Stress and temperature dependence of screw dislocation mobility inα-Fe by molecular dynamics

Abstract

The low-temperature plastic yield of α-Fe single crystals is known to display a strong temperature dependence and to be controlled by the thermally activated motion of screw dislocations. In this paper, we present molecular dynamics simulations of 1 2 111 {112} screw dislocation motion as a function of temperature and stress in order to extract mobility relations that describe the general dynamic behavior of screw dislocations in pure α-Fe. We find two dynamic regimes in the stress-velocity space governed by different mechanisms of motion. Consistent with experimental evidence, at low stresses and temperatures, the dislocations move by thermally activated nucleation and propagation of kink pairs. Then, at a critical stress, a temperature-dependent transition to a viscous linear regime is observed. Critical output from the simulations, such as threshold stresses and the stress dependence of the kink activation energy, are compared to experimental data and other atomistic works with generally very good agreement. Contrary to some experimental interpretations, we find that glide on {112} planes is only apparent, as slip always occurs by elementary kink-pair nucleation/propagation events on {110} planes. Additionally, a dislocation core transformation from compact to dissociated has been identified above room temperature, although its impact on the general mobility is seen to be limited. This and other observations expose the limitations of inferring or presuming dynamic behavior on the basis of only static calculations. We discuss the relevance and applicability of our results and provide a closed-form functional mobility law suitable for mesoscale computational techniques.

Atomistic Simulations in bcc Metals

“…5.2 that the dissipation parameter for a kink is proportional to the dissipation parameter for the host dislocation line, which should therefore also be temperature independent. Although this agrees with several [7,[29][30][31], though not all [32,33], atomistic simulations of dislocations and other defect systems in bcc Iron, decades of theoretical work [10,34,35] conclude that the dissipation parameter for a dislocation should increase linearly with temperature due to the increasing phonon population. We return to this important issue concerning the coupling of dislocations to the heat bath in the following section on screw dislocations, as the diffusive form (4.7) they exhibit allows an even more direct investigation of γ kink .…”

“…The vast majority of existing potentials predict a screw dislocation has multiple core structures [38,39], leading many authors to suggest that a screw dislocation may pass through a metastable core structure during the kink nucleation process [37]. Under an applied stress this can produce a new kink formation pathway leading to a discontinuity in the flow stress [6,14,30]. However, this discontinuity is not shown in experiment, and recent ab initio calculations [40,41] rule out any metastable core structure, with the nucleation pathway seen to occur almost entirely in the {110} slip planes.…”

Kink pair production and dislocation motion

“…4 below), and is never linear in the force, however small the force is taken to be. This mathematical effect is not captured by existing nonlinear forms for H * such as the elasticity-derived expression H 0 (1 −  F p ) q   2425.…”

Crystal plasticity finite element analysis of ferritic stainless steel for sheet formability prediction

“…The material parameters may be obtained from finer scale simulations such as dislocation dynamics (Queyreau et al, 2009;Wang and Beyerlein, 2011) and molecular dynamics (Gilbert et al, 2011;Lim et al, 2015). A few parameters such as the critical resolved shear stress may also be obtained directly from tests on single crystals (Franciosi, 1983).…”