Dynamical Effects on Dislocation Glide through Weak Obstacles
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Dynamical Effects on Dislocation Glide through Weak Obstacles Masato Hiratani and Vasily V. Bulatov Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. ABSTRACT Underdamped dislocation motion through local pinning obstacles is studied computationally using a stochastic dislocation dynamics scheme. The global dislocation velocity is observed to be non-linearly stress dependent. Strongly non-Arrhenius dynamics are found at a higher stress range. The statistical analysis indicates that the correlation of the local dislocation kinetic energy is extended and exceeds the average obstacle spacing as temperature decreases, which can lead to the inertial dislocation bypass of the obstacles. INTRODUCTION The problem of dislocation-local obstacle interaction has been intensively investigated for many years due to its significant importance in the engineering applications of strengthening materials. It has also attracted considerable attention for its manifestation of various complex dynamical phases, also commonly predicted for other topological defects, e.g., vortices in superfluid, or fluxoids in superconductors [1]. Previous studies, however, have been mainly focused on the overdamped system, and the kinetic energy of the defects has been neglected. In the early deformation stage of a well-annealed sample with dilute impurities, each dislocation is well separated from the others, but an individual dislocation encounters random arrays of local obstacles during its glide. Under a constant driving force, each dislocation moves forward and eventually is pinned by surrounding obstacles by their combined pinning effects when the driving force does not exceed the percolation threshold. At finite temperature, dislocations can maintain forward motion over macroscopic distances in a sequence of thermally induced depinning and flight to the next metastable pinned configuration. At low temperature and a low drive, the waiting time tw for the thermal activation is much longer than the flight time tf between the metastable configurations, and the dislocation motion has been often described as hops. In such a case, the activation enthalpy G has a strongly non-linear stress dependence, and the temperature dependence of the dislocation velocity v follows the Arrhenius law as v ∝ exp(− G kT ) . In the limit of small driving force, the material response in this dislocation system is expected to be elastic [2]. Under a higher force, the tw is reduced and can be comparable to tf particularly when the drag is high. In such a case, the flight process is not negligible anymore, and the tf has to be included into the elapsed travel time. Provided that the thermal activation and the flight are independent, one could estimate the average dislocation velocity with these two processes as v ~ Λ (t w + t f ) where Λ is the average distance between the successive metastable configurations. While the flight velocity in steady motion is determined by the kinetic damping B and the local dr
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