M. S. Duesbery

Screw dislocation in a model sodium lattice

Abstract The deformation stacking-fault energies and screw dislocation core structures in a model sodium lattice have been calculated for body-centred-cubic and hexagonal close-packed phases, using a fundamental effective ion-ion potential, the reliability of which has been established elsewhere. Particular emphasis is placed upon deviations from continuum elasticity, and it is shown that the use of linear elasticity theory over distances smaller than about ten lattice spacings can lead to serious error. It is shown that the screw dislocation core structures in b.c.c. and h.c.p. lattices have markedly different characteristics, and relative dislocation mobilities in the two phases are discussed in the light of these differences.

Transients in steady-state plastic deformation produced by changes of strain rate

Abstract Transient changes in load following a decrease in strain rate have been measured with a fast recording system during tensile tests of copper crystals. Good agreement between the predicted and measured shapes of the transients have been obtained simply by taking into account the elasticity of the testing machine. A method of extrapolation to obtain the resulting change in flow stress is described. It is suggested that the transients are present because the conditions prevailing on secondary glide systems are characteristic of creep, and that the transients essentially reflect changes in creep rate on secondary systems.

Dislocations in two dimensions I. Floating systems

Abstract The properties of defects in condensed two-dimensional lattices of the inert gases Kr and Xe are examined by computer simulation, with a view to understanding the properties of defects in a novel crystal structure—the triangular lattice. Generalized stacking faults, point defects and dislocation dipoles are considered; the energies, mechanical properties and geometric configurations are calculated. It is found that dislocation dipoles with small separations tend to behave like point-defect clusters. Large dipoles show the expected elastic energy dependence, but interact strongly with the lattice, and have significant Peierls stresses. The defect properties are found to depend principally on the state of lattice expansion or compression. The participation of dislocations in the melting transition is considered; continuous melting via a dislocation-mediated mechanism is found to be unlikely.

The Motion of Screw Dislocations in a Model B.C.C. Sodium Lattice

The behavior of the screw dislocation core in the presence of an external uniaxial stress of varying sense and orientation has been examined for a bcc model lattice, using an effective ion-ion potential for sodium developed from first principles. The Peierls stress is strongly orientation-dependent, and has a minimum value of 0.0076G, where G is the shear modulus. The mechanism for dislocation movement can be planar or nonplanar, depending on the orientation of the applied stress, and can give rise to crystallographic slip on {110| or {112| planes or to noncrystallo-graphic slip.

Dislocations in two dimensions II. Modulated systems

Abstract The properties of defects in two-dimensional systems are of topical physical interest. A specific example is the inert-gas monolayer physisorbed on graphite. The substrate stabilizes a two-dimensional crystalline condensate, usually but not always incommensurate with the substrate. Atomistic computer modelling methods have been used to study the properties of dislocation dipoles and dislocation-domain wall interactions in Xe-C and Kr-C systems. Defects in both structures are found to have much in common with those in the face-centred cubic bulk lattice, such as the mode of dislocation dissociation and the existence of faulted dipoles and stacking fault polygons. In spite of similarities in the binding, the thermodynamics and topology of dislocation-domain wall interactions predict very different properties in the two systems. In particular, vacancy dipoles in incommensurate Kr-C systems are found to exhibit a novel interaction with domain walls which may be of relevance in melting.

Partial Dislocation Interactions in a Face-Centred Cubic Model Sodium Lattice

Preliminary results presented deal with the mutual interaction and individual core structures of Shockley partial dislocations in a fcc model sodium lattice. Deviations from the predictions of linear elasticity are apparent for separations greater than about 10 Burgers vectors, due to boundary conditions, and also for separations less than about 8 Burgers vectors. The nonlinearity at small separations is of an unexpected sense, such that the partial separation is greater than the linear elastic value. The possible sources for nonlinearity are discussed in the light of the results.