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

Development of bond-order potentials for BCC transition metals

In this paper we present bond-order potentials (BOPs) based on the tight-binding method. The potentials have been developed for bcc non-magnetic metals of group V.B (V, Nb, Ta) and group VI.B (Cr, Mo, W) as well as for the ferromagnetic bcc iron. The testing of the transferability of BOPs involves energies of alternate structures, formation energies of vacancies and self-interstitials, transformation paths between different structures and phonon dispersion relations. An example of the application of these potentials is modeling of the structure and glide of 1⁄2 screw dislocations under the effect of applied shear and tensile/compressive stresses

Angular-dependent interatomic potential for large-scale atomistic simulation of the Fe-Cr-H ternary system

The recently developed angular-dependent potential for pure iron was advanced to the interatomic potential of the Fe-Cr-H ternary system. The new potential allows to simulate Fe-Cr alloys for a wide range of compositions and different concentrations of hydrogen. The angular-dependent format of the model and the development procedure based on the machine learning approach allow to achieve a favorable balance between the computation cost and the reliability of the created parametrization. As part of potential validation, we performed a large number of tests of both the binary metallic alloys and hydrogen interactions. The applicability of the potential is demonstrated by large-scale simulations of hydrogen diffusion in the vicinity of crystal defects. © 2022 American Physical Society

Environmentally dependent bond-order potentials: New developments and applications

The bond-order potentials (BOPs) idea employs the orthogonal two-centre tight-binding (TB) representation for the bond energy and the Harris-Foulkes approximation for the repulsive pairwise contribution. In the last ten years, although many efforts have been focused on theoretical calculations of the bond order expression, the BOPs still suffers from the uncertainty of how best to choose the semi-empirical TB parameters that enter the scheme. In this paper, we review recent developments to obtain the reliable and transferable BOP's which help to extend the accuracy and applicability to technologically important multicomponent systems. Firstly, we have found that a simple pair potential is unsuitable for describing the environmental screening effects due to the s and p orbital overlap repulsion in transition metal alloys and therefore the inability to reproduce the negative Cauchy pressures exhibiting in strong covalent systems. By adding the environmental dependent repulsive term, the Cauchy pressure problem has been removed and we are now able to get the BOPs for studying dislocations, extended defects and mechanical properties of high-temperature intermetallic Ti-Al alloys. In particular, new results on the core structures and possible dissociation of different type of dislocations will be discussed. Secondly, we present the first derivation of explicit analytic expressions for environmental dependence of sigma, pi and delta bond integrals by inverting the non-orthogonal matrix. We illustrate the power of this new formalism by showing that it not only captures the transferability of bond integrals between Mo, Si and MoSi2 but also predicts the large discontinuities between first and second nearest neighbours for ppsigma, pppi and ddpi even though absence of any discontinuity for the ddsigma bond integral. A new environmentally dependent BOPs has been developed for bcc-Mo indicating that the core structure of 1/2 screw dislocations is narrower than structures found in previous studies in agreement with recent ab initio calculations. Finally, the new formalism will allow us to study the problem of medium range order found recently in amorphous materials with covalent bonding at large and realistic nanoscale. For the case of a-C where the issue of sp(2) VS sp(3) is very crucial for modelling amorphous structure we found that the a and pi bond integrals are not only transferable between graphite and diamond structures but they are also strongly anisotropic due to inter-plan bonding between graphite sheets

Environmentally dependent bond-order potentials: New developments and applications

The bond-order potentials (BOPs) idea employs the orthogonal two-centre tight-binding (TB) representation for the bond energy and the Harris-Foulkes approximation for the repulsive pairwise contribution. In the last ten years, although many efforts have been focused on theoretical calculations of the bond order expression, the BOPs still suffers from the uncertainty of how best to choose the semi-empirical TB parameters that enter the scheme. In this paper, we review recent developments to obtain the reliable and transferable BOP's which help to extend the accuracy and applicability to technologically important multicomponent systems. Firstly, we have found that a simple pair potential is unsuitable for describing the environmental screening effects due to the s and p orbital overlap repulsion in transition metal alloys and therefore the inability to reproduce the negative Cauchy pressures exhibiting in strong covalent systems. By adding the environmental dependent repulsive term, the Cauchy pressure problem has been removed and we are now able to get the BOPs for studying dislocations, extended defects and mechanical properties of high-temperature intermetallic Ti-Al alloys. In particular, new results on the core structures and possible dissociation of different type of dislocations will be discussed. Secondly, we present the first derivation of explicit analytic expressions for environmental dependence of sigma, pi and delta bond integrals by inverting the non-orthogonal matrix. We illustrate the power of this new formalism by showing that it not only captures the transferability of bond integrals between Mo, Si and MoSi2 but also predicts the large discontinuities between first and second nearest neighbours for ppsigma, pppi and ddpi even though absence of any discontinuity for the ddsigma bond integral. A new environmentally dependent BOPs has been developed for bcc-Mo indicating that the core structure of 1/2<111> screw dislocations is narrower than structures found in previous studies in agreement with recent ab initio calculations. Finally, the new formalism will allow us to study the problem of medium range order found recently in amorphous materials with covalent bonding at large and realistic nanoscale. For the case of a-C where the issue of sp(2) VS sp(3) is very crucial for modelling amorphous structure we found that the a and pi bond integrals are not only transferable between graphite and diamond structures but they are also strongly anisotropic due to inter-plan bonding between graphite sheets