Reaction Pathway Analysis of Homogeneous Dislocation Nucleation in a Perfect Molybdenum Crystal
- PDF / 276,229 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 113 Downloads / 169 Views
Reaction Pathway Analysis of Homogeneous Dislocation Nucleation in a Perfect Molybdenum Crystal Hasan A. Saeed1, Satoshi Izumi1, Shotaro Hara1 and Shinsuke Sakai1 1 Department of Mechanical Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8654, Japan. ABSTRACT Reaction pathway analysis was carried out for homogeneous dislocation nucleation in perfect crystal Mo. The reaction sampling method employed was based on the Nudged Elastic Band algorithm and other extended schemes. Results obtained were compared with corresponding results for Cu and Si. The stress range for activation energies less than 5 eV is found to be considerably higher for Mo than those for Cu as well as Si. Stresses in excess of 12 GPa make homogeneous dislocation nucleation in Mo an unrealistic transition. The results also show the dislocation cores under this stress range to be diffused, with shear displacement of particles being considerably less than the Burgers vector. Depending on the applied stress, displacement of extra slip-plane atoms can be considerable in Mo. This is in contrast to Cu, in which dislocation nucleation is essentially a two-plane phenomenon. INTRODUCTION Dislocations are responsible for crystalline solids’ fundamental mechanical properties of ductility and strength. Dislocation nucleation, being an atomistic phenomenon, can be best approached by atomistic studies. To date, most of the atomistic level simulation work done on dislocation nucleation has been on heterogeneous systems [1-4]. However, there are an infinite number of configurations of heterogeneous systems, each with its own peculiar complexities, making it difficult to organize the subject in a coherent manner. To observe the phenomenon of dislocation nucleation in a fundamental way, free from complex effects such as stress concentrations, surfaces, etc., analysis of homogeneous dislocation is necessary. Homogeneous dislocation nucleation has also assumed great importance in view of the recent focus on nanoindentation as a means [5] of ascertaining the onset of plasticity in single crystals. From this point of view, there have been efforts of understanding dislocation nucleation using Molecular Dynamics (MD) [6]. A serious drawback of MD, however, is the limited time-scale at its disposal. Consequently, very high strain-rates become necessary, resulting in a wide separation from the experimental time-scale of dislocation nucleation, which, being a stress-mediated thermally activated transition, occurs at stress levels much lower than the athermal stress [7]. The Nudged Elastic Band (NEB) method is a reaction pathway sampling algorithm that makes it possible to directly determine activation parameters (activation energy and activation volume) by searching the minimum energy path (MEP) of the transition. The highest point on the MEP is called the saddle-point and the energy difference between the saddle-point and the initial configuration is the activation energy input required in the form of thermal fluctuation if the transition is to occur. The
Data Loading...
