19 research outputs found
Cement As a Waste Form for Nuclear Fission Products: The Case of ⁹⁰Sr and Its Daughters
No full textOne of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of90Sr insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its β-decay on the cement paste structure. We show that ⁹⁰Sr is stable when it substitutes the Ca²⁺ ions in C-S-H, and so is its daughter nucleus ⁹⁰Y after β-decay. Interestingly,⁹⁰Zr, daughter of ⁹⁰Y and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for ⁹⁰Sr storage
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Modeling Solute-Dislocation Interactions in Body-Centered Cubic Alloys Using Kinetic Monte Carlo Simulations
No full textInteractions among dislocations and solute atoms are the basis of several important processes in metal plasticity. In body-centered cubic (bcc) metals and alloys, low-temperature plastic flow is controlled by screw dislocation glide, which is known to take place by thenucleation and sideward relaxation of kink pairs across two consecutive Peierls valleys. In alloys, dislocations and solutes affect each others kinetics via long-range stress field coupling and short-range inelastic interactions. It is known that in certain substitutional bcc alloys a transition from solute softening to solute hardening is observed at a critical concentration. In the first part of this work, we develop a kinetic Monte Carlo model of screw dislocation glide and solute diffusion in substitutional W–Re alloys. We find that dislocation kinetics is governed by two competing mechanisms. At low solute concentrations, nucleation is enhanced by the softening of the Peierls stress, which dominates over the elastic repulsion of Re atoms on kinks. This trend is reversed at higher concentrations, resulting in a minimum in the flow stress that is concentration and temperature dependent. This minimum marks the transition from solute softening to hardening, which is found to be in reasonable agreement with experiments.In the second part of this work, we extend our model to interstitial W–O alloys. we report for the first time on simulations of jerky flow in W-O as a representative bcc interstitial solid solution. The simulations are carried out in a stochastic framework that naturally capturesrare events in a rigorous manner, enabling the study of solute diffusion and dislocation motion concurrently. The model has no adjustable parameters, with all coefficients calculated using first principles methods. We find that three regimes emerge from the stress-temperature space: one representative of standard solid solution strengthening, another mimicking solute cloud formation, and a third one, where the dynamic interaction of solutes and dislocations results in jerky flow and dynamic strain aging. We show how the symbiosis between quantum mechanical calculations and mesoscopic methods capable of furnishing diffusive timescales is a powerful demonstration of the capacities of physical models to explain macroscopic behavior
Phase field modeling of the growth of an interaction compound in research reactors fuels
No full textDans le contexte de la réduction de l'enrichissement des combustibles des réacteurs nucléaires de recherche, certaines nouvelles solutions plus denses en uranium sont testées, parmi lesquelles U₃Si₂-Al. Lors de son utilisation en réacteur, en raison de l'irradiation et de la température, des phénomènes d'interdiffusion entraînent une amorphisation du combustible U₃Si₂ ainsi que le développement d'un composé d'interaction amorphe entre les particules de U₃Si₂ et la matrice aluminium. La faible conductivité thermique du composé d'interaction entraîne un échauffement de la plaque de combustible et peut entraîner une dégradation du matériau. Afin de mieux comprendre et anticiper ces phénomènes, nous avons étudié la croissance du composé d'interaction sous irradiation par champs de phase ajusté sur des calculs à l'échelle atomique. À l'échelle atomique, nous avons modélisé l'amorphisation sous irradiation des particules de combustible U₃Si₂ par dynamique moléculaire. Un modèle atomique pour U₃Si₂ amorphe a été utilisé pour des calculs Monte-Carlo afin de déterminer les concentrations d'équilibre entre uranium et silicium en fonction de différences de potentiel chimique entre les deux espèces. Nous avons ainsi calculé la courbure de l'énergie libre chimique du combustible amorphe. Cette étude à l'échelle atomique, combinée aux données thermodynamiques de la littérature, nous a permis d'obtenir les courbes des énergies libres chimiques de U₃Si₂ amorphe, de Al, et du composé d'interaction amorphe. Elles sont ellipsoïdales dans le repère (cU, cSi, fchim), et non paraboliques comme cela est généralement utilisé. Afin de modéliser la croissance du composé d'interaction à l'échelle mésoscopique, un modèle en champs de phase triphasé a été adapté à la forme singulière des énergies libres chimiques trouvées à l'échelle atomique. Pour valider les développements apportés à ce modèle, nous avons étudié les profils des concentrations aux interfaces et la cinétique de l'évolution du système vers l'équilibre thermodynamique. Les résultats des simulations concordent avec les calculs théoriques analytiques. À l'issue de la paramétrisation du modèle en champs de phase, la croissance sous irradiation du composé d'interaction a été modélisée. Les coefficients d'interdiffusion jouent le rôle de variables d'ajustement. Nous trouvons que la croissance du composé d'interaction sous irradiation se caractérise par un état transitoire hyper-parabolique dû à la mise en équilibre des phases U₃Si₂ et Al avant un état permanent diffusif, régi par le composé d'interaction, avec une croissance plus classique en racine carrée du temps. Pour des durées nominales d'irradiation, le régime permanent n'est pas encore atteint, ce qui différent des lois de croissance de la littérature, obtenues à partir d'équations de diffusion. Les lois de croissance fournissent une bonne approximation de l'évolution su système sans en traduire l'ensemble des complexités mises en avant par notre méthode multi-échelles.In the context of reducing fuel enrichment in research nuclear reactors, new fuels with a higher uranium density are being tested, including U₃Si₂-Al. When used in a reactor, irradiation and temperature produce interdiffusion phenomena, that lead to the amorphization of the U₃Si₂ fuel and to the development of an amorphous interaction compound between U₃Si₂ particles and the aluminium matrix. The interaction compound low thermal conductivity leads to the heating of the fuel plate and can contribute to the degradation of the material. We have modelled the interaction compound growth using a phase field model parametrized on atomic-scale simulations in order to better understand this phenomenon. At the atomic scale, we modelled the amorphization under irradiation of U₃Si₂ fuel particles using molecular dynamics. An atomic model for amorphous U₃Si₂ was then used in Monte Carlo calculations to determine the equilibrium concentrations between uranium and silicon as a function of the differences in chemical potential between the two species. Using this method, we calculated the curvature of the chemical free energy of the amorphous fuel. This atomic-scale study, combined with thermodynamic data from the literature, enabled us to obtain chemical free energy curves for amorphous U₃Si₂, Al and the amorphous interaction compound. They are ellipsoidal in the 3D-space (cU, cSi, fchim), and not parabolic as generally used. In order to model the growth of the interaction compound at the mesoscopic scale, a three-phase phase-field model was adapted to the singular shape of the chemical free energies found at the atomic scale. To validate the developments made to this model, we studied the concentration profiles at the interfaces and the kinetics of the evolution of the system towards thermodynamic equilibrium. The results of the simulations are consistent with the theoretical analytical calculations. After the parameterization of the phase field model, we modelled the growth of the interaction compound under irradiation. The interdiffusion coefficients were used as fitting parameters. We find that the growth of the interaction compound under irradiation is characterised by a hyper-parabolic transient state due to the equilibration of the U₃Si₂ and Al phases before reaching a permanent diffusive state, governed by the interaction compound, with a classical square-root variation as a function of time. For nominal irradiation times, the permanent state is not reached, which is different from the growth laws found in the literature that were obtained from diffusion equations. The growth laws provide a good approximation of the evolution of the system but without reflecting the transitional state highlighted by our multi-scale method
Ab initio modelling of screw dislocations in body-centered cubic transition metals
No full textNous avons réalisé des calculs de structure électronique ab initio, basés sur la théorie de lafonctionnelle de la densité (DFT), pour étudier les propriétés des dislocations vis h111i dansles métaux de transition cubiques centrés (V, Nb, Ta, Mo, W et Fe). Dans tous ces éléments,le coeur facile non-dégénéré est la configuration d’énergie minimale et la configuration de coeurdissociée a une énergie très élevée, comparable ou plus élevée que celle du coeur difficile, encontradiction avec les prédictions des potentiels interatomiques. Nous avons mis en évidence destendances de groupe marquées sur l’énergie de coeur de la dislocation facile, reliées à la positiondu niveau de Fermi par rapport au minimum du pseudo-gap de la densité d’états électroniques.Notre travail fait aussi apparaitre un comportement atypique du fer, avec une énergie relativedu coeur difficile basse, proche de celle du point col entre deux coeurs faciles, conduisant à unpotentiel de Peierls plat autour de la configuration difficile, contrairement aux autres éléments.A partir de ces calculs DFT, nous avons construit le paysage énergétique à deux dimensionsdans le plan {111} (potentiel de Peierls) et nous avons étudié plusieurs propriétés relativesau glissement des dislocations, et en particulier l’énergie de formation de la paire de décrochementset la dépendance de la contrainte de Peierls en fonction de l’orientation cristalline.Nous proposons une modification simple de la loi de Schmid, qui prend en compte la trajectoirenon rectiligne de la dislocation et qui permet d’expliquer qualitativement pourquoi l’asymétriemaclage/antimaclage est moins marquée dans Fe que dans les autres métaux cubiques centrés.We performed electronic structure ab initio calculations based on density functional theory(DFT) to study the h111i screw dislocation properties in body-centered cubic transition metals(V, Nb, Ta, Mo, W and Fe). In all investigated elements, the nondegenerate easy coreis the minimum energy configuration and the split core configuration has a high energy nearor above that of the hard core, contrary to interatomic potential predictions. A strong groupdependence of the core energy of the easy dislocation is also evidenced, related to the positionof the Fermi level with respect to the minimum of the pseudogap of the electronic density ofstates. Our work also reveals an atypical behavior in Fe, with a low relative energy at the hardcore position, close to that of the saddle configuration between easy cores, resulting in a flatPeierls potential around the hard core configuration, at variance with other elements. Fromthese DFT calculations, the two-dimensional energetic landscape in the {111} plane (Peierlspotential) is constructed and we investigated several properties of dislocation glide and in particular,the kink-pair formation enthalpy, as well as the dependence of the Peierls stress oncrystal orientation. We proposed a simple modification to the Schmid law that takes accountof the non-straight trajectory of the dislocation and that qualitatively explains why the twinning/antitwinning asymmetry is less pronounced in Fe than in other body-centered cubic metals
Electronic Structure Calculations of Oxygen Atom Transport Energetics in the Presence of Screw Dislocations in Tungsten
No full textPlastic flow in body-centered cubic (bcc) alloys is governed by the thermally-activated motion of screw dislocations in close-packed planes. In bcc interstitial solid solutions, solute diffusion can occur at very fast rates owing to low migration energies and solute concentrations. Under mechanical loading, solutes may move on the same or similar time scale as dislocations glide, even at low temperatures, potentially resulting in very rich co-evolution processes that may have important effects in the overall material response. It is therefore important to accurately quantify the coupling between interstitial impurities and dislocations, so that larger-scale models can correctly account for their interactions. In this paper, we use electronic structure calculations to obtain the energetics of oxygen diffusion under stress and its interaction energy with screw dislocation cores in bcc tungsten. We find that oxygen atoms preferentially migrate from tetrahedral to tetrahedral site with an energy of 0.2 eV. This energy couples only weakly to hydrostatic and deviatoric deformations, with activation volumes of less than 0.02 and 0.02 b 3 , respectively. The strongest effect is found for the inelastic interaction between O atoms and screw dislocation cores, which leads to attractive energies between 1.2 and 1.9 eV and sometimes triggers a transformation of the screw dislocation core from an easy core configuration to a hard core configuration
Screw dislocations in BCC transition metals: from <i>ab initio</i> modeling to yield criterion
No full textPlastic anisotropy and dislocation trajectory in BCC metals
No full textInternational audiencePlasticity in body-centred cubic (BCC) metals at low temperatures is atypical, marked in particular by an anisotropic elastic limit in clear violation of the famous Schmid law applicable to most other metals. This effect is known to originate from the behaviour of the screw dislocations; however, the underlying physics has so far remained insufficiently understood to predict plastic anisotropy without adjustable parameters. Here we show that deviations from the Schmid law can be quantified from the deviations of the screw dislocation trajectory away from a straight path between equilibrium configurations, a consequence of the asymmetrical and metal-dependent potential energy landscape of the dislocation. We propose a modified parameter-free Schmid law, based on a projection of the applied stress on the curved trajectory, which compares well with experimental variations and first-principles calculations of the dislocation Peierls stress as a function of crystal orientation
Simulating the mechanisms of serrated flow in interstitial alloys with atomic resolution over diffusive timescales
No full textSimulating the mechanisms of serrated flow in interstitial alloys with atomic resolution over diffusive timescales
No full textThe Portevin-Le Chatelier (PLC) effect is a phenomenon by which plastic slip in metallic materials becomes unstable, resulting in jerky flow and the onset of inhomogeneous deformation. The PLC effect is thought to be fundamentally caused by the dynamic interplay between dislocations and solute atoms. However, this interplay is almost always inaccessible experimentally due to the extremely fine length and time scales over which it occurs. In this paper, simulations of jerky flow in W-O interstitial solid solutions reveal three dynamic regimes emerging from the simulated strain rate-temperature space: one resembling standard solid solution strengthening, another one mimicking solute cloud formation, and a third one where dislocation/solute coevolution leads to jerky flow as a precursor of dynamic strain aging. The simulations are carried out in a stochastic framework that naturally captures rare events in a rigorous manner, providing atomistic resolution over diffusive time scales using no adjustable parameters
Exploring the potential of dislocation density fields for the discrete-to-continuum crossover in nanomechanics
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