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Quey R.,Cornell University | Quey R.,Ecole Nationale Superieure des Mines de Saint - Etienne CMP | Dawson P.R.,Cornell University | Barbe F.,CNRS Material Physics Group
Computer Methods in Applied Mechanics and Engineering

A methodology is presented for the generation and meshing of large-scale three-dimensional random polycrystals. Voronoi tessellations are used and are shown to include morphological properties that make them particularly challenging to mesh with high element quality. Original approaches are presented to solve these problems: (i) "geometry regularization", which consists in removing the geometrical details of the polycrystal morphology, (ii) "multimeshing" which consists in using simultaneously several meshing algorithms to optimize mesh quality, and (iii) remeshing, by which a new mesh is constructed over a deformed mesh and the state variables are transported, for large strain applications. Detailed statistical analyses are conducted on the polycrystal morphology and mesh quality. The results are mainly illustrated by the high-quality meshing of polycrystals with large number of grains (up to 105), and the finite element method simulation of a plane strain compression of ε=1.4 of a 3000-grain polycrystal. The presented algorithms are implemented and distributed in a free (open-source) software package: Neper. © 2011 Elsevier B.V. Source

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

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. Source

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. Source

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

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. Source

Silaeva E.P.,CNRS Material Physics Group | Karahka M.,Dalhousie University | Kreuzer H.J.,Dalhousie University
Current Opinion in Solid State and Materials Science

After reviewing the physics and chemistry in high electrostatic fields and summarizing the theoretical results for Atom Probe Tomography of metallic tips, we turn to the new challenges associated with insulators and semiconductors with regard to local fields inside and on the surface of such materials. The recent (theoretical) discovery that in high fields the band gap in these materials is drastically reduced to the point where at the evaporation field strength it vanishes will be crucial in our discussion. © 2013 Elsevier Ltd. All rights reserved. Source

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