Dislocation Nucleation at a Surface Step by a Multiscale Approach
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Dislocation Nucleation at a Surface Step by a Multiscale Approach 1
Chengzhi Li, 2Darren Segall, and 1Guanshui Xu
1 Department 2
of Mechanical Engineering, University of California, Riverside, CA 92521 Department of Applied Physics, California Institute of Technology, Pasadena, CA 91125
ABSTRACT Nucleation of a screw dislocation at a step from a {112} surface of tantalum has been studied by a multiscale approach. The profile of the dislocation, represented by the relative displacement between two adjacent atomic layers along the slip plane, is solved based on the variational boundary integral formulation of the Peierls-Nabarro dislocation model, in which the interatomic potential is incorporated based on atomic calculations of generalized stacking fault energies. The results show that the atomic scale step significantly facilitates dislocation nucleation from the surface. The results from all atom calculations are also presented to corroborate the results obtained by this multiscale approach. INTRODUCTION The energetics of dislocation nucleation from surface heterogeneities such as steps remains a key for understanding a wide variety of scientific and engineering problems, such as deformation mechanisms of nanoscale contacts and the control of threading dislocations in strained heteroepitaxial thin films in microelectronic devices. Many previous studies have pointed out that the stress concentration at surface heterogeneities such as steps is likely the major source to facilitate dislocation nucleation from the surface (e.g. [1]). Recently, atomistic simulations have been carried out to study the influence of steps on dislocation nucleation for Al [2] and Si [3]. The computational demand of all atom methods, however, restricts the height of the step and the simulation cell size; making studies of technological relevant geometries from all atom methods a difficult task. Alternatively, the multiscale approach based on the variational boundary integral formulation of the Peierls-Nabarro model has proven to be an effective approach to study dislocation nucleation problems [4-6] in complicated configurations. In this paper, we use this approach to study dislocation nucleation at a surface step of tantalum. Tantalum has a number of interesting properties which make it a good system to study. It is part of the bcc family of metals whose low temperature plasticity is controlled by long, straight, low mobile screw dislocations and has many active slip planes. These interesting features have inspired numerous atomic level investigations of the motion of such screw dislocations under stress using empirical potential and ab-initio methods [7]. Furthermore, the results from all atom calculations are also presented to corroborate the results obtained by this multiscale approach. ATOMIC SCALE INFORMATION AND CALCULATION In our multiscale approach the many atomic degrees of freedom are reduced to fewer, yet
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more physically intuitive, degrees of freedom. This allows us to gain a greater understanding of the r
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