Atomistic Simulation of kinks for 1/2a<111> Screw Dislocation in Ta
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Atomistic Simulation of kinks for 1/2a Screw Dislocation in Ta *XRIHQJ:DQJ$OHMDQGUR6WUDFKDQ7DKLUdD÷ÕQ:LOOLDP$*RGGDUG,,,
Materials and Process Simulation Center, Beckman Institute (139-74) California Institute of Technology, Pasadena, 91125, California ABSTRACT We study the structure and formation energy of kinks in 1/2a screw dislocation in metallic Ta Embedded Atom Model Force Field parameterized using quantum mechanical computations. We studied a/3 kinks using a simulation cell containing four dislocations in a quadrupole arrangement. We impose periodic boundary conditions in the directions perpendicular to [111] and fixed boundaries in the [111] direction. We find that two, energetically equivalent, core configurations for the 1/2a dislocation lead to 8 distinguishable single kinks and 16 kink pairs. The different mismatches of core configurations along [111] direction cause variations in kink formation energy. The lowest formation energy of a kink pair is determined to be 0.73 eV. The geometric features of such kink pair have been studied with the help of structural analysis of the atomistic model. We also compare the activation energy for dislocation motion via the double kink mechanism with the activation energy for a rigid dislocation motion from a dipole annihilation simulation. We find that the migration energy for dislocation motion via double kink formation is 0.016 eV/b, which is less than the quarter of the migration energy associated with the kink free motion of a straight dislocation, 0.073 eV/b. INTRODUCTION Dislocations, and their interactions with other defects, are responsible for the plastic deformation in materials. The studies on structural, energetic and dynamic properties of dislocations are of importance in the design and processing of materials, especially for metals. Although new experimental techniques, such as the scanning tunneling microscope (STM), highresolution transmission electron microscope (HRTEM), etc., are able to observe the structure of the defects in crystals at atomistic level, most of the details on the structure and mobility of dislocations are currently beyond the reach of experiment. Hence, the atomistic level simulation becomes a reliable approach to unveil the secrets about dislocation. In bcc metals, the screw dislocations have lower mobility than the edge components. They have a prevalent role on the low temperature plastic deformation. Especially for this reason, the property and behavior of a/2 screw dislocation have been studied extensively by atomistic simulations. [1,2] Dislocations structure and the mechanism by which they move at T=0 K are different from those at finite temperatures. At T=0 K, dislocations are straight linear defects. When they move, all atoms along the dislocation move in unison. While, at finite temperatures, dislocations are no longer perfect straight lines. Different segments of dislocation may be in different equilibrium positions on its potential energy surface. Kinks, the region connecting neighboring dislocation segments
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