Core structure and Peierls Barrier of screw dislocations in α-Fe from first-principles

Full text of publication follows: The specificity of the plastic behavior of body-centered cubic (bcc) metals at low temperature is attributed to the non-planar core structure of 1/2 <111> screw dislocations which induce a high lattice-friction Peierls stress. The goal of this work is to obtain a quantitative description of the structure and properties of these dislocation cores in bcc iron by performing first principles electronic structure calculations. These calculations are carried out within the DFT framework using localized basis functions as implemented in the SIESTA code. The results obtained for the {211} and {110} γ-surfaces suggest a non-degenerate core structure, symmetrically spread across the three {110} planes, in contrast with most empirical potential results. We have successfully set up two geometries for the study of the dislocation core structure and Peierls potential: the cluster approach with a single dislocation in a cylindrical slab with fixed atomic positions at the free surfaces; and a periodic array of dipolar or quadrupolar distributions of dislocations. The DFT calculations yield a nondegenerate core structure and the two sets of results are in excellent agreement with each other. The easy core is found to be more stable than the hard core by about 50 meV/b. The very weak deviation from the purely elastic displacement field and the change in magnetic moment have been analyzed. In the dipolar distribution, the interaction between dislocations is dominated by elastic effects, but significant anisotropic core-core interactions have been evidenced. The Peierls barrier has been determined using the drag method within both cluster and quadrupolar arrangements and a very good agreement is obtained between the two geometries on the energy barrier, amounting to 30 meV/b. Empirical potentials for iron have been benchmarked on these results; the EAM potential proposed by Mendeleev et al. compares well with the DFT results for the y-surfaces and the dislocation core structure, but not for the deviation from elasticity and for Peierls potential. This work was performed in the framework of the European Fusion Materials Modelling programme. (authors)