Mobility and stability of large vacancy and vacancy-copper clusters in iron: An atomistic kinetic Monte Carlo study

The formation of Cu-rich precipitates under irradiation is a major cause for changes in the mechanical response to load of reactor pressure vessel steels. In previous works, it has been shown that the mecha- nism under which precipitation occurs is governed by diffusion of vacancy-copper (VCu) complexes, also in the absence of irradiation. Coarse-grained computer models (such as object kinetic Monte Carlo) aimed at simulating irradiation processes in model alloys or steels should therefore explicitly include the mobil- ity of Cu precipitates, as a consequence of vacancy hops at their surface. For this purpose, in this work we calculate diffusion coefficients and lifetimes for a large variety of VCu complexes. We use an innovative atomistic model, where vacancy migration energies are calculated with little approximations, taking into account all effects of static relaxation and long-range chemical interaction as predicted by an interatomic potential. Our results show that, contrary to what intuition might suggest, saturation in vacancies tend toslow down the transport of Cu atoms.Fil: Castin, N.. Centre d’Etudes de l’énergie Nucléaire; BélgicaFil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Malerba, L.. Centre d’Etudes de l’énergie Nucléaire; Bélgic

Obstruction to movement in edge dislocations ½ <111> {110} by Cu, Mn and Ni solutes

El principal mecanismo de fragilización de los aceros bainíticos por la irradiación es la obstrucción al movimiento de las dislocaciones. Los defectos estructurales responsables de esta obstrucción son producto de la irradiación a la que está sometido el material. En particular los precipitados de Cu, Ni y Mn son considerados como los principales contribuyentes a la fragilización y la evidencia experimental indica que se ubican preferentemente en la zona de la línea de dislocación. En este trabajo se estudia el efecto de la segregación alrededor de las dislocaciones de borde con vector de Burgers b = 1/2[111] y la tensión necesaria para el movimiento. Para redistribuir los átomos de soluto minimizando la energía se aplica un algoritmo de Monte Carlo en el ensamble semi-gran-canónico. La tensión de corte crítica se estudia mediante la técnica de dinámica molecular y se analiza como afecta la presencia de Cu en una matriz de Fe con 1%Ni-1%Mn. Los resultados muestran que la tensión requerida para mover las dislocaciones aumenta sustancialmente con la presencia de solutos segregados.The main mechanism of embrittlement of bainitic steels by irradiation is the impediment of dislocation movement. Structural defects responsible for this obstruction are a product of irradiation. In particular, Cu, Ni and Mn precipitates are considered as major contributors to embrittlement and experimental evidence indicates that preferentially locate in the area of the dislocation line. In this paper we study the effect of this segregation around edge dislocations with Burgers vector b = 1/2 [111] and the stress required for the movement. A Monte Carlo algorithm in the semi-grand canonical ensemble is used to study the alloying elements segregation to the dislocation core. The critical resolved shear stress is studied by molecular dynamics to analyze the effect of Cu atoms in a Fe-matrix containing 1%Ni-1%Mn. The results show that the stress required for the dislocation to break-away substantially increases with the presence of segregated solutes.Fil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; ArgentinaFil: E. Martínez. Los Alamos National Laboratory; Estados Unido

Many-body interatomic U and Al-U potentials

In the present work, an interatomic potential in the framework of the embedded atom method (EAM) is developed for the Al–U binary system. A methodology is detailed to fit the U potential that reproduces the stability of the a phase at low temperatures and the g phase at high ones. The thermal stability of both phases, thermal expansion and vacancy driven self diffusion are studied. The Al–U potential is fit to first principles calculated formation energies of the experimentally observed intermetallic phases, Al2U (cubic C15), Al3U (cubic L12) and Al4U (orthorhombic D1b). As a first validation the potentials are tested against available experimental measurements.Fil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Bonny, G.. No especifíca;Fil: Fernandez, Julian Roberto. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Exact mean field concept to compute defect energetics in random alloys on rigid lattices

In modern materials science modeling, the evolution of the energetics of random alloys with composition are desirable input parameters for several meso-scale and continuum scale models. When using atomistic methods to parameterize the above mentioned concentration dependent function, a mean field theory can significantly reduce the computational burden associated to obtaining the desired statistics in a random alloy. In this work, a mean field concept is developed to obtain the energetics of point-defect clusters in perfect random alloys. It is demonstrated that for a rigid lattice the concept is mathematically exact. In addition to the accuracy of the presented method, it is also computationally efficient as a small box can be used and perfect statistics are obtained in a single run. The method is illustrated by computing the formation and binding energy of solute and vacancy pairs in FeCr and FeW binaries. Also, the dissociation energy of small vacancy clusters was computed in FeCr and FeCr-2%W alloys, which are considered model alloys for Eurofer steels. As a result, it was concluded that the dissociation energy is not expected to vary by more than 0.1 eV in the 0?10% Cr and 0?2% W composition range. The present mean field concept can be directly applied to parameterize meso-scale models, such as cluster dynamics and object kinetic Monte Carlo models.Fil: Bonny, G.. Sck-Cen Centre Detude de Lénergie Nucléaire; FranciaFil: Castin, N.. Sck-Cen Centre Detude de Lénergie Nucléaire; FranciaFil: Pascuet, Maria Ines Magdalena. Comision Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Çelik, Y.. Sck-Cen Centre Detude de Lénergie Nucléaire; Franci

On the accurate description of uranium metallic phases: A MEAM interatomic potential approach

A new interatomic potential in the framework of the modified embedded atom method (MEAM) to model U metal is presented. The potential acceptably reproduces the lattice parameters and cohesive energy of the orthorhombic αU. The relative stability of the experimentally observed phase at low temperatures with respect to several other structures (bct, bcc, simple cubic, tetragonal β Np, fcc and hcp) is also taken into account. Intrinsic point defect properties compare reasonably well with data from the literature. To determine the quality of the interaction, the potential is used to study a number of properties for the pure metal at finite temperatures and the results are compared with the available data. The obtained allotropic αU↔γU transformation and melting temperatures are in good agreement with experimental values. Based on the simulations, a new αU↔γU transformation mechanism is proposed.Fil: Fernandez, Julian Roberto. Universidad Nacional de San Martín. Instituto Sabato; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; ArgentinaFil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Atomic interaction of the MEAM type for the study of intermetallics in the Al–U alloy

Interaction for both pure Al and Al–U alloys of the MEAM type are developed. The obtained Al interatomic potential assures its compatibility with the details of the framework presently adopted. The Al–U interaction fits various properties of the Al2U, Al3U and Al4U intermetallics. The potential verifies the stability of the intermetallic structures in a temperature range compatible with that observed in the phase diagram, and also takes into account the greater stability of these structures relative to others that are competitive in energy. The intermetallics are characterized by calculating elastic and thermal properties and point defect parameters. Molecular dynamics simulations show a growth of the Al3U intermetallic in the Al/U interface in agreement with experimental evidence.Fil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Fernandez, Julian Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Universidad Nacional de San Martín; Argentin

Understanding why dislocation loops are visible in transmission electron microscopy: The tungsten case

Dislocation loops finely disperse in bulk W are generally visible to the transition electron microscopy (TEM) after irradiation. In the absence of strong interactions, these loops would normally diffuse very fast until being sunk at grain boundaries or at the dislocation network. In this work, we evaluate the strength of two pining effects that can explain the reason why they are nevertheless observed by TEM in bulk. On the one hand, we evaluate with density functional theory (DFT) the strength of binding between isolated loops and dissolved chemical impurities. Employing classical equations of diffusion, we estimate the resulting effective diffusion coefficient of loops. On the other hand, we consider the effect of mutual elastic interactions (MEI) between the loops, applying linear elasticity. We perform a large set of kinetic Monte Carlo (KMC) simulations, aimed at evaluating the effective diffusion coefficient, accounting for multiple interactions. Finally, we draw a map that indicates the dominant pinning effect given the experimental conditions (loop size and loop number density). Comparing with a large database of experimental TEM evidence, we conclude that pinning by dissolved impurities is the dominant effect.Fil: Castin, N.. Studiecentrum voor Kernenergie; BélgicaFil: Bakaev, A.. Studiecentrum voor Kernenergie; BélgicaFil: Terentyev, D.. Studiecentrum voor Kernenergie; BélgicaFil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Unidad de Actividad de Materiales (CAC); ArgentinaFil: Bonny, G.. Studiecentrum voor Kernenergie; Bélgic

The influence of grain size on the hydrogen diffusion in bcc Fe

This work studies the diffusion of Hydrogen (H) in bcc Fe, containing a high-angle symmetric tilt grain boundary (GB), as a function of both the temperature and the average grain size. For this purpose, we propose a microscopic effective model which includes diffusion in bulk and in the GB. The model distinguishes between diffusion along the GB, in parallel with the bulk, while diffusion through the GB is to be considered in series. The bounding and migration energies of the H interstitial sites are derived through an extensive study of H atoms dissolved in a high-angle symmetric tilt GB. This is undertaken in the framework of a set of classical interatomic potentials, and partially from Density Functional Theory (DFT) calculations, in order to check the consistency of equilibrium atomic structures. We find that preferential trapping sites for H in the GB delay the H migration, thus enhancing its solubility. The derived H diffusion coefficients are in agreement with experimental evidence, however various kinds of GBs are present in real samples. In addition, we see that at high temperature, H diffusion does not depend on the grain size, as similar results than in bulk are found. In contrast, at room temperatures (290 K) and nano-sized grains (100 nm) the effective diffusion can slow down up to two orders of magnitude.Fil: Ramunni, Viviana Patricia. Comisión Nacional de Energía Atómica. Centro Atómico Ezeiza. Gerencia de Materiales y Combustibles Nucleares; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Castin, Nicolas. Centre d'Etude de l'Energie Nucléaire; BélgicaFil: Rivas, Alejandro Mariano Fidel. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Density functional theory-based cluster expansion to simulate thermal annealing in FeCrW alloys

In this work, we develop a rigid lattice cluster expansion as an ultimate goal to track the micro-structural evolution of Eurofer steel under neutron irradiation. The fact that all (defect) structures are mapped upon a rigid lattice allows a simplified computation and fitting procedure, thus enabling alloys of large chemical complexity to be modelled. As a first step towards the chemical complexity of Eurofer steels, we develop a cluster expansion (CE) for the FeCrW-vacancy system based on density functional theory (DFT) calculations in the dilute alloy limit. The DFT calculations suggest that only CrW clusters containing vacancies are stabilised. The cluster expansion was used to simulate thermal annealing in Fe?20Cr?xW alloys at 773 K. It is found that the addition of W to the alloy results in a non-linear decrease in the precipitation kinetics. The CE was found suitable to describe the energetics of the FeCrW-vacancy system in the Fe-rich limit.Fil: Bonny, G.. Nuclear Materials Science Institute; BélgicaFil: Castin, Nicolas. Nuclear Materials Science Institute; BélgicaFil: Domain, C.. Les Renardières; FranciaFil: Olsson, P.. KTH Royal Institute of Technology; SueciaFil: Verreyken, B.. Nuclear Materials Science Institute; BélgicaFil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Terentyev, D.. Nuclear Materials Science Institute; Bélgic