Screw dislocation structure and mobility in body centered cubic Fe predicted by a Gaussian Approximation Potential. (December 2018) - Maresca, Francesco
Abstract The plastic flow behavior of bcc transition metals up to moderate temperatures is dominated by the thermally activated glide of screw dislocations, which in turn is determined by the atomic-scale screw dislocation core structure and the associated kink-pair nucleation mechanism for glide. Modeling complex plasticity phenomena requires the simulation of many atoms and interacting dislocations and defects. These sizes are beyond the scope of first-principles methods and thus require empirical interatomic potentials. Especially for the technological important case of bcc Fe, existing empirical interatomic potentials yield spurious behavior. Here, the structure and motion of the screw dislocations in Fe are studied using a new Gaussian Approximation Potential (GAP) for bcc Fe, which has been shown to reproduce the potential energy surface predicted by density-functional theory (DFT) and many associated properties. The Fe GAP predicts a compact, non-degenerate core structure, a single-hump Peierls potential, and glide on {110}, consistent with DFT results. The thermally activated motion at finite temperatures occurs by the expected kink-pair nucleation and propagation mechanism. The stress-dependent enthalpy barrier for screw motion, computed using the nudged-elastic-band method, follows closely a form predicted by standard theories with a zero-stress barrier of ~1 eV, close to the experimental value of 0.84 eV, and a Peierls stress of ~2 GPa consistent with DFT predictions of the Peierls potential. Iron: simulating screw dislocations A Gaussian Approximation Potential (GAP) can successfully reproduce the structure and motion of screw dislocations in iron. A team led by Francesco Maresca at the EPFL in Lausanne, Switzerland, used the recently developed GAP for iron to simulate aspects of screw dislocation behavior via both molecular statics and molecular dynamics simulations, and validated their results against density-functional theory calculations. The GAP for iron also successfully simulated kink-pair nucleation and screw dislocation glide along the {110} plane, while the stress-dependence of the enthalpy barrier for kink-pair nucleation was consistent with long-standing theories. This potential could be used to identify the atomic-scale origins of many other important plasticity phenomena such as dislocations interacting with radiation damage and cracks in iron and other body centered cubic materials.