Screw dislocation structure and mobility in body centered cubic Fe predicted by a Gaussian Approximation Potential

Abstract

The plastic flow behavior of bcc transition metals up to moderate 3 temperatures is dominated by the thermally activated glide of screw 4 dislocations, which in turn is determined by the atomic-scale screw dis- 5 location core structure and the associated kink-pair nucleation mech- 6 anism for glide. Modeling complex plasticity phenomena requires the 7 simulation of many atoms and interacting dislocations and defects. 8 These sizes are beyond the scope of first-principles methods and thus 9 require empirical interatomic potentials. Especially for the technolog- 10 ical important case of bcc Fe, existing empirical interatomic potentials 11 yield spurious behavior. Here, the structure and motion of the screw 12 dislocations in Fe are studied using a new Gaussian Approximation 13 Potential (GAP) for bcc Fe, which has been shown to reproduce the 14 potential energy surface predicted by density-functional theory (DFT) and many associated properties. The Fe GAP predicts a compact, 16 non-degenerate core structure, a single-hump Peierls potential, and 17 glide on {110}, consistent with DFT results. The thermally-activated 18 motion at finite temperatures occurs by the expected kink-pair nucle- 19 ation and propagation mechanism. The stress-dependent enthalpy 20 barrier for screw motion, computed using the nudged-elastic-band 21 method, follows closely a form predicted by standard theories with a 22 zero-stress barrier of ∼ 1eV, close to the experimental value of 0.84eV, 23 and a Peierls stress of ∼ 2GPa consistent with DFT predictions of the 24 Peierls potential.