Michel Perez

Dislocation interaction with C in α-Fe: A comparison between atomic simulations and elasticity theory

Abstract The interaction of C atoms with a screw and an edge dislocation is modelled at an atomic scale using an empirical Fe–C interatomic potential based on the embedded atom method and molecular statics simulations. Results of atomic simulations are compared with predictions of elasticity theory. It is shown that a quantitative agreement can be obtained between both modelling techniques as long as anisotropic elastic calculations are performed and both the dilatation and the tetragonal distortion induced by the C interstitial are considered. Using isotropic elasticity allows the prediction of the main trends of the interaction, whereas considering only the interstitial dilatation will lead to a wrong interaction.

Predictors of Cavitation in Glassy Polymers under Tensile Strain: A Coarse-Grained Molecular Dynamics Investigation

The nucleation of cavities in a homogenous polymer under tensile strain is investigated in a coarse-grained molecular dynamics simulation. In order to establish a causal relation between local microstructure and the onset of cavitation, a detailed analysis of some local properties is presented. In contrast to common assumptions, the nucleation of a cavity is neither correlated to a local loss of density nor, to the stress at the atomic scale and nor to the chain ends density in the undeformed state. Instead, a cavity in glassy polymers nucleates in regions that display a low bulk elastic modulus. This criterion allows one to predict the cavity position before the cavitation occurs. Even if the localization of a cavity is not directly predictable from the initial configuration, the elastically weak zones identified in the initial state emerge as favorite spots for cavity formation.

Microscopic modelling of simultaneous two-phase precipitation: application to carbide precipitation in low-carbon steels

Abstract A thermodynamically-based precipitation model, employing the classical nucleation and growth theories, has been adapted to deal with simultaneous precipitation of metastable and stable phases. This model gives an estimation of the precipitation kinetics (time evolution of radius and density of precipitates for both phases, as well as the evolution of solute fraction) in a wide range of temperature. Results were successfully compared with an experimental isothermal precipitation diagram (Time–Temperature-Transformation, TTT) from the literature for the precipitation of ϵ carbide and Fe 3 C in low-carbon steels.

Mechanical testing of glassy and rubbery polymers in numerical simulations: Role of boundary conditions in tensile stress experiments

We use coarse-grained molecular dynamics simulations to perform tensile test deformation on glassy and rubbery polymer samples using two types of driving for the deformation. We compare the outcome from a standard homogeneous deformation procedure with that of a boundary driven procedure in which the sample is driven by the nanometric equivalent of grips. No significant difference is observed in both uniaxial and triaxial tensile experiments. Implications for testing the behavior of nonhomogeneous polymer materials are briefly discussed.

Nanoscale buckling deformation in layered copolymer materials

In layered materials, a common mode of deformation involves buckling of the layers under tensile deformation in the direction perpendicular to the layers. The instability mechanism, which operates in elastic materials from geological to nanometer scales, involves the elastic contrast between different layers. In a regular stacking of “hard” and “soft” layers, the tensile stress is first accommodated by a large deformation of the soft layers. The inhibited Poisson contraction results in a compressive stress in the direction transverse to the tensile deformation axis. The hard layers sustain this transverse compression until buckling takes place and results in an undulated structure. Using molecular simulations, we demonstrate this scenario for a material made of triblock copolymers. The buckling deformation is observed to take place at the nanoscale, at a wavelength that depends on strain rate. In contrast to what is commonly assumed, the wavelength of the undulation is not determined by defects in the microstructure. Rather, it results from kinetic effects, with a competition between the rate of strain and the growth rate of the instability.

Atomistic modeling of carbon Cottrell atmospheres in bcc iron

Atomistic simulations with an EAM interatomic potential were used to evaluate carbon-dislocation binding energies in bcc iron. These binding energies were then used to calculate the occupation probability of interstitial sites in the vicinity of an edge and a screw dislocation. The saturation concentration due to carbon-carbon interactions was also estimated by atomistic simulations in the dislocation core and taken as an upper limit for carbon concentration in a Cottrell atmosphere. We obtained a maximum concentration of 10 ± 1 at.% C at T = 0 K within a radius of 1 nm from the dislocation lines. The spatial carbon distributions around the line defects revealed that the Cottrell atmosphere associated with an edge dislocation is denser than that around a screw dislocation, in contrast with the predictions of the classical model of Cochardt and colleagues. Moreover, the present Cottrell atmosphere model is in reasonable quantitative accord with the three-dimensional atom probe data available in the literature.

High-temperature contactless viscosity measurements by the gas–film levitation technique: Application to oxide and metallic glasses

In the field of thermophysical characterization of materials at high temperature, a crucial issue is to limit the effect of chemical or physical phenomena occurring at the interface between the sample and the container. Therefore, contactless techniques are well adapted to high-temperature measurements. The gas–film levitation method has recently proved to be applicable to viscosity measurements. Our purpose in this article is to derive viscosity values by the observation of the dynamical response of a perturbed levitating drop. We present here recent improvements in this technique, with particular attention paid to measurement accuracy issues and temperature calibration problems. Viscosity measurements performed on oxide and metallic glasses reveal that a gas–film levitation based viscometer is able to provide measurements with good accuracy (±10%) in a wide viscosity range, from very viscous (up to several kPa s, aperiodic relaxation of the drop) to fluid liquids (a few mPa s, damped oscillation of the ...

Assessment of consolidation of oxide dispersion strengthened ferritic steels by spark plasma sintering: from laboratory scale to industrial products

Abstract Oxide dispersion strengthened steels are new generation alloys that are usually processed by hot isostatic pressing (HIP). In this study, spark plasma sintering (SPS) was studied as an alternative consolidation technique. The influence of the processing parameters on the microstructure was quantified. The homogeneity of the SPSed materials was characterised by electron microprobe and microhardness. A combination of limited grain growth and minimised porosity can be achieved on semi-industrial compact. Excellent tensile properties were obtained compared to the literature.

Rolling bearing applications: some trends in materials and heat treatments

Abstract Rolling contact fatigue resistance and dimensional stability are the two important requirements for rolling bearing materials. Rolling contact fatigue can be initiated under the surface due to the presence of elastic inhomogeneities (inclusions) in the material, and, on the surface due to starved or contamination lubrication conditions. Two classical solutions in terms of material or heat treatment are presented and criticised. First, it is shown that ceramic balls, classically used to reduce contact stress, may induce deeper dents under contaminated environment. Second, an integrated approach based on thermoelectric power measurement is used to predict and understand the dimensional stability of bainitic 100Cr6 steel that exhibits dimension change proportional to its retained austenite content.

Characterization of precipitates size distribution: validation of low-voltage STEM

The size distribution of second phase precipitates is frequently determined using conventional transmission electron microscopy (CTEM). However, other techniques, which present different advantages, can also be used for this purpose. In this paper, we focus on high angle annular dark field (HAADF) in TEM and scanning TEM (STEM) in scanning electron microscopy (SEM) imaging modes. The mentioned techniques will be first described, then compared to more conventional ones for the measurement of carbides size distribution in two FeCV and FeCVNb model alloys. This comparative study shows that STEM in SEM, a technique much easier to undertake compared to TEM, is perfectly adapted for size distribution measurements of second phase particles, with sizes ranging between 5 and 200 nm in these systems.