Dynamic aging of dislocations in materials with a high crystalline relief: Competition between diffusion and impurity en
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STRUCTURE OF CRYSTALS
Dynamic Aging of Dislocations in Materials with a High Crystalline Relief: Competition between Diffusion and Impurity Entrainment B. V. Petukhov Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiœ pr. 59, Moscow, 119333 Russia e-mail: [email protected] Received June 5, 2008
Abstract—A model of the dynamic interaction of dislocations with the impurity subsystem of crystals that have a high lattice potential relief (Peierls barriers) has been developed. It is shown that the microscopic structure of migration barriers for impurities near a dislocation core may cause qualitatively different behavior of the impurity atmosphere on a moving dislocation. It is justified that the impurity kinetics during atmosphere formation includes two stages. The first (initial) stage is fast and significantly nonequilibrium; it is followed by the second stage, characterized by a slower approach to equilibrium. The initial stage manifests itself at a sufficiently fast dislocation motion and may lead to an anomalous increase in the driving force (or the yield strength of the material) with an increase in the temperature in some range. Blocking of the dislocation motion by impurities may cause inverse brittle–ductile transition, which is observed in some materials with an increase (rather than the usual decrease) in temperature. PACS numbers: 61.72.Lk DOI: 10.1134/S1063774509010143
INTRODUCTION
motion, are different for the two noted types of materials. In fcc crystals, the barrier is low and dislocations overcome barriers formed by forest dislocations or particular local centers. In high-barrier materials, the socalled kink mechanism of dislocation motion is implemented [16] and the kinetics of the impurity subsystem perturbation caused by moving dislocations is quite different. The key problem of the theory of dynamic strain aging of materials is the variation in the impurity content in dislocation lines. However, the consideration of this problem in the literature is too simplified; in many cases, the following kinetic law is postulated [14]:
The effect of impurities on the dislocation dynamics significantly modifies the mechanical properties of crystalline materials. This effect is thoroughly studied, both experimentally and theoretically. Static impurities “frozen” into the crystal lattice form individual obstacles to dislocation motion. Mobile impurities diffuse to dislocations and form enriched atmospheres around them which modify the dynamic characteristics of dislocations by their collective effect, thus leading to the so-called dislocation “aging.” At the macroscopic level, this causes “dynamic strain aging” of the whole material. When the dislocation and impurity mobilities are comparable, plastic flow instability often arises, which manifests itself in the form of jumps in the stress–strain curves (the Portevin–Le Chatelier effect [1]). The dynamic aging of dislocations is also considered a possible reason for the temperature anomalies in the yield strengths of material
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