Assessment of interatomic potentials for atomistic analysis of static and dynamic properties of screw dislocations in W

Screw dislocations in bcc metals display non-planar cores at zero temperature which result in high lattice friction and thermally activated strain rate behavior. In bcc W, electronic structure molecular statics calculations reveal a compact, non-degenerate core with an associated Peierls stress between 1.7 and 2.8 GPa. However, a full picture of the dynamic behavior of dislocations can only be gained by using more efficient atomistic simulations based on semiempirical interatomic potentials. In this paper we assess the suitability of five different potentials in terms of static properties relevant to screw dislocations in pure W. As well, we perform molecular dynamics simulations of stress-assisted glide using all five potentials to study the dynamic behavior of screw dislocations under shear stress. Dislocations are seen to display thermally-activated motion in most of the applied stress range, with a gradual transition to a viscous damping regime at high stresses. We find that one potential predicts a core transformation from compact to dissociated at finite temperature that affects the energetics of kink-pair production and impacts the mechanism of motion. We conclude that a modified embedded-atom potential achieves the best compromise in terms of static and dynamic screw dislocation properties, although at an expense of about ten-fold compared to central potentials

Prediction of the kink-pair formation enthalpy on screw dislocations in alpha-iron by a line tension model parametrized on empirical potentials and first-principles calculations

International audienceThe thermally activated glide of screw dislocations in bcc crystals proceeds through the formation of kinks by pairs, a mechanism named after Peierls for his early work on dislocation theory. A method is proposed to compute the dislocation kink-pair formation enthalpy from density functional theory (DFT) calculations. This method consists of properly adjusting the parameters of a one-dimensional line tension model from atomistic calculations performed in small simulation cells. This model is applied to bcc iron to determine the kink-pair formation enthalpy at different applied stresses from DFT calculations

Attractive interaction between interstitial solutes and screw dislocations in bcc iron from first principles

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First-principles prediction of kink-pair activation enthalpy on screw dislocations in bcc transition metals: V, Nb, Ta, Mo, W, and Fe

International audienceThe atomistic study of kink pairs on screw dislocations in body-centered cubic (bcc) metals is challenging because interatomic potentials in bcc metals still lack accuracy and kink pairs require too many atoms to be modeled by first principles. Here, we circumvent this difficulty using a one-dimensional line tension model whose parameters, namely the line tension and Peierls barrier, are reachable to density functional theory calculations. The model parameterized in V, Nb, Ta, Mo, W, and Fe, is used to study the kink-pair activation enthalpy and spatial extension. Interestingly, we find that the atomistic line tension is more than twice the usual elastic estimates. The calculations also show interesting group tendencies with the line tension and kink-pair width larger in group V than in group VI elements. Finally, the present kink-pair activation energies are shown to compare qualitatively with experimental data and potential origins of quantitative discrepancies are discussed

Ab initio investigation of the Peierls potential of screw dislocations in bcc Fe and W

International audienceThe easy, hard and split core configurations of the screw dislocation and the energy pathways between them are studied in body-centered cubic (bcc) Fe and W using different density functional theory (DFT) approaches. All approaches indicate that in Fe, the hard core has a low relative energy, close to or even below that of the saddle configuration for a straight path between two easy cores. This surprising result is not a direct consequence of magnetism in bcc Fe. Moreover, the path followed by the dislocation core in the (111) plane between easy cores, identified here using two different methods to locate the dislocation position, is almost straight, while the energy landscape between the hard core position and the saddle configuration for a straight path is found to be very flat. These results in Fe are in contrast with predictions from empirical potentials as well as DFT calculations in W, where the hard core has an energy about twice that of the maximum energy along the Peierls barrier, and where the dislocation trajectory between easy cores is curved. Also, the split core configuration is found to be unstable in DFT and of high energy in both Fe and W, in contrast with predictions from most empirical potentials. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Stability of self-interstitial clusters with C15 Laves phase structure in iron

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Ab initio modeling of the two-dimensional energy landscape of screw dislocations in bcc transition metals

International audienceA density functional theory (DFT) study of the 1/2⟨111⟩ screw dislocation was performed in the following body-centered cubic transition metals: V, Nb, Ta, Cr, Mo, W, and Fe. The energies of the easy, hard, and split core configurations, as well as the pathways between them, were investigated and used to generate the two-dimensional (2D) Peierls potential, i.e. the energy landscape seen by the dislocation as a function of its position in the (111) plane. In all investigated elements, the nondegenerate easy core is the minimum energy configuration, while the split core configuration, centered in the immediate vicinity of a ⟨111⟩ atomic column, has a high energy near or above that of the hard core. This unexpected result yields 2D Peierls potentials very different from the usually assumed landscapes. The 2D Peierls potential in Fe differs from the other transition metals, with a monkey saddle instead of a local maximum located at the hard core. An estimation of the Peierls stress from the shape of the Peierls barrier is presented in all investigated metals. A strong group dependence of the core energy is also evidenced, related to the position of the Fermi level with respect to the minimum of the pseudogap of the electronic density of states

First principles investigation of carbon-screw dislocation interactions in body-centered cubic metals

International audienceUsing ab initio density functional theory calculations, we investigate the effect of interstitial carbon solutes on $1/2\langle 111\rangle$ screw dislocations in non-magnetic body-centered cubic transition metals from group 5 (V, Nb, Ta) and group 6 (Mo, W). The two groups are found to display different solute–dislocation interaction behaviors. Group 6 shows a core reconstruction similar to that previously reported in Fe(C): the dislocation adopts a hard-core configuration with the carbon atoms at the center of regular trigonal prisms formed by the metal atoms. The solute–dislocation interaction energies are strongly attractive, ranging from −1.3 to −1.9 eV depending on the metal and the carbon–carbon distance. By way of contrast, the configuration of lowest energy in group 5 consists of the dislocation in its easy core and the carbon atom in a fifth nearest neighbor octahedral site. The configuration is attractive, but less than in group 6. We show that this group dependence is consistent with the carbon local environment in the stable stoichiometric carbide structures, namely cubic NaCl-type for group 5 and hexagonal WC-type for group 6: in both cases the carbon atoms are at the center of octahedra and prisms respectively

Ab initio modeling of dislocation core properties in metals and semiconductors

International audienceDislocation cores, the regions in the immediate vicinity of dislocation lines, control a number of properties such as dislocation mobility, cross-slip and short-range interactions with other defects. The quantitative modeling of dislocation cores requires an electronic-level description of atomic bonding. Ab initio quantum mechanical calculations of dislocation cores based on the density functional theory have progressed rapidly thanks to the steady increase in computing capacities and the development of dedicated numerical methods and codes. Our aim in this overview paper is, after a description of the methodology regarding in particular the boundary conditions, to review the new and unexpected results obtained on dislocation cores from first principles, including the identification of unforeseen stable and metastable cores and the quantitative evaluation of both interaction energies and energy pathways, in pure metals and alloys of different crystallography (FCC, BCC, HCP) as well as semiconductors. We also identify key challenges to be explored in this rapidly growing field