Polycrystal models, beginning with the work of Sachs (1928) and Taylor (1938), have been used to predict very complex material behavior. The basis of these models is single crystal plasticity theory, which is then extended to model an actual (polycrystalline) material composed of a large number of single crystals or grains. Crystal plasticity models are formulated at the scale of the individual grain, which is viewed as a fundamental material element. To first order this is a reasonable approximation, and results in qualitatively good predictions. However, it is also well known that the grain is not a uniform entity, and that a great deal of non-uniform activity, including the development of well-defined dislocation structures, occurs within individual grains. The goals of this research are to complete an experimental data set for validation of material modeling, and to then improve the physical basis of predictive polycrystal plasticity models. Preferred orientations (textures) of oxygen free high conductivity (OFHC) copper were measured using reflection x-ray diffraction techniques. Monotonic strain paths included a variety of strain levels for both compression and torsion. One of the significant contributions of this research was the measurement of textures resulting from non-monotonic deformation histories, specifically compressive prestrain (to two different levels) followed by torsion to an effective plastic strain of 1.00. We also concluded synchrotron radiation experiments to map Laue images to examine subgrain microtexture formation at various stages of finite deformation. The second major contribution is to polycrystal plasticity modeling. Improvements to the plasticity model were achieved by including the effects of gradually developing, sub-grain scale microstructures, without explicitly modeling the structures, in terms of both crystallographic texture formation and work hardening. The effects of these microstructures were incorporated through the use of new internal state variables. They result in a broadening of the peaks of the macroscopic texture and a reduction of the rate of texture formation. Predictions of crystallographic orientation distributions were verified by plotting stereographs, which were shown to match measured crystallographic textures. The microstructural hardening law was introduced through a new form of latent hardening, which was shown to match experimental stress-strain behavior more closely than the basic model of Pierce, Asaro, and Needleman (1982). This latent hardening form augmented a Taylor-type term, which reflected statistically stored dislocations in the slip system hardness. Significantly, this improvement was also noted in the case of non-monotonic loading, which the standard model could not predict even to first order. Also, in the course of this research a planar double slip model was used as a precursor to the full three-dimensional modeling. The objective was to use the planar model to test various formulations, at least qualitatively, since it is a simpler model. As a result of comparisons between the three-dimensional simulations and the planar ones, the planar model was shown to be an insufficient tool for developing new texture and hardening evolution schemes as compared to the three-dimensional models. The planar model was unsuitable for modeling any but the most basic crystal plasticity relations and most simple deformation paths in a qualitative manner.

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Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2000. / David L. McDowell. Part of the SMARTech Electronic Thesis and Dissertation Collection.

La mobilité de l’hydrogène dans les métaux est un paramètre clef pour la compréhension des mécanismes de base de la fragilisation par l’hydrogène (FPH). Cette problématique est directement associée aux mécanismes de diffusion et de piégeage de l’hydrogène au sein d’un réseau cristallin. Ces derniers dépendent des diverses hétérogénéités microstructurales et en particulier des défauts cristallins présents au sein du matériau. Dans le cadre de nos travaux nous nous sommes restreints à étudier la diffusion et le piégeage de l’hydrogène au sein de deux systèmes élémentaires : des monocristaux et des bi-cristaux de nickel. Nous avons développé une méthodologie associant des outils expérimentaux (Perméation électrochimique/TDS, METHR, EBSD) et numériques (FEM-COMSOL/EAM-LAMMPS). Les résultats obtenus sur monocristaux montrent une dépendance du coefficient de diffusion de l’hydrogène avec l’orientation cristallographique et la teneur en hydrogène. L’analyse thermodynamique du système nickel-hydrogène-lacune démontre une dépendance du potentiel chimique de l’hydrogène à l’état de contrainte induit par la formation d’amas de lacunes associés à la présence de l’hydrogène. Le caractère anisotrope de la diffusion est alors expliqué par l’anisotropie des propriétés d’élasticité du réseau cristallin et la présence de ces amas. D’autre part nous avons caractérisé les processus de diffusion et de piégeage de l’hydrogène pour des bi-cristaux de nickel présentant différents volumes libres. L’énergie de ségrégation de l’hydrogène dépend de la nature du site (volume libre local et énergie mécanique associée à l’incorporation du soluté). La diffusion de l’hydrogène est influencée directement par la nature de joint de grain (excès de volume et distribution des sites). Nos résultats, à l’échelle atomique, montrent une corrélation entre la solubilité et le volume libre du joint de grain. Les joints de grains avec un volume libre important présentent des chemins de diffusion plus favorables pour l'hydrogène que dans le réseau cristallin et dans le même temps un nombre plus important de sites de ségrégation. / The mobility of hydrogen in metals is a key parameter for understanding the basic mechanisms of hydrogen embrittlement (HE). This problem is directly related to the mechanisms of diffusion and trapping of hydrogen within a crystal lattice. These mechanisms depend on the various microstructural heterogeneities and in particular the crystalline defects. In our work, we have focused on the diffusion and trapping of hydrogen in two elementary systems: nickel single crystals and bi-crystals. We developed a methodology combining experimental tools (electrochemical permeation / TDS, HRTEM, EBSD) and numerical methods (FEM-COMSOL / EAM-LAMMPS). The results obtained on the single crystals show a dependence of the diffusion coefficient of hydrogen with the crystallographic orientation and the hydrogen content. The thermodynamic analysis of the nickel-hydrogen-vacancy system shows a dependence of the chemical potential of hydrogen with the stress state induced by the formation of clusters of vacancies associated with the presence of hydrogen. The anisotropic character of the diffusion is then explained by the anisotropy of the elastic properties of the crystal lattice and the presence of these clusters. Moreover, we have characterized the processes of diffusion and trapping of hydrogen for nickel bi-crystals with different free volumes. The segregation energy of hydrogen depends on the nature of the site (the local free volume and the mechanical energy associated with the incorporation of solute). The diffusion of hydrogen is directly influenced by the nature of the grain boundary (the free volume and the distribution of the segregation sites). Our results, at the atomic scale, show a correlation between the solubility and the free volume of the grain boundary. The grain boundaries with a higher free volume have more favorable diffusion paths for hydrogen than in the crystal lattice and at the same time more segregation sites.

abstract: Hydrogen embrittlement (HE) is a phenomenon that affects both the physical and chemical properties of several intrinsically ductile metals. Consequently, understanding the mechanisms behind HE has been of particular interest in both experimental and modeling research. Discrepancies between experimental observations and modeling results have led to various proposals for HE mechanisms. Therefore, to gain insights into HE mechanisms in iron, this dissertation aims to investigate several key issues involving HE such as: a) the incipient crack tip events; b) the cohesive strength of grain boundaries (GBs); c) the dislocation-GB interactions and d) the dislocation mobility. The crack tip, which presents a preferential trap site for hydrogen segregation, was examined using atomistic methods and the continuum based Rice-Thompson criterion as sufficient concentration of hydrogen can alter the crack tip deformation mechanism. Results suggest that there is a plausible co-existence of the adsorption induced dislocation emission and hydrogen enhanced decohesion mechanisms. In the case of GB-hydrogen interaction, we observed that the segregation of hydrogen along the interface leads to a reduction in cohesive strength resulting in intergranular failure. A methodology was further developed to quantify the role of the GB structure on this behavior. GBs play a fundamental role in determining the strengthening mechanisms acting as an impediment to the dislocation motion; however, the presence of an unsurmountable barrier for a dislocation can generate slip localization that could further lead to intergranular crack initiation. It was found that the presence of hydrogen increases the strain energy stored within the GB which could lead to a transition in failure mode. Finally, in the case of body centered cubic metals, understanding the complex screw dislocation motion is critical to the development of an accurate continuum description of the plastic behavior. Further, the presence of hydrogen has been shown to drastically alter the plastic deformation, but the precise role of hydrogen is still unclear. Thus, the role of hydrogen on the dislocation mobility was examined using density functional theory and atomistic simulations. Overall, this dissertation provides a novel atomic-scale understanding of the HE mechanism and development of multiscale tools for future endeavors. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2015

Polycrystalline materials are widely used for industrial applications. These materials are highly anisotropic with different responses under different loading conditions. This dissertation uses a crystal plasticity scheme in the finite element framework (CPFEM) to study deformation mechanisms in alumina, aluminium and stainless steel – all polycrystalline. Four research cases in this dissertation have been presented in the form of manuscripts for publication. When possible, modelling predictions have been compared against various experimental techniques such as Diffraction Contrast Tomography (DCT), Neutron Diffraction (ND) and Electron Back Scatter Diffraction (EBSD). After an introduction (Chapter 1) and a literature review (Chapter 2) on plastic deformation and modelling techniques, the methodology and results are presented and discussed (Chapters 3 and 4). Measurements of elastic strains for individual grain families (ND) and local rotations (DCT and EBSD) are compared against corresponding predictions by the model following different loading modes. Each study reveals different degrees of agreement between predictions and measurements. The individual conclusions to each study are presented in Chapter 4. Some overall conclusions and suggestions for further work are presented in Chapter 5.

Two-dimensional van der Waals materials have been a subject of intense research interest in recent years. High thermal conductivity of graphene can be utilized for many thermal management applications. In spite of possessing very high electron mobility, graphene can’t be used as transistors because of the absence of band gap; however transition metal dichalcogenides are another class of two-dimensional van der Waals materials with inherent band gap and show a great promise for future nanoelectronic applications. But in order to tailor these properties for commercial applications, we should develop a better understanding of the effect of extrinsic factors like size, rough edges, grain boundaries, mass-impurities, interaction with substrate etc. on thermal and electrical transport. Most materials exhibit a smooth ballistic-to-diffusive type of thermal transport in which when the sample size is small as compared to mean-free-path of phonons the transport is ballistic, whereas, when the sample size is large as compared to phonon mean-free-path, phonons undergo multiple scattering events and the thermal transport becomes diffusive in nature. However, graphene exhibits an atypical thermal transport behavior where thermal conductivity shows an increasing logarithmic trend even for samples far greater than the mean-free-path of phonons. We show that this anomalous behavior can be attributed to the significant contribution coming from momentum-conserving normal phonon-phonon scattering. Secondly, graphene grain boundaries have been found to significantly reduce thermal conductivity even in the presence of substrates. In spite of numerous studies on the effect of grain boundaries (GBs) on thermal conductivity in graphene, there lacks a complete correlation between GB resistance and misorientation angle across graphene GBs. We show a direct correlation between thermal GB resistance and mismatch angles with low angle mismatch can be captured only by GB roughness, whereas, large mismatch angles will lead to the formation of a disordered patch at the interface and it could significantly deteriorate the overall thermal conductivity even in the presence of substrates. GBs are found to affect electrical transport in two-dimensional systems as well. Owing to the excellent electronic properties and compactness of these two-dimensional materials, high quality 2D heterojunctions are the subject of intense research interest in recent years. Graphene-MoS2 heterojuctions are found to form ohmic contacts and show great potential for future nanoelectronic applications. We show that the interface resistance in Gr-MoS2 heterojuctions can affect the overall resistance of the device if the channel (MoS2) length is small at low carrier densities, whereas, at high carrier densities interface resistance do not play much role in determining the resistance of the entire device. However, if graphene and MoS2 grains are misorientated then interface resistance can play a crucial role in determining the overall resistance of the device. We also show a weak dependence of misorientation angles on GB resistance across MoS2 grain boundaries.

In this work, the relationship between grain boundary crystallography and grain boundary atomic structure is examined, using [1 0 0] - symmetric tilt grain boundaries in nickel. The structural unit model is used as a benchmark to evaluate the atomic structure description capacities of an emerging structural descriptor, the local environment representation, which itself is a refinement of the also-emergent Smooth Overlap of Atomic Positions (SOAP) descriptor. We show that the local environment representation encodes both the information of the structural unit model and additional information, such as distortion in the structural units and the arrangement of the structural units at the interface. The use of the local environment representation permits the use of a visualization tool known as SPRING to represent structural similarities between grain boundaries. With the SPRING representation, we produce objective evidence of a relationship between crystallography and atomic structure, at least for [1 0 0] - symmetric tilt grain boundaries.

abstract: Niobium is the primary material for fabricating superconducting radio-frequency (SRF) cavities. However, presence of impurities and defects degrade the superconducting behavior of niobium twofold, first by nucleating non-superconducting phases and second by increasing the residual surface resistance of cavities. In particular, niobium absorbs hydrogen during cavity fabrication and promotes precipitation of non-superconducting niobium hydride phases. Additionally, magnetic flux trapping at defects leads to a normal conducting (non-superconducting) core which increases surface resistance and negatively affects niobium performance for superconducting applications. However, undelaying mechanisms related to hydride formation and dissolution along with defect interaction with magnetic fields is still unclear. Therefore, this dissertation aims to investigate the role of defects and impurities on functional properties of niobium for SRF cavities using first-principles methods. Here, density functional theory calculations revealed that nitrogen addition suppressed hydrogen absorption interstitially and at grain boundaries, and it also decreased the energetic stability of niobium hydride precipitates present in niobium. Further, hydrogen segregation at the screw dislocation was observed to transform the dislocation core structure and increase the barrier for screw dislocation motion. Valence charge transfer calculations displayed a strong tendency of nitrogen to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen leading to a very unstable state for interstitial hydrogen and hydrides. Thus, presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties. First-principles methods were further implemented to gain a theoretical perspective about the experimental observations that lattice defects are effective at trapping magnetic flux in high-purity superconducting niobium. Full-potential linear augmented plane-wave methods were used to analyze the effects of magnetic field on the superconducting state surrounding these defects. A considerable amount of trapped flux was obtained at the dislocation core and grain boundaries which can be attributed to significantly different electronic structure of defects as compared to bulk niobium. Electron redistribution at defects enhances non-paramagnetic effects that perturb superconductivity, resulting in local conditions suitable for flux trapping. Therefore, controlling accumulation or depletion of charge at the defects could mitigate these tendencies and aid in improving superconductive behavior of niobium. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2019

This short communication is devoted to the room temperature processes of diffusion and redistribution of dissolved carbon atoms in martensite to the nanosegregation regions at dislocations and grain boundaries. It is related to the DF7 contribution of M. Lavrskyi et al. on the carbon kinetics in martensite [1] and to the DF7 contribution of Yu. Nechaev on the compound-like nanosegregation at dislocations and grain boundaries in metallic materials.