Showing papers by "David Bacon published in 2006"

Atomic-scale study of dislocation–stacking fault tetrahedron interactions. Part I: mechanisms

Abstract: Stacking fault tetrahedra (SFTs) are formed under irradiation in fcc metals and alloys. The high number density of SFTs observed suggests that they should contribute to radiation-induced hardening and, therefore, be taken into account when estimating mechanical property changes of irradiated materials. The key issue in this is to describe the interaction between a moving dislocation and an individual SFT, which is distinguished by a small physical size of the order of ∼1–10 nm. We have performed atomistic simulations of edge and screw dislocations interacting with SFTs of different sizes at different temperatures and strain rates. Five possible interaction outcomes have been identified, involving either partial absorption, or shearing or restoration of SFTs. The mechanisms that give rise to these processes are described and their dependence on interaction parameters, such as SFT size, dislocation–SFT geometry, temperature and stress/strain rate are determined. Mechanisms that help to explain the formation...

Computer simulation of reactions between an edge dislocation and glissile self-interstitial clusters in iron

Abstract: Clusters of self-interstitial atoms (SIAs) are formed in metals by high-energy displacement cascades, often in the form of small dislocation loops with a perfect Burgers vector, b. Atomic-scale computer simulation is used here to investigate their reaction with an edge dislocation gliding in α-iron under stress for the situation where b is inclined to the dislocation slip plane. The b of small loops (37 SIAs here) changes spontaneously and the interstitials are absorbed as a pair of superjogs. The line glides forward at critical stress τc when one or more vacancies are created and the jogs adopt a glissile form. A large loop (331 SIAs here) reacts spontaneously with the dislocation to form a segment with b = 〈100 〉, which is sessile on the dislocation slip plane, and as applied stress increases the dislocation side arms are pulled into screw orientation. At low temperature (100 K), the 〈100〉 segment remains sessile and the dislocation eventually breaks free when the screw dipole arms cross-slip and annihi...

Computer simulation of dislocation–solute interaction in dilute Fe–Cu alloys

Abstract: The effects of the substitutional element copper in solution in ?-iron on glide of a ? 111 { 110 } edge dislocation are investigated by atomic-scale computer simulation. Under static conditions (temperature T = 0?K), single copper atoms and nearest-neighbour pairs in the first atomic plane below the dislocation slip plane provide the strongest barrier to slip, in partial agreement with continuum theory. This contrasts with recent simulation results for the Ni?Al fcc alloy (Rodary et al 2004 Phys. Rev. B 70 054111), where Al atoms displaced into nearest-neighbour coordination across the slip plane form the strongest obstacles. The dynamics of dislocation glide in Fe?Cu solid solution at T > 0?K are determined as a function of solute concentration. Parameters such as velocity, critical stress and drag coefficient are analysed. Again, there are differences from the Ni?Al system. The results are discussed in terms of the static strength of solute configurations and the different crystal structure of iron and nickel.

Statistics of primary damage creation in high-energy displacement cascades in copper and zirconium

Abstract: A comparison of radiation defect creation in high-energy displacement cascades in two metals with close-packed crystal structures, namely FCC (Cu) and HCP (α-Zr) has been conducted. The report consolidates the results of molecular dynamic simulations of over 400 displacement cascades in copper and 240 cascades in zirconium. The simulations were carried out for a wide range of temperature (100–900 K), and primary knock-on atom (PKA) energy (5–25 keV). For both metals the dependence of the number of Frenkel pairs and the fraction of point defects in clusters on temperature and PKA energy were obtained. Similarities and differences in point defect cluster stability are contrasted for the two metals and the influence of material on the accumulation of primary damage is considered.

Identification and morphology of point defect clusters created in displacement cascades in α-zirconium

Abstract: Atomic scale computer simulation of high-energy displacement cascades in α-zirconium has been carried out for a wide range of primary knock-on atom energy (10–25 keV) and temperature (100–600 K) A large number of vacancy and self-interstitial atom (SIA) clusters of various shapes and sizes was generated in more than 240 cascades In spite of the variety of cluster structures, they can be categorized into three distinct configurations for vacancy and SIA clusters Internal atom arrangements for all point defect clusters have been established and the type of stacking faults that can be assigned to vacancy clusters identified The explanation of different relaxation patterns of prismatic vacancy loops occupying either even or odd number of neighbouring close-packed planes is suggested Transformation of the pyramid-like vacancy cluster with a basal-plane extrinsic fault into prismatic vacancy loop is considered

Weak gravitational shear and flexion with polar shapelets

Abstract: We derive expressions, in terms of "polar shapelets", for the image distortion operations associated with weak gravitational lensing. Shear causes galaxy shapes to become elongated, and is sensitive to the second derivative of the projected gravitational potential along their line of sight; flexion bends galaxy shapes into arcs, and is sensitive to the third derivative. Polar shapelets provide a natural representation, in which both shear and flexion transformations are compact. Through this tool, we understand progress in several weak lensing methods. We then exploit various symmetries of shapelets to construct a range of shear estimators with useful properties. Through an analogous investigation, we also explore several flexion estimators. In particular, some of the estimators can be measured simultaneously and independently for every galaxy, and will provide unique checks for systematics in future weak lensing analyses. Using simulated images from the Shear TEsting Programme (STEP), we show that we can recover input shears with no significant bias. A complete software package to parametrize astronomical images in terms of polar shapelets, and to perform a full weak lensing analysis, is available at this http URL .

Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni3Al

Abstract: Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni3Al are simulated using molecular dynamics methods. The threshold energy for sputtering, E sp, and adatom creation, E ad, are determined as a function of recoil direction. The study is relevant to FEG STEM (a scanning transmission electron microscope fitted with a field emission gun) experiments on preferential Al sputtering and/or enhancement of the Ni–Al ratio near boundaries. Surfaces intersected by {110} and {111} APBs have minimum E sp of 6.5 eV for an Al atom on the Ni–Al mixed (M) surface, which is close to the value of 6.0 eV for a perfect M surface. High values of E sp of an Al atom generally occur at a large angle to the surface normal and depend strongly on the detailed atomic configuration of the surface. The mean E sp, averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for E ad imply that an...