Twins as barriers to basal slip in hexagonal-close-packed metals
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I. INTRODUCTION
TWINNING is an important mode of plastic deformation in hexagonal-close-packed (hcp) metals, but larger strains require additional deformation by slip. The interactions between gliding dislocations and twin boundaries are, therefore, matters of practical interest because the deformation-induced twins may act as barriers to further slip. The present project is aimed at understanding the barrier mechanisms for commonly observed twinning modes, namely {1011}, {1012}, {1121}, and {1122}. The interface atomic structure of these twins and the interaction of crystal slip dislocations with the first two twin types have been described previously, and a mechanism by which such interaction can actually assist twin boundary motion under stress has been found (discussed subsequently). The present work is concerned with slip interaction with the {1121} and {1122} twins. Yoo[1] presented a comprehensive review of this problem from analyses based on crystallography and elasticity theories. However, dislocation movement along or into a boundary depends on the atomic arrangement at the interface and the atomic structure of the dislocation core, and these cannot be treated by approaches based purely on crystallography or the continuum description. Atomistic computer simulation has proved to be a suitable tool for understanding these interactions. In previous work,[2] we showed that when a shear stress resolved for basal slip is applied to a {1012} twinned bicrystal, screw dislocations with Burgers vector, b ⫽ 1/3具1120典, and line direction parallel to the boundary are able to cross the twin interface, and no residual dislocations are left A. SERRA, Catedra´tica (Professor), is with the Departament de Matematica Aplicada III, Universitat Politecnica de Catalunya, 08034 Barcelona, Spain. D.J. BACON and R.C. POND, Professors, are with the Department of Engineering, Materials Science and Engineering, The University of Liverpool, Liverpool L69 3GH, United Kingdom. Contact e-mail:[email protected] This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
behind. The cross-slip process is facilitated by core constriction at the boundary, as expected by Yoo, but the interface still represents a significant barrier to glide. In contrast to this, similar modeling of the {1011} twins revealed a reaction not anticipated from the simple crystallographic analysis, namely total absorption of the screw dislocation by its transformation into a pair of twinning dislocations. Another important result found by computer simulation[3] arises from the interaction
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