Gauge transformations to combine multi-component many-boby interatomic potentials

Many-body interatomic potentials play an important role in atomistic modelling of materials. For pure elements it is known that there exist gauge transformations that can change the form of the potential functions without modifying its properties. These same transformations, however, fail when applied to alloys. Even though different research groups may use the same potentials to describe pure elements, the gauges employed for fitting alloys will generally be different. In this scenario, it is a priori impossible to merge them into one potential describing the combined system, and thus no advantage is taken from state-of-the-art developments in the literature. Here, we generalise the gauge transformations applied to pure species in order to leave the properties of alloys invariant. Based on these transformations, a strategy to merge potentials developed within different gauges is presented, aiming at the description of the combined system. Advantage of existing state-of-the-art potentials is so taken, thus focusing the efforts on fitting only the missing interactions. Such a procedure constitutes a helpful tool for the development of potentials targeted to alloys of increased complexity, while maintaining the description quality of their constituents.Fil: Bonny, G.. Nuclear Materials Science Institute; BélgicaFil: Pasianot, Roberto Cesar. Comision Nacional de Energia Atomica. Centro Atomico Constituyentes. Departamento de Materiales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Instituto Sabato; Argentin

Iron chromium potential to model high-chromium ferritic alloys

We present an Fe­Cr interatomic potential to model high-Cr ferritic alloys.The potential is fitted to thermodynamic and point-defect propertiesobtained from density functional theory (DFT) calculations and experi-ments. The developed potential is also benchmarked against otherpotentials available in literature. It shows particularly good agreementwith the DFT obtained mixing enthalpy of the random alloy, the formationenergy of intermetallics and experimental excess vibrational entropy andphase diagram. In addition, DFT calculated point-defect properties,both interstitial and substitutional, are well reproduced, as is the screwdislocation core structure. As a first validation of the potential, we studythe precipitation hardening of Fe­Cr alloys via static simulations of theinteraction between Cr precipitates and screw dislocations. It is concludedthat the description of the dislocation core modification near a precipitatemight have a significant influence on the interaction mechanisms observedin dynamic simulations.Fil: Bonny, G.. No especifíca;Fil: Pasianot, Roberto Cesar. Universidad Nacional de San Martín. Instituto Sabato; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; ArgentinaFil: Terentyev, D.. No especifíca;Fil: Malerba, L.. No especifíca

Kinetics versus thermodynamics in materials modeling: The case of the di-vacancy in iron

Monte Carlo models are widely used for the study of microstructural and microchemical evolution of materials under irradiation. However, they often link explicitly the relevant activation energies to the energy difference between local equilibrium states. We provide a simple example (di-vacancy migration in iron) in which a rigorous activation energy calculation, by means of both empirical interatomic potentials and density functional theory methods, clearly shows that such a link is not granted, revealing a migration mechanism that a thermodynamics-linked activation energy model cannot predict. Such a mechanism is, however, fully consistent with thermodynamics. This example emphasizes the importance of basing Monte Carlo methods on models where the activation energies are rigorously calculated, rather than deduced from widespread heuristic equations.Fil: Djurabekova, F.. No especifíca;Fil: Malerba, L.. No especifíca;Fil: Pasianot, Roberto Cesar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; ArgentinaFil: Olsson, P.. No especifíca;Fil: Nordlund, K.. No especifíca

THEORETICAL AND NUMERICAL STUDY OF HYDROGEN DIFFUSION IN Zr-NbALLOYS

Within the framework of the delayed hydride cracking phenomena reported for the pressure tubes of CANDU-typenuclear reactors, we study the influence of the Zr-β(∼20 %Nb -∼80 %Zr) continuous phase on the hydrogen diffusion coefficient,DH, through the Zr-α/ Zr-βbiphasic alloy. We propose an improved phenomenological model forDHwithrespect to those found in the literature. Furthermore, we study the influence of the Nb content onDHfor the cubic phase Zr-β, employing the transition state theory furnished with ab-initio parameters provided by the SIESTA code. In particular, 9 ordered alloys are considered with different Nb content and effective activation energies are computed by Arrhenius fits for each alloy. We find that activation energies vary in a non-monotonic way as Nb content increases, reaching a maximum value at about Zr-50 %Nb. Finally, we observe that the predicted and measured larger diffusivity along the tube axis vs. the radial direction, is consistent with the material texture. Moreover, we conclude that the loss of continuity of the Zr-βsheets present in the tube microstructure, is consistent with the decrease ofDHin time at a given temperature

Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press

Self-interstitials structure in the hcp metals: A further perspective from first-principles calculations

We study the structure of several standard and non-standard self-interstitial configurations in a series of hcp metals, by using Density Functional Theory as embodied in the computer codes SIESTA and WIEN2k. The considered metals include Be, Mg, Ti, Zr, Co, Zn, and Cd, thus spanning the whole range of experimental c/a ratios, different kinds of bonding, and even magnetism (Co). The results show the importance of low symmetry configurations, closely related to the non-basal crowdion, in order to rationalize the experimental data on self-interstitial structure and migration.Fil: Pasianot, Roberto Cesar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional de San Martín. Instituto Sabato; Argentin

On the determination of defect dipoles from atomistic simulations using periodic boundary conditions

By solving the problem of a periodic distribution of point defects in general anisotropic media, we give an alternative, more direct proof, of the relatively recent procedure that extracts dipole tensors from the stress acting on the cell of atomistic simulations performed under periodic boundary conditions. Moreover, we show that naive superposition of individual defect fields is not a solution of the problem, though correction terms can be identified; as a byproduct, analysis of the latter allows us to reveal a spurious contribution to the elastic interaction energy as calculated in current literature procedures, that therefore must be subtracted in order to obtain correct results.Fil: Pasianot, Roberto Cesar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional de San Martín; Argentin