Les méthodes conçues pour être incorporées dans des polycristaux de modélisation multi-échelles sont présentées dans ce travail en deux tâches. Ce travail contient des méthodes à moyenne échelle pour capturer les effets des interactions de dislocations de glissement rencontrant des joints de grains maclage et la croissance simultanée de plusieurs fractions de volume de grains maclage sur le durcissement mécanique et l’évolution de la texture. Celles-ci sont mises en œuvre dans un cadre de plasticité cristalline utilisant le code visco-plastic-self consistent de Los Alamos, VPSC-7. Présentés ici, les effets de la croissance simultanée de multiples variantes maclage sur l’évolution de la texture sont suivis à l’aide d’un schéma de transfert de volume double de type Kalidindi. Dans la tâche 1, la mise en œuvre de ce schéma afin de simuler la texture des aciers à plasticité induite par maclage (TWIP) soumis au pressage angulaire à canal égal (ECAP) est résumée. Dans la tâche 2, les effets de durcissement de deux types d’interaction entre les dislocations de glissement et les joints de grain maclage rencontrés, à savoir la transmutation et la dissociation de dislocation, sont capturés au moyen de la modification du modèle de durcissement basé sur la densité de dislocation de [11]. Les interactions du premier type sont présentées dans une relation constitutive calculant la quantité de densité de dislocations attribuée à un système de glissement donné contenu dans la fraction de volume maclage rencontrée à partir de chaque système de glissement en interaction dans la fraction de volume mère. La quantité transmutée à partir de chaque système de glissement en interaction décrit à l’aide de la méthode de correspondance, sur la cartographie des systèmes de glisse- ment d’un grain parent à des systèmes de glissement dans des grains maclage considérés. Des interactions du second type sont ensuite introduites dans cette relation constitutive en tant que paramètre de dissociation, dont la valeur est établie par les observations tirées des résultats des simulations de dynamique moléculaire de [8] et [53]. Ces méthodes sont implantées pour simuler le comportement de durcissement anisotrope du magnésium HCP sous plusieurs chemins de charge / Methods designed for incorporation into multiscale modeling polycrystals are presented in this work in two tasks. This work contains mesoscale methods for capturing the effects of both the interactions of slip dislocations encountering twin grain boundaries and the simultaneous growth of multiple twin grain volume fractions on mechanical hardening and texture evolution. These are implemented in a crystal plasticity framework using the Los Alamos viscoplastic self-consistent code, VPSC-7. Presented here, the effects of simultaneous growth in multiple twin variants on textural evolution is tracked using a Kalidindi-type twin volume transfer scheme. In Task 1, the implementation of this scheme in order to simulate the texture of Twinning Induced Plasticity steels (TWIP) subjected to Equal Channel Angular Pressing (ECAP) are summarized. In Task 2, the hardening effects of two types of interactions between slip dislocations and encountered twin grain boundaries, namely dislocation transmutation and dissociation, are captured by way of modifying the dislocation density based hardening model of [11]. Interactions of the first type are presented in a constitutive relation calculating the amount of dislocation density apportioned to a given slip system contained within the encountered twin volume fraction from each interacting slip system in the parent volume fraction. The amount transmuted from each interacting slip system described using the Correspondence Method, an on to mapping of slip systems in a parent grain to slip systems in considered twin grains. Interactions of the second type are then introduced into this constitutive relation as a disassociation parameter, the value of which is established by observations gleaned from the results of the molecular dynamics simulations of [8] and [53]. These methods are implanted to simulate the anisotropic hardening behavior of HCP magnesium under multiple load paths

The motivation to use lightweight materials in the construction of the automotive structure is the resultant increased fuel efficiency. However, these materials possess certain drawbacks that make it challenging to adopt them into current automobile manufacturing processes. In this dissertation the microstructural response observed in a magnesium alloy, AZ31, and an advanced high strength steel alloy, QP1180, to uniaxial deformation is analyzed and the results are presented. In AZ31 the required slip modes are not activated at room temperature leading to its low ductility at room temperature. The resulting activity of these twins in response to uniaxial tension is analyzed and its correlations with the microstructure features is reported. Additionally, a neighborhood viscoplastic self-consistent model is developed that will allow more accurate simulation of twin response to outside deformation. Furthermore, activity of slip modes that are usually observed at high temperatures (>200°C) are also observed at lower temperatures (<125°C) and they are compared to the relative twin activity at these temperatures. It is observed that larger grains, with high schmid factors, longer grain boundaries and have misorientation with its neighboring grain greater than 27° are more favorable for twin formation and transmission in the AZ31 microstructure in response to uniaxial tension. The nature of retained austenite (RA) transformation into martensite that gives QP1180 its enhanced ductility, is not clearly understood primarily because of challenges present in characterization of these metastable RA. Further, a 2 dimensional characterization method does not provide the complete information of the RA grain. These challenges are overcome by characterization of a 3 dimensional volume element using serial sectioning and EBSD followed by reconstruction using DREAM3D. The influence of 3d morphology and orientation direction on RA transformation is studied using as-is and uniaxially deformed samples. A novel shear affinity factor is introduced as a metric to describe the ease of RA transformation under uniaxial tension. The 3d nature of the information collected allows a new classification of disk shape in addition to globular and lamellar shapes for RA. It is found that RA that are low volume laths and have low shear affinity factor transform later compared to disk shaped RA’s. Through these guidelines the preparation of a microstructure that is conducive to RA transformation under uniaxial tension is possible.

Methods designed for incorporation into multiscale modeling polycrystals are presented in this work in two tasks. This work contains mesoscale methods for capturing the effects of both the interactions of slip dislocations encountering twin grain boundaries and the simultaneous growth of multiple twin grain volume fractions on mechanical hardening and texture evolution. These are implemented in a crystal plasticity framework using the Los Alamos visco-plastic self consistent code, VPSC-7. Presented here, the effects of simultaneous growth in multiple twin variants on textural evolution is tracked using a Kalidindi-type twin volume transfer scheme. In Task 1, the implementation of this scheme in order to simulate the texture of Twinning Induced Plasticity steels (TWIP) subjected to Equal Channel Angular Pressing (ECAP) are summarized. In Task 2, the hardening effects of two types of interactions between slip dislocations and encountered twin grain boundaries, namely dislocation transmutation and dissociation, are captured by way of modifying the dislocation density based hardening model of [14]. Interactions of the first type are presented in a constitutive relation calculating the amount of dislocation density apportioned to a given slip system contained within the encountered twin volume fraction from each interacting slip system in the parent volume fraction. The amount transmuted from each interacting slip system described using the Correspondence Method, an onto mapping of slip systems in a parent grain to slip systems in considered twin grains. Interactions of the second type are then introduced into this constitutive relation as a disassociation parameter, the value of which is established by observations gleaned from the results of the molecular dynamics simulations of [11] and [36]. These methods are implanted to simulate the anisotropic hardening behavior of HCP magnesium under multiple load paths.

El interés industrial por la formabilidad de chapas de aceros de doble fase (DP) se ha incrementado en las últimas décadas, impulsado principalmente por la reciente popularidad de los aceros avanzados de alta resistencia (AHSS) para reducir el peso de partes automotrices. Esto resulta en una fuerte necesidad de determinar la respuesta límite del material frente a solicitaciones típicas de operaciones de conformado y el estudio de los factores que la influencian. La presente Tesis Doctoral aborda el estudio numérico de los factores microestructurales que influyen en el diagrama límite de conformado (FLD) de chapas de acero DP-780. El comportamiento límite del material se modela mediante la técnica de Marciniak-Kuczynski (MK), la cual asume la presencia de una imperfección inicial precursora del proceso de localización; mientras que la descripción constitutiva del material se realiza en el marco de la plasticidad cristalina. El comportamiento anisótropo, la presencia de una distribución preferencial de orientaciones y el efecto de las fases constituyentes – ferrita/martensita – se obtiene mediante una homogeneización autoconsistente de la respuesta viscoplástica a nivel del cristal simple (VPSC). El acople de ambas técnicas (MK-VPSC) permite modelar exitosamente la respuesta límite de las chapas de acero DP-780. Se investiga numéricamente el efecto de parámetros microestructurales típicos de aceros DP, la influencia de la anisotropía y su evolución, así como el efecto del comportamiento del endurecimiento post-estricción en las deformaciones límite. Tanto la fracción en volumen como la plasticidad de la martensita presentan una influencia significativa en la predicción del diagrama FLD, mientras que la evolución de la textura cristalográfica sólo afecta las deformaciones límite bajo solicitaciones biaxiales. El mejor acuerdo con los datos experimentales se encuentra cuando se utiliza una ley de endurecimiento de saturación y cuando la deformación de la martensita es impedida o es retardada hasta el punto de estricción. Un análisis de la actividad de los sistemas de deslizamiento sugiere que, dentro del marco de trabajo del modelo MK-VPSC, la localización ocurre mucho más rápido en la ferrita que en la martensita. Se presenta una extensión del modelo MK-VPSC que permite evitar problemas de convergencia y reducir el costo computacional. Esto se alcanza aplicando directamente las condiciones en velocidad de deformación y tensión, resultantes de las restricciones de equilibrio y compatibilidad, en la banda de inestabilidad del modelo MK. Además, los estados mecánicos dentro y fuera de ésta se resuelven en el marco de referencia de la muestra, evitando rotar las orientaciones cristalográficas y las variables internas a la orientación de la banda para cada incremento, mejorando la eficiencia computacional. Las condiciones de borde generalizadas incorporadas al modelo permiten calcular diagramas FLD basados en trayectorias de carga en deformación (FLDρ) como en tensión (FLDα). / Triggered by the recent popularity of advanced high strength steels (AHSS) for weight-reduction in automotive components, industrial interest in the formability of dual-phase (DP) steel sheets has increased in the last decades. Thus, there is a strong need in the determination of the material’s limit behavior for typical loading conditions in sheet forming operations, as well as the analysis of the influencing factors. This thesis addresses the numerical study of microstructural factors influencing the forming limit diagram (FLD) of DP-780 steel sheets. The material’s limit behavior is modeled by the Marciniak-Kuczynski (MK) model, which assumes an initial imperfection, precursor of the localization process; whereas the material’s constitutive description is performed within the crystal plasticity framework. The anisotropic behavior, the presence of preferred orientation distributions and the effect of the constituent phases – ferrite/martensite – is obtained by a self-consistent homogenization of the single crystal viscoplastic response (VPSC). The coupled techniques (MK-VPSC) can successfully model the limit response of the DP-780 steel sheet. The effect of typical microstructural parameters of DP steels, the influence of anisotropy and its evolution with deformation, as well as the extrapolated post-necking hardening behavior, on the forming limits is numerically investigated. Both the martensitic volume fraction and plasticity have a significant influence on the FLD prediction, while the evolution of crystallographic texture only affects the limit strains under biaxial deformation. The best agreement with experimentation is found when using the saturation hardening law and when the martensite deformation is either not allowed or retarded to occur after the point of necking. An analysis of the slip systems activity suggests that, within the MK-VPSC framework, localization occurs much faster in the ferritic than in the martensitic phase. An extension to the MK-VPSC model is presented in this thesis in order to avoid convergence problems and reduce the computational cost. This is achieved by directly applying the stress and strain-rate boundary conditions, resulting from the equilibrium and compatibility restrictions, at the MK instability band. Moreover, the mechanical states outside and inside the groove are solved in the sample reference frame. This avoids rotating the crystallographic orientations and the internal variables to the current groove orientation for each increment, improving the computational performance. The generalized boundary conditions in the polycrystal model allow calculating either strain ratio (FLDρ) or stress ratio (FLDα) based FLDs.

Texture evolution of depleted uranium is investigated using a viscoplastic self-consistent model. Depleted uranium, which has the same structure as alpha-uranium, is difficult to model as it has an orthorhombic symmetry structure, as well as many twin systems which must be addressed in order to properly simulate the textural evolution. The VPSC method allowed for a flexible model which could not only encompass the low symmetry component but also the twinning components of depleted uranium. The model focuses on the viscoplastic regime only, neglecting the elastic regime of deformation, and uses a self-consistent method to solve the model. Different deformation processes, such as torsion, rolling, and swaging, are simulated and the theoretical textures, plotted as pole figures or inverse pole figures, are compared with previous experimental textures found for alpha uranium from previous experimental sources. A specific twin system, the (176)[512] system, is also given special consideration. This twin system is a dominant deformation mode for alpha uranium at high strain rates, but is quite elusive in general. Different deformation processes are compared where this twin system is active and not active. This allows comparison on the effect of this twin on the overall texture of depleted uranium. In addition, a sample of depleted uranium from Y12 which was analyzed for (176)[512] twins is compared to theoretical results from a VPSC simulation where the (176)[512] twin is active.

La déformation plastique du monocristaI de glace se produit essentiellement par glissement de dislocations dans les plans de base. Le glissement sur d'autres systèmes ou une éventuelle montée de dislocations basales limite la vitesse de déformation de la glace polycristalline. Dans les glaces des calottes polaires, une orientation préferentielle des axes c se développe au cours de la déformation, induisant une forte anisotropie viscoplastique. Un modèle auto-cohérent viscoplastique (VPSC) est utilisé pour calculer le comportement mécanique de la glace polycristalline et le développement des textures. Les résultats sont comparés à ceux du modèle à contraintes uniformes (borne inférieure) et à déformations uniformes (borne supérieure). Dans les modèles, il est supposé que le monocristal de glace se déforme par glissement basal, prismatique, et pyramidal. La résistance des systèmes de glissement d'un grain in-situ est déterminée à partir de résultats d'essais mécaniques sur des échantillons polycristallins fortement anisotropes. D'après le modèle VPSC, le comportement d'un grain in-situ est semblable à celui d'un monocristal isolé. Ce modèle reproduit parfaitement le comportement expérimental des glaces polycristallines anisotropes. Lorsqu'un polycristal est déformé de telle manière à ce que le glissement basal soit difficilement activé, la direction de la vitesse de déformation est très sensible à la direction de la contrainte, appliquée. De telles conditions de déformation devraient se retrouver dans le voisinage des dômes des calottes polaires. Dans les couches de surface des calottes polaires (zone de grossissement de grains), le modèle VPSC permet d'obtenir une bonne estimation du développement des textures. Dans les glaces plus profondes, on montre que la recristallisation par rotation a une influence significative sur le développement des textures. Une série d'essais mécaniques en laboratoire indique une tendance au comportement newtonien sous de très faibles contraintes.

Hexagonal closed packed (hcp) materials, in contrast to cubic materials, possess several processing challenges due to their anisotropic structural response, the wide variety of deformation textures they exhibit, and limited ductility at room temperature. The aim of this work is to investigate, both experimentally and theoretically, the effect os severe plastic deformation, ultrafine grain sizes, crystallographic textures and number of phases on the flow stress anisotropy and tension compression asymmetry, and the mechanisms responsible for these phenomena in two hcp materials: AZ31B Mg alloy consisting of one phase and Zn-8wt.% Al that has an hcp matrix with a secondary facecentered cubic (fcc) phase. Mg and its alloys have high specific strength that can potentially meet the high demand for light weight structural materials and low fuelconsumption in transportation. Zn-Al alloys, on the other hand, can be potential substitutes for several ferrous and non-ferrous materials because of their good mechanical and tribological properties. Both alloys have been successfully processed using equal channel angular extrusion (ECAE) following different processing routes in order to produce samples with a wide variety of microstructures and crystallographic textures for revealing the relationship between microstructural parameters, crystallographic texture and resulting flow stress anisotropy at room temperature. For AZ31B Mg alloy, the texture evolution during ECAE following conventional and hybrid ECAE routes was successfully predicted using visco-plastic self-consistent (VPSC) crystal plasticity model. The flow stress anisotropy and tension-compression (T/C) asymmetry of the as received and processed samples at room temperature were measured and predicted using the same VPSC model coupled with a dislocation-based hardening scheme. The governing mechanisms behind these phenomena are revealed as functions of grains size and crystallographic texture. It was found that the variation in flow stress anisotropy and T/C asymmetry among samples can be explained based on the texture that is generated after each processing path. Therefore, it is possible to control the flow anisotropy and T/C asymmetry in this alloy and similar Mg alloys by controlling the processing route and number of passes, and the selection of processing conditions can be optimized using VPSC simulations. In Zn-8wt.% Al alloy, the hard phase size, morphology, and distribution were found to control the anisotropy in the flow strength and elongation to failure of the ECAE processed samples.

No description available.