SURFACE EFFECTS ON THE VELOCITY OF DISLOCATIONS AT THE NANOSCALE

We have observed prominent change in the velocity of dislocation in thin films due to the proximity of free surfaces using molecular dynamics simulations. A simple model has been introduced and the numerical implementation of our model is in accord with the simulation results. Such surface effects have been attributed to the change in lattice resistance to dislocation motion.

Frank Reginald Nunes Nabarro MBE. 7 March 1916 — 20 July 2006

Frank Nabarro is remembered as one of the great pioneers who developed the theory of dislocations in solids and thereby strikingly advanced understanding of the mechanical behaviour of metals. In two crucial areas he addressed the problem of the stress required to initiate plastic flow: first in metals alloyed to produce precipitates (precipitation hardening), and second in pure crystals of all types in which dislocation movement is impeded only by the intrinsic lattice resistance (Peierls–Nabarro stress). He predicted the slow creep of crystals at high temperature by the movement of single vacancies (Nabarro–Herring creep), a universal phenomenon of great engineering importance. Working from the University of Witwatersrand he edited Dislocations in solids , a series of review articles that continue to be essential keys to the huge unruly literature on dislocations. In South Africa he had an important role not only in developing solid state physics but also in promoting educational opportunities for students of all races, particularly in planning university expansion to accommodate the much larger student numbers expected after the end of apartheid, a policy that he vehemently opposed.

Energy Changes and the Lattice Resistance

The aim of this paper is to study the effect of relaxing the assumption in the Peierls analysis that the dislocation must be wide compared to the atom spacing. To do this the use of the continuum description of the in-plane strains caused by the presence of an edge dislocation is replaced by an atomistic interaction taken to be linear elastic. It is found that in this case the inplane interactions give a contribution to the overall misfit energy changes that are not present in the Peierls analysis because of the use of a continuum approach. This contribution modifies these energy changes so that the total misfit energy is a minimum at the conventional low energy positions (whereas in the Peierls analysis it is a maximum) and gives values of the Peierls stress in reasonable agreement with those measured.