Twin nucleation mechanisms at a crack tip in an hcp material: Molecular simulation
L Kucherov and EB Tadmor, ACTA MATERIALIA, 55, 2065-2074 (2007).
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 (1) over bar2 (1) over bar0 and 1 (1) over bar 00 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 11 (2) over bar1 twin is nucleated homogeneously or a 10 (1) over bar1 twin is nucleated heterogeneously by Shockley partials that nucleate inside the fec region and penetrate the hcp matrix. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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