Showing papers by "Adrian P. Sutton published in 2013"

Theory and simulation of the diffusion of kinks on dislocations in bcc metals

Abstract: Isolated kinks on thermally fluctuating $1/2\ensuremath{\langle}111\ensuremath{\rangle}$ screw, $\ensuremath{\langle}100\ensuremath{\rangle}$ edge, and $1/2\ensuremath{\langle}111\ensuremath{\rangle}$ edge dislocations in bcc iron are simulated under zero stress conditions using molecular dynamics (MD). Kinks are seen to perform stochastic motion in a potential landscape that depends on the dislocation character and geometry, and their motion provides fresh insight into the coupling of dislocations to a heat bath. The kink formation energy, migration barrier, and friction parameter are deduced from the simulations. A discrete Frenkel-Kontorova-Langevin model is able to reproduce the coarse-grained data from MD at $\ensuremath{\sim}$${10}^{\ensuremath{-}7}$ of the computational cost, without assuming an a priori temperature dependence beyond the fluctuation-dissipation theorem. Analytical results reveal that discreteness effects play an essential role in thermally activated dislocation glide, revealing the existence of a crucial intermediate length scale between molecular and dislocation dynamics. The model is used to investigate dislocation motion under the vanishingly small stress levels found in the evolution of dislocation microstructures in irradiated materials.

A dynamic discrete dislocation plasticity method for the simulation of plastic relaxation under shock loading

Abstract: In this article, it is demonstrated that current methods of modelling plasticity as the collective motion of discrete dislocations, such as two-dimensional discrete dislocation plasticity (DDP), ar

Quantum-classical simulations of the electronic stopping force and charge on slow heavy channelling ions in metals

Abstract: By simulating the passage of heavy ions along open channels in a model crystalline metal using semi-classical Ehrenfest dynamics we directly investigate the nature of non-adiabatic electronic effects Our time-dependent tight-binding approach incorporates both an explicit quantum mechanical electronic system and an explicit representation of a set of classical ions The coupled evolution of the ions and electrons allows us to explore phenomena that lie beyond the approximations made in classical molecular dynamics simulations and in theories of electronic stopping We report a velocity-dependent charge-localization phenomenon not predicted by previous theoretical treatments of channelling This charge localization can be attributed to the excitation of electrons into defect states highly localized on the channelling ion These modes of excitation only become active when the frequency at which the channelling ion moves from interstitial point to equivalent interstitial point matches the frequency corresponding to excitations from the Fermi level into the localized states Examining the stopping force exerted on the channelling ion by the electronic system, we find broad agreement with theories of slow ion stopping (a stopping force proportional to velocity) for a low velocity channelling ion (up to about 0:5 nm fs 1 from our calculations), and a reduction in stopping power attributable to the charge localization effect at higher velocities By exploiting the simplicity of our electronic structure model we are able to illuminate the physics behind the excitation processes that we observe and present an intuitive picture of electronic stopping from a real-space, chemical perspective (Some figures may appear in colour only in the online journal)

A Dynamic Discrete Dislocation Plasticity Method for the Dimulation of Plastic Relaxation under Shock Loading

Abstract: In this article, it is demonstrated that current methods of modelling plasticity as the collective motion of discrete dislocations, such as two-dimensional discrete dislocation plasticity (DDP), are unsuitable for the simulation of very high strain rate processes (106 s−1 or more) such as plastic relaxation during shock loading. Current DDP models treat dislocations quasi-statically, ignoring the time-dependent nature of the elastic fields of dislocations. It is shown that this assumption introduces unphysical artefacts into the system when simulating plasticity resulting from shock loading. This deficiency can be overcome only by formulating a fully time-dependent elastodynamic description of the elastic fields of discrete dislocations. Building on the work of Markenscoff & Clifton, the fundamental time-dependent solutions for the injection and non-uniform motion of straight edge dislocations are presented. The numerical implementation of these solutions for a single moving dislocation and for two annihilating dislocations in an infinite plane are presented. The application of these solutions in a two-dimensional model of time-dependent plasticity during shock loading is outlined here and will be presented in detail elsewhere.

Huang Diffuse Scattering from Small Planar Dislocation Loops

Abstract: This paper gives out a theoretical framework of electron/X-ray Huang diffuse scattering intensity at the immediate vicinity of Bragg reflection in reciprocal space. Nodal lines of two types in the simulated patterns of Huang diffuse scattering intensity are discussed in connection with a loop shape factor and the Huang diffuse scattering intensity from infinitesimal loops. It is suggested that the Huang diffuse scattering method is supplementary to the conventional TEM amplitude contrast imaging techniques and it has advantages in characterizing the morphology of very small dislocation loop when other methods fail.