Dislocation Dynamics Simulations of Junctions in Hexagonal Close-Packed Crystals
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Dislocation Dynamics Simulations of Junctions in Hexagonal Close-Packed Crystals Chi-Chin Wu 1,2*, Sylvie Aubry 3, Peter W. Chung2, Athanasios Arsenlis3 1
Oak Ridge Affiliated Universities Maryland, 4692 Millennium Drive, Suite 101, Belcamp MD 21017, U.S.A. * Email: [email protected] 2 Computational and Information Sciences Directorate, U. S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005, U.S.A. 3 High Performance Computational Materials Science and Chemistry Group, Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, P.O. Box 808, L-367, Livermore, CA 94551, U.S.A. ABSTRACT The formation and strength of dislocations in the hexagonal closed-packed material are studied through dislocation junctions and the critical stress required to completely break them. Dislocation dynamics calculations of junctions are compared to an analytical line tension approximation in order to verify the simulations. Results show agreements between the models. Also the critical shear stress necessary to break a short and a long dislocation junction is computed numerically. Unzipping envelopes are mapped out for these junctions to describe their stability regions as functions of resolved shear stresses on the glide planes. The example of two non-coplanar binary dislocation junctions with slip systems [2 -1 -1 0] (0 1 -1 0) and [-1 2 -1 0] (0 0 0 1) corresponding to a prismatic and basal slip respectively is chosen to verify and validate our implementation. INTRODUCTION Dislocation mechanisms in hexagonal closed-packed (HCP) crystals have traditionally not been as well studied as those in cubic systems such as face-centered cubic (FCC) and bodycentered cubic (BCC) crystals. HCP possesses reduced crystal symmetry and different slip modes caused by the hexagonal lattice and material-dependent c/a ratios (where c and a are the lattice spacings on the basal plane and its normal direction, respectively). Instead of unanimously gliding on the close-packed basal planes (0 0 0 1) as all dislocations do in FCC and BCC, the slip systems for dislocations in HCP also involve higher-order non-basal slip planes including {1 0 -1 0} prismatic, first-order {1 0 -1 1} pyramidal, and second-order {1 1 -2 2} pyramidal slip planes, and several possible Burgers vectors including , , or [1 - 2]. Despite these complexities, the HCP structure and all slip systems and cross-slip planes can still be conveniently described by employing a simple three-index orthogonal coordinate system based on the double tetrahedron notation for DD simulations [3 - 4]. In plastic deformation, dislocation junctions can form energetically (the so called “zipping” phenomena) but also can be destroyed via “unzipping” by an applied stress [5 - 15]. When an external stress is applied, all dislocation segments involved in the junction may bow out via the Frank-Read (F-R) mechanism [16]. The changing strain/stress field surrounding dislocations induced by the dynamically changing dislocation configurations can affect the process of junction unz
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