CNRS Material Physics Group

Rouen, France

CNRS Material Physics Group

Rouen, France
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Keller C.,CNRS Material Physics Group | Hug E.,CNRS Crystallography and Material Science Laboratory
International Journal of Plasticity | Year: 2017

In this article, the Kocks-Mecking formalism is employed to analyze the grain size and thickness effects which modify the mechanical behavior of nickel polycrystals. To this aim, a two-internal variable Kocks-Mecking model, based on the evolution of forest and mobile dislocation densities, is numerically optimized using a wide experimental database of tensile curves of nickel polycrystals already published. The original model has been modified to take into account the grain size contribution to strain hardening in addition to the conventional dislocation production and annihilation terms. Using this improved model, the tensile curves of samples with different grain sizes, thicknesses and number of grains across the thickness are well reproduced. By means of an analysis of the model parameters, i.e. dislocation mean free path, cross-slip rate and dislocation densities, the strain mechanisms of nickel polycrystals are investigated as a function of their microstructural characteristics. For specimens with few grains across the thickness, the rate of dislocation annihilation is increased which, in turn, decreases the forest dislocation density and stress level. These results show, first, that the well-known Kocks-Mecking model is able to reproduce size effect and, second, confirm previous assumptions about the mechanical behavior of miniaturized samples. Eventually, the modeling of the mechanical behavior for samples concerned by miniaturization taking into account the surface effect contribution is discussed. © 2017 Elsevier Ltd.

Boisse J.,Rutgers University | Zapolsky H.,CNRS Material Physics Group | Khachaturyan A.G.,Rutgers University
Acta Materialia | Year: 2011

The Fe-Ga body-centered cubic (bcc) alloys within the 15-20 at.% Ga composition range have abnormally high magnetostriction. There is growing evidence that this effect is associated with the magnetic field-induced flip of tetragonal axes of nanoparticles of the ordered phase formed in this range. We studied structural transformations within this composition range at 550 °C by using computer modeling of the atomic-scale ordering and clustering in the atomic density field approximation. It is shown that the initial stage of equilibration of the compositionally homogeneous bcc solid solution with 19 at.% Ga results in bcc → B2 congruent ordering followed by a precipitation of Ga-rich B2 particles, which eventually transform to particles of the DO 3 phase. At the composition 21 at.% Ga, the congruently ordered B2 phase undergoes further B2 → DO3 congruent ordering, which is followed by decomposition into an equilibrium mixture of the bcc and DO 3 phases. An important result is that the phase transformations at 0.15 < c < 0.19 produce nanoparticles of transient B2 phase. We assume that the nanoprecipitates of the transient B2 phase undergo a diffusionless cubic → tetragonal transformation, forming the L10 phase during cooling to the room temperature, and that this involves a magnetic field-induced flipping of tetragonality of these nanoprecipitates which may be responsible for the giant magnetostriction. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Taleb L.,CNRS Material Physics Group | Cailletaud G.,MINES ParisTech Center of materials
International Journal of Plasticity | Year: 2010

In this paper we consider the elastoplastic behavior of the 304L stainless steel under cyclic loading at room temperature. After the experimental investigations presented in Taleb and Hauet (2009), the present work deals with modeling in the light of the new observations. An improved version of the multimechanism model is proposed in which the isotropic variable is revisited in order to take into account the non-proportional effect of the loading as well as the strain memory phenomenon. A particular attention has been paid to the identification process in order to capture the main important phenomena: relative parts of isotropic and kinematic hardening, time dependent effects, non-proportionality effect, strain amplitude dependence. Only strain controlled tests have been used for the identification process. The capabilities of the model with "only" 17 parameters are evaluated considering a number of proportional and non-proportional stress and strain controlled tests. © 2009 Elsevier Ltd. All rights reserved.

Taleb L.,CNRS Material Physics Group | Cailletaud G.,MINES ParisTech Center of materials
International Journal of Plasticity | Year: 2011

The main objective of the work is to investigate the importance of time dependent phenomena in the cyclic behavior of the 304L SS at room temperature. The contribution of creep in the cyclic evolution of the inelastic strain observed under stress control is particularly studied through various tests using proportional and non proportional loading paths. The test results demonstrate that the behavior is strongly rate dependent. Moreover, most of the cyclic accumulation of the inelastic strain is due to creep which implies that ratcheting is very small for the different conditions considered. The study clearly shows that a correct description of the behavior of the 304L SS at room temperature cannot be obtained without time-dependent constitutive equations. The tests performed have been simulated with the multimechanism constitutive equations after the identification of its 17 parameters. The quality of the simulations is in good agreement with the experimental results especially for the non proportional loading paths. © 2011 Elsevier Ltd. All rights reserved.

Chbihi A.,CNRS Material Physics Group | Sauvage X.,CNRS Material Physics Group | Blavette D.,CNRS Material Physics Group | Blavette D.,Institut Universitaire de France
Acta Materialia | Year: 2012

The early stage of chromium precipitation in copper was analyzed at the atomic scale by atom probe tomography (APT). Quantitative data about the precipitate size, three-dimensional shape, density, composition and volume fraction were obtained in a Cu-1Cr-0.1Zr (wt.%) commercial alloy aged at 713 K. Surprisingly, nanoscaled precipitates exhibit various shapes (spherical, plates and ellipsoid) and contain a large amount of Cu (up to 50%), in contradiction to the equilibrium Cu-Cr phase diagram. APT data also show that some impurities (Fe) may segregate along Cu/Cr interfaces. The concomitant evolution of the precipitate shape and composition as a function of the aging time is discussed. Special emphasis is given to the competition between interfacial and elastic energy, and to the role of Fe segregation. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Meslin E.,CEA Saclay Nuclear Research Center | Radiguet B.,CNRS Material Physics Group | Loyer-Prost M.,CEA Saclay Nuclear Research Center
Acta Materialia | Year: 2013

The radiation embrittlement of low-alloyed reactor pressure vessel steels is partly due to the formation of nanometer-sized solute clusters (Mn, Si, Ni, Cu and P). In order to determine if radiation-induced mechanisms can take part in the solute clustering, an under-saturated binary Fe-1 at.% Mn alloy was irradiated with Fe ions at 400 C. After irradiation, atom probe tomography experiments revealed that a high density of Mn-rich clusters is formed. This observation clearly demonstrates that, under these irradiation conditions, Mn clustering in this model alloy is radiation-induced and not radiation-enhanced. Mn-rich clusters were not distributed homogeneously in the analyzed volume but were heterogeneously precipitated on a planar object, suggesting a grain boundary (GB) or a dislocation loop. In parallel, a rate theory model calibrated on the population of point defect (PD) clusters measured by transmission electron microscopy has shown that the dominant sinks for mobile PDs are PD clusters. Thus Mn clustering could be explained by Mn atoms dragged by mobile PD fluxes towards sinks such as PD clusters or GBs. According to the model, most of the dragging occurs via isolated interstitials. These results are in very good agreement with previous studies, suggesting a correlation of position between solute-rich clusters and sinks. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Lecoq N.,CNRS Material Physics Group
Journal of Physics: Conference Series | Year: 2012

Hydrodynamic interactions between particles and walls are relevant for the open problem of specifying boundary conditions for suspension flows. The Reynolds number around a small particle close to a wall is usually low and creeping flow equations apply. From the solution of these equations, the drag coefficient on a sphere becomes infinite when the gap between the sphere and a smooth wall vanishes, so that contact may not occur. Physically, the drag is finite because of various reasons, one of them being the particle and wall roughness. Then, for vanishing gap, even though some layers of fluid molecules may be left between the particle and wall roughness peaks, it may conventionally be said that contact occurs. In this paper, we are considering the example of a smooth sphere moving towards a rough wall. The roughness considered here consist of random rough planes or parallel periodic wedges, the characteristic length of which is small compared with the sphere radius. This problem is considered both experimentally and theoretically. The motion of a millimetre size bead settling towards a corrugated horizontal wall in a viscous oil is measured with laser interferometry giving an accuracy on the displacement of 0.2μm. Several random rough planes and wedge shaped walls were used, with various wavelengths and wedge angles. From the results, it is observed that the velocity of the sphere is, except for small gaps, similar to that towards a smooth plane that is shifted down from the top of corrugations. For the periodic wedges, the creeping flow is calculated as a series in the slope of the roughness grooves. The convergence of the series for the shift distance in term of the slope is accelerated by use of Euler transformation and of the existence of a limit for large slope. The cases of a flow along and across the grooves are considered separately. The shift is larger in the former case. Slightly flattened tops of the wedges used in experiments are also considered in the calculations. The effective theoretical shift for a sphere approaching a wall is obtained from Lorentz reciprocal theorem with an expansion for small roughness compared with the gap between the sphere and the wall. The effective shift is found to be the average of the shifts for shear flows in the two perpendicular directions. A good agreement is found between theory and experiment. The theoretical description of the flow close to the random rough wall represents a difficult, nearly insurmountable problem except in lattice Boltzmann simulations. Statistical analysis is presented in this paper to deduce the effective shift for sand-blasted rough surfaces. To overcome the difficulties of modelling, a regular perturbation expansion is developed, and from Lorentz reciprocal theorem, the first order correction to the drag force due to random roughness is evaluated. © Published under licence by IOP Publishing Ltd.

Vieille B.,CNRS Material Physics Group | Taleb L.,CNRS Material Physics Group
Composites Science and Technology | Year: 2011

Could thermoplastic-based composites be used to replace thermosetting-based composites in high-temperature secondary aircraft structures? The purpose of this work is to establish the ability of a material system to be used in aircraft engine nacelles when subjected to static loadings, with a key upper temperature of 120°C. In order to provide answers to this question, the thermo-mechanical behaviors of carbon fiber fabric reinforced PPS or epoxy laminates have been compared specifically within the temperature change with 120°C at the upper bound. The temperature-dependent ductile behavior of laminates is more or less exacerbated, depending on polymers glass transition temperature, and laminates stacking sequence. For both materials, the degree of retention of tensile mechanical properties is quite high in notched and unnotched quasi-isotropic laminates. A Digital Image Correlation technique has been used in order to understand the influence of temperature and matrix ductility on the mechanisms of overstresses accommodation near the hole. In fabric reinforced laminates, the high-temperature results suggest a competition between the mechanisms of damage, and the mechanism of plasticization, enhanced in angle-ply lay-ups. Thus, the highly ductile behavior of TP-based laminates, at temperatures higher than their Tg, is very effective to accommodate the overstresses near the hole. © 2011 Elsevier Ltd.

The evaporation mechanisms of surface atoms in laser assisted atom probe tomography (LA-APT) are reviewed with an emphasis on the changes in laser-matter interaction when the sample is a nanometric tip submitted to a high electric field. The nanometric dimensions induce light diffraction, the tip shape induces field enhancement and these effects together completely change the absorption properties of the sample from those of macroscopic bulk materials. Moreover, the high electric field applied to the sample during LA-APT analysis strongly modifies the surface optical properties of band gap materials, due to the band bending induced at the surface. All these effects are presented and studied in order to determine the physical mechanisms of atoms evaporation in LA-APT. Moreover, LA-APT is used as an original experimental setup to study: (a) the absorption of nanometric tips; (b) the contribution of the standing field to this laser energy absorption and (c) the heating and cooling process of nanometric sample after the interaction with ultra fast laser. © 2013 Elsevier B.V.

Taleb L.,CNRS Material Physics Group
International Journal of Plasticity | Year: 2013

This work discusses the sources of the accumulated inelastic strain observed under unsymmetrical stress cycling. Different metallic materials have been chosen for their large variety of elasto-visco-plastic behaviors: two austenitic stainless steels (304L and 316L), two ferritic steels (35NiCrMo16 and XC18), one aluminum alloy (Al 2017) and one copper-zinc alloy (CuZn27). Each material has been tested at room temperature under two loading paths: one proportional (tension-compression) and the other non proportional following a triangular path in the axial stress-shear strain space. The results demonstrate that ratcheting under multiaxial loading is clearly present even if it may be associated with other phenomena. Indeed, the multiaxial stress state governs the direction of the inelastic strain rate through the normality rule. Under unsymmetrical tension-compression load control, the cyclic growth of the inelastic strain is almost absent in the aluminum and copper-zinc alloys under the loading considered while for the austenitic steels, the cyclic accumulation seems mainly due to creep. Namely, 1D ratcheting seems rather very small for these four materials. However, for the ferritic steels, a cyclic progressive deformation is observed and may be associated with ratcheting even if the mode of control (load) may also have a contribution. © 2012 Elsevier Ltd. All rights reserved.

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