Sidney Yip

Molecular dynamics calculation of free energy

The results of a systematic study of a recently proposed method by Frenkel and Ladd for calculating free energies via molecular dynamics are reported. Internal measures of the error, the effect of varying parameters, and comparison of the relative computational efficiency of the method compared to other methods is considered. In particular, agreement with the quasiharmonic method is shown for temperatures up to 75% of melting.

On the mechanism of grain-boundary migration in metals : a molecular dynamics study

The migration of a (100) {theta}=43.6{degree} ({Sigma}29) twist grain boundary is observed during the course of a molecular-dynamics simulation. The atomic-level details of the migration are investigated by determining the time dependence of the planar structure factor, a function of the planar interparticle bond angles, and the location of the center of a mass of planes near the grain boundary. It is found that a migration step consists of local bond rearrangements which, when the simulation cell is made large enough, produce domain-like structures in the migrating plane. Although no overall sliding is observed during migration, a local sliding of the planes near the migrating grain boundary accompanies the migration process. It is suggested that a three-dimensional cloud of thermally produced Frenkel-like point defects near the boundary accompanies, and facilitates, its migration.

On the relevance of extrinsic defects to melting: A molecular dynamics study using an embedded atom potential

Three basic classes of theories of melting have been proposed. The first treats the phenomenon as a homogeneous, bulk process involving a lattice instability, in which the (temperature-dependent) normal modes of the lattice become unstable at sufficiently high temperature. The second involves a mechanical instability occurring when the concentration of intrinsic (i.e., thermally-generated) defects reaches a critical concentration. The third class, originating from experimental observation, describes melting as nucleating at extrinsic defects such as free surfaces, grain boundaries (GBs) etc. Several recent measurements demonstrate that when the surface conditions are modified, the melting point can be depressed or the solid can be substantially superheated. The implication is that melting is basically a heterogeneous process and the mechanism of nucleation at extrinsic defects generally determines the kinetics.