A. Legris

Influence of the interatomic potentials on molecular dynamics simulations of displacement cascades

Abstract Molecular dynamics (MD) is a powerful tool to study the displacement cascades initiated by the neutrons when they interact with matter. Key components of this technique are the interatomic potentials which model the binding of the different constitutive atoms. There exist many interatomic potentials dedicated to α-Fe and we have tested three of them for the study of radiation damage. We have found that the primary damage is potential sensitive. From our study, it appears that some characteristics of the potentials, not always considered, can be correlated to the type of damage produced by displacement cascades. The repulsive part of the potential has a strong influence on the cascade morphology. Moreover, equilibrium properties such as the atoms mean square displacements, the vacancy migration and vacancy–vacancy binding energies also appear to have some influence and should be investigated carefully when simulating radiation damage. It is therefore very important to use extreme care when trying to obtain quantitative results from MD simulations.

Atomic-scale Ab-initio study of the Zr-H system: I. Bulk properties

Bulk properties of the Zr-H system were studied in the framework of the density functional theory. The local density approximation (LDA) is found to be insufficient for a proper description of interactions between Zr and H atoms and the generalized gradient approximation (GGA) is required. In α Zr, H atoms preferentially occupy tetrahedral (T) sites at low temperatures, and can be regarded as being independent of each other up to very short distances, except for repulsive interactions between dumbbells in the same interstitial site. The Zr density of electronic states is perturbed by the presence of H, which induces the emergence of localized states. H diffusion occurs along the c axis preferentially in octahedral (O) sites, and in the basal plane by alternate jumps into T and O sites. In the γ(ZrH), δ (ZrH1.5) and e(ZrH2) hydrides, H-H interactions cannot be neglected, the nearly equal formation energies of these compounds indicate that their relative stabilities probably depend on mechanical and thermal contributions to free energies, and in fcc Zr, H atoms tend to adopt planar arrangements for compositions close to ZrH.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Identification and characterization of a new zirconium hydride

Zirconium alloys are currently used in nuclear power plants where they are susceptible to hydrogen pick‐up. Hydride precipitation may occur when the hydrogen solubility limit is reached. Various Zr hydride phases, γ, δ and ɛ have been identified since the 1950s. Combining electron precession microdiffraction, electron energy loss spectroscopy and ab initio electronic calculations, a new Zr hydride named ζ has been identified and characterized. It belongs to the trigonal crystal system with space group P3 m1 and it is fully coherent with the αZr matrix.

Liquid metal embrittlement of the martensitic steel 91: influence of the chemical composition of the liquid metal. Experiments and electronic structure calculations

Abstract In previous works [Scripta Mater. 43 (2000) 997; J. Nucl. Mater. 296 (2001) 256], we showed that the martensitic steel 91 is prone to liquid metal embrittlement (LME) by liquid lead provided that some metallurgical conditions are fulfilled. In this work, we report results of LME of the steel 91 in contact with Pb–Bi and other low melting temperature metals such as Sn and Hg. Our experimental results can be interpreted within the framework of the surface energy reduction models for LME. To account for the experimental observations, we performed electronic structure calculations to assess the chemical interaction between low melting temperature metal atoms and iron surfaces. Our results allow to establish a simple criterion that can give trends on the embrittlement power of a liquid metal in contact with iron.

Embrittlement of the martensitic steel 91 tested in liquid lead

Two potential problems are encountered in the case of intimate contact between liquid metals and metallic substrates: grain boundary wetting and liquid metal embrittlement (LME) which both induce a degradation of the mechanical properties. Tensile tests were carried out on a 9% Cr 1% Mo martensitic (Grade 91) steel in a liquid lead environment at temperatures ranging between 623 and 773 K. The Grade 91 steel was submitted to heat treatments in order to modify its hardness and also to produce either ferritic or martensitic grains. Smooth and notched specimens were used. We found out that by combining adapted heat treatments and the notch effect, it is possible to create conditions severe enough that lead to LME. Our experimental observations (transgranular failure) are compatible with the expectations of traditional mechanisms based on a reduction of the surface energy and/or adsorption induced chemical bond softening at the steel surface in contact with liquid lead.

Influence of crystal chemistry on ideal plastic shear anisotropy in forsterite: First principle calculations

Abstract We present ab initio calculations of ideal shear strengths (ISS) in forsterite at zero temperature using pseudopotential density functional theory within the generalized gradient approximation. A localized rigid-body shear is imposed on a given plane of an infinite defect-free crystal. The energy increase associated with this shear (called the generalized stacking fault energy) gives access to the ISS. The goal of this study is to assess the influence of crystal chemistry on the intrinsic resistance of plastic shear of a mineral like forsterite. ISS have been calculated for plastic shear along [100], [010], and [001] in various potential glide planes of forsterite. We show that the [001] slip, which corresponds experimentally to an easy glide at low temperature, exhibits the lowest energy barrier. The [010] glide is precluded because it involves very unfavorable atom impingements.

Characterization of Zirconium Hydrides and Phase Field Approach to a Mesoscopic-Scale Modeling of Their Precipitation

Zirconium alloys are currently used in nuclear power plants where they are submitted to hydrogen pick-up. Hydrogen in solid solution or hydride precipitation can affect the behavior of zirconium alloys during service but also in long term storage and in accidental conditions. Numerical modeling at mesoscopic scale using a “phase field” approach has been launched to describe hydride precipitation and its consequences on the mechanical properties of zirconium alloys. To obtain realistic results, it should take into account an accurate kinetic, thermodynamic, and structural database in order to properly describe hydride nucleation, growth, and coalescence as well as hydride interaction with external stresses. Therefore, an accurate structural characterization was performed on Zircaloy-4 plates and it allowed us to identify a new zirconium hydride phase called ζ. The ζ phase has a trigonal symmetry and is fully coherent with hcp αZr. The consequences of this new zirconium hydride phase on hydride transformation process and stress-reorientation phenomenon are discussed. A first attempt to numerically model the precipitation of this new zirconium hydride phase has been undertaken using the phase field approach.

Simulation of Irradiation Effects in Reactor Pressure Vessel Steels: the Reactor for Virtual Experiments (REVE) Project

Components of commercial nuclear reactors are subjected to neutron bombardments that can modify their mechanical properties. Prediction of in-service and post-service behaviors generally requires irradiation in so-called “test reactors” as well as subsequent mechanical testing in specialized hot cell facilities. However, the use of these research facilities is becoming more problematic, in particular due to increasing costs and decreasing availability. One way of partially mitigating these problems is to complement the empirical approach by developing tools for numerical simulation of irradiation effects in materials. The development of such tools is clearly an ambitious task that will require a long-term international collaborative effort. In this paper, we present an outline of the Reactor for Virtual Experiments (REVE) project, a collaborative European and American effort aimed at developing quantitative simulations of irradiation effects in materials. The first demonstration phase of REVE will target embrittlement of reactor pressure vessel (RPV) steels, since the effects and mechanisms of irradiation damage in this material are relatively well understood and many modeling tools have been developed or are under development in this field. As for any experiment, the input variables of the REVE simulation will be the neutron spectrum, time and temperature of irradiation, the alloy composition (e.g., Cu, Ni, Mn, and C contents) and microstructure and the unirradiated mechanical properties. The simulations will predict the irradiation-induced increases of yield stress and Charpy transition temperature as well as the decrease of toughness due to the concomitant evolution of the microstructure.

Investigation of Glide Properties in Hexagonal Titanium and Zirconium: An Ab Initio Atomic Scale Study

In this work we present ab initio atomic-scale simulations based on the density functional theory of stacking faults and of the structure of the (a) screw dislocation core in hexagonal Zr and Ti. The basal, prismatic, pyramidal π1 and π2 gamma-surfaces were investigated and the energy profiles along (a) and (c+a) Burgers vectors were determined. The results clearly indicate preferential prismatic spreading of screw dislocation cores suggesting a primary prismatic glide. The ab initio simulations are in an overall good agreement with previous tight binding ones1 although differences concerning the atomic relaxation around prismatic faults have been observed. Some environment effects on dislocation glide properties have been investigated through the study of the hydrogen effect on the stacking fault excess energies. Hydrogen in solid solution induces significant reductions of the stacking fault energy and should promote enhanced planar prismatic glide.

Recent Advances in Point Defect Studies Driven by Density Functional Theory

We highlight some of the most salient recent advances in point defects studies obtained from atomic-scale simulations performed in the framework of the density functional theory. The refinement of the theory, combined with its efficient numerical implementations and the (until now) everlasting growth of computer power allowed the transition from qualitative (in the beginning of the 90’) to quantitative results. Some of the longstanding controversies in the field have been tackled, and as far as aluminum is concerned, it has been shown that the curvature in the Arrheniusplot is due to anharmonic effects rather than to a two-defect diffusion mechanism. The anomalous diffusion in the b (bcc) phase of the group-IV elements has been related to the strong structural relaxation around vacancies, which significantly reduces their formation energy. Self-interstitials have been studied in materials of technological interest, their structure and mobility have been analyzed allowing a better interpretation of experimental results and an improved understanding of processes occurring under irradiation. Dilute interstitial solid solutions have been investigated. The strong binding between C and vacancies in bcc Fe may partially explain the observed influence of low amounts of C on Fe self-diffusion; the attraction of H to stacking faults in a Zr should favor planar dislocations glide. Intermetallics involving Fe (Fe-Al, Fe-Co) behave like highly correlated systems requiring methodological improvements of the DFT for a quantitative description. However, valuable trends concerning the structural point defects (those that allow nonstoichiometric compositions at low temperature) as well as the temperature dependence of point defects concentrations have been obtained.