Critical Conditions for Dislocation Nucleation from Surface Steps
- PDF / 142,445 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 25 Downloads / 257 Views
P8.40.1
Critical Conditions for Dislocation Nucleation from Surface Steps Chengzhi Li and Guanshui Xu Department of Mechanical Engineering University of California at Riverside, Riverside, CA 92521 Abstract The critical conditions for dislocation nucleation from the surface steps of various geometries are analyzed based on the Peierls-Nabarro dislocation model. By modeling a surface step as part of a threedimensional crack surface, the half space problem is transferred into an equivalent three dimensional crack problem in an infinite medium. The profiles of embryonic dislocations, corresponding to the relative displacements between the two adjacent atomic layers along slip planes, are then rigorously solved through the variational boundary integral method. The critical conditions for dislocation nucleation are determined by solving the stress dependent activation energies required to activate embryonic dislocations from their stable to unstable saddle point configurations. For a given slip plane, the effects of step geometry such as the step height and inclined angle on dislocation nucleation are analyzed in detail. The results show that the atomic scale steps may reduce the critical stress required for dislocation nucleation from the surface by several factors. Compared to previous analyses of this type of problem based on continuum elastic dislocation theory, the presented analysis eliminates the uncertain core cutoff parameter by allowing for the existence of an extended dislocation core as the embryonic dislocation evolves. Because of the serious limitation of direct atomic simulation for this type of problem, the presented methodology of incorporating atomic information into continuum approach appears to be particularly noteworthy for providing insights of energetics of the atomic processes involved in dislocation nucleation. Introduction The critical conditions for dislocation nucleation from the stressed surface have been of considerable interest for understanding a wide variety scientific and engineering problems such as deformation mechanisms of nanoscale contact and control of dislocation density in strained heteroepitaxial thin films in microelectronic devices. Previous analyses have been focused on critical conditions for dislocation nucleation from the perfectly flat surface using either atomic simulation, the combined atomistic and continuum model such as the Peierls-Nabarro dislocation model, or continuum elastic dislocation theory. Many researches pointed out that the stress concentration caused by surface heterogeneities such as steps and microcracks should significantly facilitate dislocation nucleation from the stressed surface [1-4]. The effect of an atomic scale surface crack on dislocation nucleation has been previously studied based on the Peierls and Nabarro dislocation model [4]. The effects of the surface steps on dislocation nucleation, however, have not been addressed in the literature in detail. We present an analysis of this problem based on the Peierls-Nabarro dislocation model [5, 6]
Data Loading...
