-74