Equilibrium and Dynamic Properties of Dislocation Dipoles
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0976-EE02-04
Equilibrium and Dynamic Properties of Dislocation Dipoles Patrick Veyssiere LEM, CNRS-ONERA, BP 72, Chatillon, 92322, France
ABSTRACT This paper embraces several aspects related to dislocation self-organization in single slip. The formation of prismatic loops, seminal to the nucleation and growth of dislocation braids, is a dynamical process involving elastic dipolar interactions. Preliminary simulations of the concerted process of loop generation are presented. Elastic properties of dislocation dipoles are analysed as a function of dipole character. Equilibrium properties are investigated under the isotropic and anisotropic elastic approximations. The orientation dependence of the so-called dipole passing stress is examined, and its implication in the elementary mechanisms that take place under constrained deformation conditions, such as fatigue cycling, are discussed. INTRODUCTION The property that dislocations belonging to a given slip system can spontaneously engender obstacles to their own propagation is a major outcome of the early transmission electron microscope (TEM) explorations of deformation microstructures in relation with the plastic behaviour of materials. This property, which was totally unexpected at that time [1], has been the object of an almost uninterrupted concern since then. It has recently gained further interest with the rapid development of numerical experiments and, subsequently, with the flourishing investigations of single crystal pillar properties. The scenario that reflects TEM observations best is due to Kratochvil and co-workers [2-7]. It relies on binding interactions between prismatic loops and mobile dislocations and, more specifically, on the sweeping of the former by the latter. A substantial part of the work devoted to this investigation has concentrated on the origin of a length scale within the self-organized microstructure [3-5]. Comparatively little is established, however, as to the origin of the prismatic loops and to the local mechanism(s) by which stable multipolar tangles are created. The present work focuses on selected aspects related to dislocation self-organization under single slip conditions in an effort to clarify the role played by dipolar interactions and loop string generation. In particular, a preliminary investigation by dynamic simulations of dislocation interactions yielding prismatic loops is presented. In addition, the orientation dependence of equilibrium and dynamic properties of dislocation dipoles is examined under isotropic and anisotropic elasticity.
THE DYNAMICAL GENERATION OF LOOP STRINGS In the early stages of single slip straining, dislocations exhibit a pronounced tendency towards self-organization forming braids. The braids are comprised of elemental features that are mostly dipolar in nature, i.e. prismatic loops, hairpins… In between the braids, the distribution of prismatic loops and hairpins follows a certain organization whereby the extremity of a given loop (hairpin) is often aligned in the screw direction with the ext
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