On the importance of prismatic/basal interfaces in the growth of (-1012) twins in hexagonal close-packed crystals

The growth process of of (-1012) twins is studied in Magnesium using atomistic simulations. Two twin seeds are considered and both cases, a specific interface, which places face-to-face prismatic and basal planes, plays an important role. This interface has a low energy corresponding to a cusp in the orientation-dependent interface energy of a twinned bicrystal. This interface appears in several published twin structures and for instance accommodates the large deviations of twin interfaces from (-1012) planes reported recently [Zhang et al., Scr. Mater. 67 (2012) 862].Comment: 11 pages, 4 figures, submitted to Scripta Materiali

Homogénéisation des matériaux hétérogènes élastoviscoplastiques basée sur la technique des « champs translatés » : extension « affine » au cas non linéaire pour des composites biphasés

Dans cette contribution, on passe tout d’abord en revue les principales étapes de la technique à « champs translatés » pour déterminer le comportement effectif de matériaux élasto-viscoplastiques dont les phases suivent un couplage spatio-temporel de type Maxwellien et dont le comportement est supposé dans un premier temps linéaire. L’application de l’approche à « champs translatés » au problème de l’inclusion d’Eshelby viscoélastique linéaire est également présentée dans cette première partie. Le traitement de cas particuliers montre la pertinence de l’approche en viscoélasticité linéaire par rapport aux solutions obtenues classiquement par transformées de Laplace-Carson. Ensuite, l’extension de la méthode à « champs translatés » au comportement local élasto-viscoplastique avec une viscoplasticité non linéaire est résolue par le biais d’une linéarisation du comportement viscoplastique des phases de type « affine ». Cette extension couplée à un schéma d’homogénéisation à champs moyens (Mori-Tanaka ou schéma autocohérent) pour le problème hétérogène élastoviscoplastique donne une nouvelle loi d’interaction qui contient les interactions mécaniques entre les champs moyens par phase et les grandeurs macroscopiques. Dans le but de situer la validité de l’approximation à « champs moyens », les réponses mécaniques du modèle sont reportées pour des composites biphasés et sont comparées aux résultats d’autres approches d’homogénéisation de type analytique ou numériques existants dans la littérature

Stress influence on high temperature oxide scale growth: modeling and investigation on a thermal barrier coating system.

International audienceIn thermal barrier coating (TBC) systems, an oxide layer develops at high temperature below the ceramic coating, leading at long term to the mechanical failure of the structure upon cooling. This study investigates a mechanism of stress-affected oxidation likely to induce the growth of a non-uniform oxide scale detrimental to the TBC lifetime. A continuum thermodynamics formulation is derived accounting for the influence of the stress and strain situation at the sharp metal/oxide phase boundary on the local oxidation kinetics. It specially includes the contributions of the large volumetric strain and the mass consumption associated with metal oxidation. A continuum mechanics/mass diffusion framework is used along with the developed formulation for the interface evolution to study the growth of an oxide layer coupled with local stress development. The implementation of the model has required the development of a specific simulation tool, based on a finite element method completed with an external routine for the phase boundary propagation. Results on an electron-beam physical vapor deposited (EB-PVD) TBC case are presented. The processes resulting in a non-uniform oxide scale growth are analyzed and the main influences are discussed

A Fast Fourier Transform-based approach for Generalized Disclination Mechanics within a Couple Stress theory

International audienceRecently, a small-distortion theory of coupled plasticity and phase transformation accounting for the kinematics and thermodynamics of generalized defects called generalized disclinations (abbreviated g- disclinations) has been proposed. Then, a first numerical spectral approach has been developed to solve the elasto-static equations of field dislocation and g-disclination mechanics set out in this theory for periodic media and for linear elastic media using the classic Hooke’s law. Here, given a spatial distribution of generalized disclination density tensors in a homogenous linear higher order elastic media described, a couple stress theory with elastic incompatibilities of first and second orders is developed. The incompatible and compatible elastic second and first distortions are obtained from the solution of Poisson and Navier-type equations in the Fourier space. The efficient Fast Fourier Transform (FFT) algorithm is used based on intrinsic Discrete Fourier Transforms (DFT) that are well adapted to the discrete grid to compute higher order partial derivatives in the Fourier space. Therefore, stress and couple stress fields can be calculated using the inverse FFT. The numerical examples are given for straight wedge disclinations and associated wedge disclination dipoles which are of importance to geometrically describe tilt grain boundaries at fine scales in polycrystalline solids

Simulative Evaluation of Intragrain Precipitate Influence on the Material Nonlinearity using Nonlinear Ultrasound

Nondestructive evaluation using ultrasonic waves is commonly used to experimentally probe for the presence of defects (i.e. dislocations, precipitates, cracks) in complex metallic microstructures. Such defects and abnormalities are evidenced by monitoring the acoustic nonlinearity parameter β. However, from the mathematical standpoint, the correlation between the microstructural behavior and the measured acoustic nonlinearity parameter is not explicit yet. The present work aims to assess the existence of statistical correlations between microstructural defects and material nonlinearity. The effects of defect geometry, density, and geometrical arrangements (i.e. relative position) on material nonlinearity are studied. To do so, the acoustic response of Fe-Cu single crystals containing 1 % Cu precipitates with radii on the order of 10 nm is simulated by means of finite element analysis. Several thousand initial microstructures with random arrangement of precipitates are virtually tested using statistical methods, such as principal component analysis. Therefore, it is expected that a causal link can be made between the acoustic nonlinearity parameter and the precipitates-induced microstructural behavior via the proposed numerical analysis

The smallest macroscale tensile test - a model to describe constrained flow at the microscale

This work addresses the strain response and plastic flow behavior of grain boundary or interface containing materials during small scale mechanical testing. We introduce a set of geometric criteria allowing us to constrain a sample to obtain macroscopic-like flow behavior on a microscale test, as shown in Figure 1. Furthermore, the featured parameter, the blocked volume ratio, provided a new description of plasticity of microscale tensile samples in a constrained volume due to external interfaces such as coating and grain boundaries. The proposed description was experimentally validated with different Ni-based materials and different constraints (grain boundary and coating interfaces). The developed theory would open new research avenues in establishing the connection between microscale response to bulk properties as follows: Please click Additional Files below to see the full abstract

Data-Driven Constitutive Model for the Inelastic Response of Metals:Application to 316H Steel

Predictions of the mechanical response of structural elements are conditioned by the accuracy of constitutive models used at the engineering length-scale. In this regard, a prospect of mechanistic crystal-plasticity-based constitutive models is that they could be used for extrapolation beyond regimes in which they are calibrated. However, their use for assessing the performance of a component is computationally onerous. To address this limitation, a new approach is proposed whereby a surrogate constitutive model (SM) of the inelastic response of 316H steel is derived from a mechanistic crystal plasticity-based polycrystal model tracking the evolution of dislocation densities on all slip systems. The latter is used to generate a database of the expected plastic response and dislocation content evolution associated with several instances of creep loading. From the database, a SM is developed. It relies on the use of orthogonal polynomial regression to describe the evolution of the dislocation content. The SM is then validated against predictions of the dead load creep response given by the polycrystal model across a range of temperatures and stresses. When the SM is used to predict the response of 316H during complex non monotonic loading, extrapolating to new loading conditions, it is found that predictions compare particularly well against those from the physics-based polycrystal model

Modeling of the size effect in the plastic behavior of polycrystalline materials

This thesis focuses on the study of the size effect in the elastic-viscoplastic response of pure face centered cubic polycrystalline materials. First, the effect of vacancy diffusion is studied via the use of a two-phase self-consistent scheme in which the inclusion phase represents grain interiors and the matrix phase represents grain boundaries. The behavior of the inclusion phase is driven by the activity of dislocations, described with typical strain hardening laws, and by the activity of Coble creep. The behavior of the matrix phase is modeled as elastic-perfect plastic. This model is then extended to account for the possible activity of Lifschitz sliding. The active role of grain boundaries to the viscoplastic deformation is studied with the introduction of a novel method allowing the scale transition from the atomistic scale to the macroscopic scale. A model describing the mechanism of grain boundary dislocation emission and penetration is informed with molecular simulations and finite element simulations. The macroscopic response of the material is then predicted with use of several self-consistent schemes, among which two novel three-phases schemes are introduced. The most refined micromechanical scheme proposed is based on a two-phase representation of the material and is valid in the elastic-viscoplastic regime and accounts for the effect of slightly weakened interfaces.Ph.D.Committee Chair: Johnson, Steven; Committee Co-Chair: Cherkaoui, Mohammed; Committee Co-Chair: Qu, Jianmin; Committee Member: Garmestani, Hamid; Committee Member: McDowell, Davi

Modélisation de l'effet de taille dans le comportement élasto-viscoplastique des matériaux nanocristallins

This study is dedicated to the modeling of the size effect in the elasticviscoplastic behavior of nanocrystalline materials. Among others. this work aims at identifying the mechanisms responsible for the breakdown of the hall Petch law as well as to propose a new map of deformation which could aver usefull for future improvement of experimental fabrication processes. A detailed state of the art is presente in the first chapter of this theses where the structure of NC materials is presented as well the fabrication process and existing models. The state of the art is followed by a first attempt model where the effect of Coble Creep as well as that of Lischik sliding are quantified. The macroscopic behavior of the material is retrieved via the use of a self consistent micromechanical scheme. The second par tof this work i dedicated to the modeling of the recently introduced grain boundary dislocation emission mechanism. A new constitutive law , based on the formalism of thennally activated mechanisms, is introduced to describe the combined effect of dislocation emission and dislocation penetration. The study is completed by both a method of evaluation of the parameters based on molecular dynamics simulations and by finite element analysesCe travail de thèse est dédie à l'étude de l'effet de taille dans le comportement élastovisocplatsique des matériaux nanocristallin purs à structure cubique face centrée. Ce dernier ce révèle notamment par le non respect de la loi de Hall de Petch se produisant dans cas de matériau à très faible taille de grain. Dans un premier temps, un état de l'art détaille présente le type de matériau considéré, leur structure, procédé de fabrication et comportement. Les effets des mécanismes de diffusion (fluage de Coble. Glissement de Lifshitz) son quantifier par l'introduction d'un modèle composite biphasé. Dans un second temps. L'effet de l'émission de dislocations par les joints de grains est étudie. Une nouvelle loi de comportement est introduite, traduisant de l'effet du mécanisme combinée d'émission et de pénétration de dislocations sur le comportement élasto-viscoplastique des joints de grains. Le modèle se base sur le formalisme des mécanismes thermiquement active et une méthode d'obtention des paramètres basée sur la dynamique moléculaire est introduite. Le comportement macroscopique du matériau est obtenu suite à l'application d'un schéma sécant autocohérent. Ce second modèle est complète par une analyse par éléments fini

GD3: generalized discrete defect dynamics

Abstract A mesoscale model is introduced to study the dynamics of material defects lying at interface junctions. The proposed framework couples the dynamics of discrete dislocation and disclination lines. Disclinations are expected to be natural defects at interface junctions; their presence serving the purpose of accommodating discontinuities in rotation fields at material interface junctions. Crystallography-based rules are proposed to describe the kinematics of disclination motion. A discrete-continuous couple-stress framework, in which discrete defect lines are introduced as plastic eigenstrains and eigencurvatures, is proposed to explicitly follow the dynamics of interfacial defects. The framework is then applied to study 101̄2 $\left (10\bar {1}2\right)$ twin transverse propagation and thickening in magnesium. Focusing first on the case of a twin domain, It is shown that a disclination based representation of twin domains allows for an appropriate mechanistic description of the kinematics of shear transformations. In what concerns twin thickening, the stability of defects at twin interfaces is further studied. To this end, a 3D crater lying on a twin interface is described as a dipole of disclination loops. Upon self-relaxation, it is found that out of plane motion of disclinations followed by the nucleation of twinning dislocations can be activated; thereby showing that conservative non-planar motion of disclinations can be thermodynamically favorable; mechanism that had been postulated some 50 years ago