Theory and simulation of the diffusion of kinks on dislocations in bcc metals (original) (raw)

Isolated kinks on thermally fluctuating 1/2 111 screw, 100 edge, and 1/2 111 edge dislocations in bcc iron are simulated under zero stress conditions using molecular dynamics (MD). Kinks are seen to perform stochastic motion in a potential landscape that depends on the dislocation character and geometry, and their motion provides fresh insight into the coupling of dislocations to a heat bath. The kink formation energy, migration barrier, and friction parameter are deduced from the simulations. A discrete Frenkel-Kontorova-Langevin model is able to reproduce the coarse-grained data from MD at ∼10 −7 of the computational cost, without assuming an a priori temperature dependence beyond the fluctuation-dissipation theorem. Analytical results reveal that discreteness effects play an essential role in thermally activated dislocation glide, revealing the existence of a crucial intermediate length scale between molecular and dislocation dynamics. The model is used to investigate dislocation motion under the vanishingly small stress levels found in the evolution of dislocation microstructures in irradiated materials. PACS number(s): 61.72. Hh, 31.15.xv Dislocation motion is limited by two general processes: the formation and migration of kinks and pinning by impurities and other defects. 1 In this paper, we investigate the motion of kink-limited screw and edge dislocations in bcc Fe, where the kink formation energy is much larger than the thermal energy. To obtain dislocation motion on the time scales accessible to molecular dynamics (MD) simulations, some researchers have resorted to inducing kink formation by applying stresses some six orders of magnitude greater than those pertaining experimentally. 2,3 But, dislocation core structures and Peierls barriers are known to be highly stress dependent, 4 making it difficult to relate simulation to the vanishingly low stress conditions found in thermally activated evolution of dislocation microstructures.