Density-based crystal plasticity : from the discrete to the continuum

Because of the enormous range of time and space scales involved in dislocation dynamics, plastic modeling at macroscale requires a continuous formulation. In this paper, we present a rigorous formulation of the transition between the discrete, where plastic flow is resolved at the scale of individual dislocations, and the continuum, where dislocations are represented by densities. First, we focus on the underlying coarse-graining procedure and show that the emerging correlation-induced stresses are scale-dependent. Each of these stresses can be expanded into the sum of two components. The first one depends on the local values of the dislocation densities and always opposes the sum of the applied stress and long-range mean field stress generated by the geometrically necessary dislocation (GND) density; this stress acts as a friction stress. The second component depends on the local gradients of the dislocation densities and is inherently associated to a translation of the elastic domain; therefore, it acts as a back-stress. We also show that these friction and back- stresses contain symmetry-breaking components that make the local stress experienced by dislocations to depend on the sign of their Burgers vector

Statistical Derivation of Basic Equations of Diffusional Kinetics in Alloys with Application to the Description of Diffusion of Carbon in Austenite

Basic equations of diffusional kinetics in alloys are statistically derived using the master equation approach. To describe diffusional transformations in substitution alloys, we derive the "quasi-equilibrium" kinetic equation which generalizes its earlier versions by taking into account possible "interaction renormalization" effects. For the interstitial alloys Me-X, we derive the explicit expression for the diffusivity D of an interstitial atom X which notably differs from those used in previous phenomenological treatments. This microscopic expression for D is applied to describe the diffusion of carbon in austenite basing on some simple models of carbon-carbon interaction. The results obtained enable us to make certain conclusions about the real form of these interactions, and about the scale of the "transition state entropy" for diffusion of carbon in austenite.Comment: 26 pages, 5 postscript figures, LaTe

Study of Damage Propagation at the Interface Localization-Delocalization Transition of the Confined Ising Model

The propagation of damage in a confined magnetic Ising film, with short range competing magnetic fields ($h$) acting at opposite walls, is studied by means of Monte Carlo simulations. Due to the presence of the fields, the film undergoes a wetting transition at a well defined critical temperature $T_w(h)$. In fact, the competing fields causes the occurrence of an interface between magnetic domains of different orientation. For $T T_w(h)$) such interface is bounded (unbounded) to the walls, while right at $T_w(h)$ the interface is essentially located at the center of the film. It is found that the spatio-temporal spreading of the damage becomes considerably enhanced by the presence of the interface, which act as a ''catalyst'' of the damage causing an enhancement of the total damaged area. The critical points for damage spreading are evaluated by extrapolation to the thermodynamic limit using a finite-size scaling approach. Furthermore, the wetting transition effectively shifts the location of the damage spreading critical points, as compared with the well known critical temperature of the order-disorder transition characteristic of the Ising model. Such a critical points are found to be placed within the non-wet phase.Comment: 22 pages, 13 figures include

Damage characterization of displacement cascades in (U,PU)O2_2 fuels by MD

International audiencePlutonium based MOX fuels (U,Pu)O$_2$ are one of the most commonly used fuel in actual nuclear plants in France. However, little is known on its microscopic behaviour during irradiation. This is mainly resulting from the fact that experiments are difficult and expensive to set up with such compounds at such scales and conditions. Hence, atomistic simulations can bring valuable information and increase the scope of knowledge in this field. Molecular dynamics simulations have proved to be an excellent tool to study point defects creation, because of its intrinsic length and time scales. Therefore, we will present in this paper the first, to our knowledge, molecular dynamics simulations of displacement cascades in (U,Pu)O$_2$ solid solution. We will first present the assessment of four of the most recent rigid ions pair interactions for (U,Pu)O$_2$ solid solution by comparing their thermo-mechanical and thermophysical properties at different temperatures and plutonium contents. Then, the primary damage state under irradiation will be discussed by analysing the results of displacement cascades initiated with energies up to 500 keV and for different plutonium contents

Damage characterization of displacement cascades in (u,pu)o2 fuels by md simulations

International audiencePlutonium based MOX fuels (U,Pu)O2 are one of the most commonly used fuel in actual nuclear plants in France. However, little is known on its microscopic behaviour during irradiation. This is mainly resulting from the fact that experiments are difficult and expensive to set up with such compounds at such scales and conditions. Hence, atomistic simulations can bring valuable information and increase the scope of knowledge in this field. Molecular dynamics simulations have proved to be an excellent tool to study point defects creation, because of its intrinsic length and time scales. We present in this paper the insight brought by molecular dynamics simulations on primary damages in (U,Pu)O2 solid solution. The efficiency of four of the most recent rigid ions pair interactions for (U,Pu)O2 solid solution will be exhibited by comparing their thermo-mechanical and thermophysical properties at different temperatures and plutonium contents. Then, the primary damage state under irradiation will be discussed based on the analysis of the results of displacement cascades initiated with energies up to 500 keV and for different plutonium contents

High resolution Transmission Electron Microscopy characterization of a milled oxide dispersion strengthened steel powder

International audienceOxide Dispersion Strengthened (ODS) steels are promising materials for generation IV fuel claddings as their dense nano-oxide dispersion provides good creep and irradiation resistance. Even if they have been studied for years, the formation mechanism of these nano-oxides is still unclear. Here we report for the first time a High Resolution Transmission Electron Microscopy and Energy Filtered Transmission Electron Microscopy characterization of an ODS milled powder. It provides clear evidence of the presence of small crystalline nanoclusters (NCs) enriched in titanium directly after milling. Small NCs (<5 nm) have a crystalline structure and seem partly coherent with the matrix. They have an interplanar spacing close tothe (011) bcc iron structure. They coexist with larger crystalline spherical precipitates of 15e20 nm in size. Their crystalline structure may be metastable as they are not consistent with any Y-Ti-O or Ti-O structure. Such detailed observations in the as-milled grain powder confirm a mechanism of Y, Ti, O dissolution in the ferritic matrix followed by a NC precipitation during the mechanical alloying process of ODS materials