Accelerating copper dissociated dislocations to transonic and supersonic speeds
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1137-EE08-08-W10-08
Accelerating copper dissociated dislocations to transonic and supersonic speeds Paulo S. Branicio,1 Hélio Tsuzuki,2 and José Pedro Rino2 1 Materials Theory and Simulation Laboratory, Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, 138632 Singapore 2 Departamento de Física, Universidade Federal de São Carlos, Via Washington Luiz km 235, São Carlos, São Paulo 13565-905, Brazil ABSTRACT The acceleration of dissociated dislocations to transonic and supersonic velocities in copper fcc crystals is investigated using molecular dynamics simulations. A thin and long system with a single stationary dislocation is constructed to study the dislocation acceleration process in anisotropic materials, which have two transverse, vT1 and vT2 , and one longitudinal acoustic velocity, vL along the dislocation glide direction. Copper is used as the representative anisotropic material and the embedded atom method is used to calculate interatomic forces. The common neighborhood parameter and local stresses are used to monitor the position and structure of the dislocations. Initial stationary dislocations on the (111) plane, aligned along the 1 1 2 direction are accelerated in the 110 direction in two different ways. By using the shear stress of a homogeneously sheared simulation box, and by using a strain-rate deformation obtained by shearing the top and bottom atomic layers in opposite directions by a given velocity. Results show that different levels of stress make dislocations accelerate and move in three distinguished regimes: i) in a plateau of velocities close to 1.6 km/s in the subsonic regime just below vT1; ii) in a narrow range of velocities around 2.6 km/s in the first transonic regime between vT1 and vT2; iii) in the second transonic regime, above vT2 but below vL, with increasing velocities for increasing stresses. Supersonic dislocations moving above vL are generated but their motion is transient. To be generated, they require high stresses above the shear strength which trigger spontaneous nucleation of dislocation dipoles throughout the system. The stacking fault width fluctuates around 35 Å in the subsonic regime but decline subsequently with velocity and fluctuates around 13 Å in the second transonic regime. In the first transonic regime however the stacking fault anomalously increases with velocity. Both the decreasing stacking fault width for fast dislocations and the plateau of velocities in the first transonic regime, indicating the existence of a radiation-free state, are in agreement with theoretical predictions. INTRODUCTION
For a long time it was believed that dislocations could not be accelerated to transonic and supersonic velocities, because the linear elasticity theory predicts that infinite amount of energy is needed for a dislocation to be accelerated and move at the shear wave barrier [1]. However, it was recently demonstrated by molecular dynamics (MD) simulations that dislocations in tungsten and aluminum could in fact be accelerated and move
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