Boundary conditions for dislocation dynamics simulations and stage 0 of BCC metals at low temperature
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Boundary conditions for dislocation dynamics simulations and stage 0 of BCC metals at low temperature Meijie Tang and Ladislas P. Kubin1 PAT Directorate, Lawrence Livermore National Lab., Livermore, CA 94551, USA 1 LEM, CNRS-ONERA, 29, Ave. de la Division Leclerc, BP72, 92322 Chatillon Cedex, France ABSTRACT In order to study the dislocation density evolution of body centered cubic (bcc) crystals at low temperature by dislocation dynamics (DD) simulations, we investigated carefully three different boundary conditions (BC) for DD, i.e., the quasi-free surface BC, the flux-balanced BC, and the periodic BC. The latter two BCs can account for the dislocation loss from the boundary of the finite simulation box. PBC can also eliminate the influence of surfaces and improve the line connectivity. We have found that the PBC provides a convenient and effective boundary condition for DD simulations and have applied it to the study of dislocation density evolution of bcc metals during stage 0 deformation at low temperature. INTRODUCTION At low temperature, the plastic deformation of bcc metals is dominated by the low mobility of the screw dislocations that move by a thermally activated kink pair mechanism. In contrast, the edge dislocations practically see no lattice friction and are extremely mobile. The strong anisotropy in the dislocation mobility of the screw and edge has certain direct consequences on the mechanical properties of bcc metals at low temperature. First, the macroyield stress is determined by the kink pair mechanism and critically depends on the kink pair activation enthalpy [1]. Second, in contrast with the usual behavior, the well established relation between the flow stress and the square root of the dislocation density breaks down [2]. Third, there exists a so-called stage 0 before macro-yield where the plastic deformation is mainly carried out by edge dislocation motion [3]. During this stage, the dislocation density increases significantly. Thus, unlike what is usually assumed, the dislocation density at yield can be substantially larger than the initial density. Our objective is to study the strain hardening of bcc metals at low temperature using DD simulations. In previous work, we have simulated the yield stress and the forest hardening quantitatively using single crystal tantalum as a model material. In this work, we present preliminary results on the dislocation density evolution during stage 0 of the plastic deformation. An essential step towards a realistic simulation consists of implementing a proper boundary condition for the DD simulations that allow realistic dislocation density increase and plastic strain during stage 0 to be reached. In this paper, we first present the three boundary conditions implemented in the DD simulations and discuss their strengths and weaknesses. We then present results on the dislocation density evolution during stage 0 at different temperatures using PBC. Finally, some discussions and conclusions are presented.
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