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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.

There is a need for simulation methodologies for multiphase three-dimensional microstructures that can be used in numerical simulations of material behavior or in exact computation of effective properties using microstructural correlation functions. Specifically, the methodology must be able to generate verifiably realistic microstructures, with complex morphology accurately represented. Striving to address that need, the research presented here develops a general microstructure simulation toolbox for multiphase two- and three-dimensional microstructures consisting of one connected phase and one or more particulate phases. Previous work by other researchers has found successful solutions to a variety of special cases of the general problem, but most of them are intended for binary microstructures, and nearly all simulate only two-dimensional microstructures. The toolbox presented here attempts to exceed those limitations. Its framework is a Metropolis stochastic-optimization routine running a simulated-anneal schedule, with particle position coordinates defining the configuration space and a range of forms available for the ﾓenergyﾔ? function. The toolbox allows several parameterizations of the microstructure, supplying all elementary properties (phase volume fractions, mean sizes, etc.) and some non-elementary properties (distributions of elementary properties, properties relating to inter-phase distances and morphology) of microstructures as possible parameters. The toolbox is able, as one special case, to simulate realistic microstructures of uniaxially compacted mixtures of elemental Al-Ti-B powders and achieve basic microstructure-processing correlation. Statistical tests involving microstructural correlation functions bear out the realism. The toolbox is also able to generate virtual microstructures for the same system, for use in the design of experiments (which are in fact high-strain-rate impact simulations), and for evaluating hypotheses involving achievable material properties. The Al-Ti-B powder compacts are potential advanced energetic materials that, when subjected to high-strain-rate impact (which may or may not constitute shock compression), explosively release heat by anaerobic reaction according as certain incompletely understood conditions are met or not. The study of those conditions and the mechanism of reaction initiation (carried out by a collaborator) is the specific application that the simulations in this work cater to. To ensure realistic morphology in simulated Al-Ti-B microstructures, this work included reconstruction (carried out by montage serial sectioning) of large three-dimensional volumes of Al-Ti and Al-B binary compacts for two sets of powders that yielded actual 3 D Ti and B particle images. Accordingly, advancement of the experimental technique of montage serial sectioning and a quantitative characterization of the real powder microstructures also formed part of this research. While only examples from Al-Ti-B powders are used throughout this work, it is clear that the methods will apply to other similar systems.

L'oxyde de zinc (ZnO) est un semi-conducteur à large gap direct (3,37 eV) qui possède de nombreuses propriétés intéressantes (piézoélectrique, optique, catalytique, chimique...). Un large champs d'applications fait de lui l'un des matériaux les plus étudiés de la dernière décennie, notamment sous forme nanostructurée. Dans ce travail, nous nous intéressons à la synthèse par électrochimie des couches minces, des micro- & nano-plots, et des nanofils de ZnO. Deux méthodes ont été utilisées : la première dite Template consiste à la fabrication des micro- et nanopores en réseau ordonné à l'aide de la technique lithographique dans lesquels a lieu la croissance du ZnO ; la seconde consiste à la croissance libre de réseau de nanofils. Les caractérisations structurales, morphologiques et optiques du ZnO ainsi élaboré ont été réalisées par diffractométrie des rayons-X (DRX), microscopie électronique à balayage (MEB), microscopie électronique en transmission (MET), spectroscopie Raman, spectroscopie UV et photoluminescence (PL). Les propriétés électriques des couches minces et des réseaux de nanofils (sous l'effet collectif) de ZnO ont été étudiées par des mesures "courant tension" (I-V) à température ambiante dans la configuration métal/semi-conducteur/métal à l'aide d'un réseau de micro-électrodes métalliques déposé en surface du ZnO. Cette étude nous a permis de déterminer qualitativement la conductivité électrique du ZnO et les différents paramètres de la jonction Schottky entre le ZnO et le substrat doré. Celle-ci est fondamentale et indispensable pour la réalisation d'un dispositif de récupération d'énergie tel que le nanogénérateur de courant piézoélectrique à base de nanofils de ZnO

The conventional route of materials development typically involves fabrication of numerous batches of specimens having a range of different microstructures generated via variations of process parameters and measurements of relevant properties of these microstructures to identify the combination of processing conditions that yield the material having desired properties. Clearly, such a trial and error based materials development methodology is expensive, time consuming, and inefficient. Consequently, it is of interest to explore alternate strategies that can lead to a decrease in the cost and time required for development of advanced materials such as composites. Availability of powerful and inexpensive computational power and progress in computational materials science permits advancement of modeling and simulations assisted materials design methodology that may require fewer experiments, and therefore, lower cost and time for materials development. The key facets of such a technology would be computational tools for (i) creating models to generate computer simulated realistic microstructures; (ii) capturing the process-microstructure relationship using these models; and (iii) implementation of simulated microstructures in the computational models for materials behavior. Therefore, development of a general and flexible methodology for simulations of realistic microstructures is crucial for the development of simulations based materials design and development technology. Accordingly, this research concerns development of such a methodology for simulations of realistic microstructures based on experimental quantitative stereological data on few microstructures that can capture relevant details of microstructural geometry (including spatial clustering and second phase particle orientations) and its variations with process parameters in terms of a set of simulation parameters. The interpolation and extrapolation of the simulation parameters can then permit generation of atlas of virtual microstructures that covers the complete range of variations of processing conditions of interest. These simulated and virtual microstructures can then be used in the micromechanical models such as FEM to analyze their constitutive properties

Additive friction stir deposition (AFSD) of AZ31B magnesium alloy was conducted to examine evolution of grain structure, phases, and crystallographic texture. AFSD was carried out using a hollow tool made from tool steel at a constant rotational velocity of 400 rpm on the AZ31B base plate. Bar stock of AZ31B was utilized as a feed material. The linear velocity of the tool was varied in the range of 4.2-6.3 mm/s. The feed rate of the material had to be maintained at a half value compared to the corresponding linear velocity for the successful deposition. The layer thickness and length of the deposits were kept constant at 1 mm and 50 mm respectively. The tool torque and actuator force values were recorded during the process and for calculation of the average input energy for each processing condition. Temperature during the AFSD experiments was monitored using a type k thermocouple located 4 mm beneath the deposition surface at the center of the deposition track. The average input energy values showed a decreasing trend with increasing tool linear velocity. The temperature values during deposition were ∼0.7 times the liquidus of the alloy. The deposited material then was examined by laser microscope and profilometer, X-ray diffraction, scanning electron microscopy, electron back scatter diffraction (EBSC), contact angle measurement and micro hardness tests. The AFSD AZ31B samples showed reduction in areal surface roughness with an increase in the tool linear velocity. The X-ray spectra revealed increase in the intensity of prismatic planes of α-Mg phase with increase in tool linear velocity. AFSD of AZ31B Mg alloy resulted in shifting of the grain size from a broader and courser distribution within the feed material to a tighter distribution. Moreover, EBSD observations confirmed the refinement in grain size distribution as well as the presence of predominantly prismatic texture for the AFSD samples when compared to the feed material. There was a marginal improvement in the hardness for the AFSD samples compared to the feed material. However, there was no significant change in the contact angle measurements in simulated body fluid for the AFSD samples compared to the feed material. The current work demonstrated ability of AFSD technique for the additive fabrication of magnesium-based alloys and provided a methodology for examining various process attributes influencing the processing-structure-property relationship.

The effect of initial microstructures of an 0.05 wt.% C low carbon steel, acicular ferrite (AF), Bainite (B), polygonal ferrite (PF), fine polygonal ferrite (FPF), and a microstructure produced by direct strip casting (DSC) (termed SC), on the deformation and recrystallization behaviour of cold rolled low carbon (LC) steel, was investigated. The initially prepared samples with the initial microstructures were cold rolled to 50, 70 and 90% reductions, then annealed isothermally in the temperature range 580 ??? 680 oC. The microstructures and textures produced by deformation and annealing were studied by optical microscopy, XRD, TEM, SEM and EBSD. The initial microstructures were characterized mainly by optical microscopy and EBSD. Using EBSD, the ferrite grain size of the AF, B and SC samples was considerably larger than that found by optical microscopy with a large fraction of low angle grain boundaries (LAGBs) observed within prior austenite grains. All samples exhibited a very weak texture close to random. After cold rolling, the microstructures of AF and SC contained shear bands with PF and FPF generating deformation bands. For AF and SC, the pearlite phase was more extensively elongated in rolling direction compared with PF and FPF. After 90% cold rolling reduction, PF, FPF and SC consist mainly of the texture component and AF and B . It was found that FPF recrystallized most rapidly followed by B, PF and AF with SC recrystallizing orders of magnitude more slowly due to the solution drag caused by its uniformly distributed higher Mn content. Very strong (???-fibre) texture was generated in cold rolled PF followed by FPF, with AF, SC and B generating very weak textures. The texture evolution during annealing 90% reduction PF was examined in further detail. The behaviour of nucleation and grain growth provides strong evidence of orientated nucleation as the dominant factor for CRA texture development in this material.

Thesis (M.S.)--Indiana University, 2008. / Department of Earh Sciences, Indiana University-Purdue University Indianapolis (IUPUI). Advisor(s): Kathy J. Licht. Includes vita. Includes bibliographical references (leaves 149-154).

O presente trabalho teve por objetivo realizar ensaios de desgaste abrasivo em juntas soldadas pelo processo MAG, em dois aços estruturais de alta resistência e baixa liga utilizados no transporte ferroviário de minério, os quais apresentavam diferentes propriedades mecânicas. No aço de menor resistência (Aço A) foram utilizados os metais de adição AWS ER70S-G (Aço A-70) e AWS ER80S-G (Aço A-80), enquanto no aço de maior resistência (Aço B) os metais de adição AWS ER80S-G (Aço B-80) e AWS ER110S-G (Aço B-110). Os conjuntos soldados foram caracterizados quanto às suas microestruturas, dureza e resistência ao desgaste abrasivo em ensaios de degaste roda de borracha e ensaios de desgaste por deslizamento a seco e com adição de um meio corrosivo (NaCl 0,05M, água arti cial ). Os resultados obtidos a partir dos ensaios de desgaste roda de borracha mostraram que para o Aço A a junta soldada do Aço A-70 apresentou melhor resistência ao desgaste, enquanto para o Aço B a maior resistência ao desgaste foi obtida na junta soldada Aço B-110. Dos resultados dos ensaios de desgaste por deslizamento a seco, as zona afetada pelo calor de grãos grosseiros (ZAC-GG) foram as regiões que apresentaram a maior resistência ao desgaste. Das condições soldadas no Aço A, o metal base (MB) apresentou a menor resistência ao desgaste, entre os metais de solda (MS), o MS do Aço A-80 apresentou a maior resistência ao desgaste. No Aço B, o MS do Aço B-80 apresentou a menor resistência ao desgaste, e o MS do Aço B-110 apresentou a resistência ao desgaste similar ao MB. Foi observado um maior desgaste para as amostras que foram submetidas ao ensaio de desgaste por deslizamento com solução, comparativamente às amostras submetidas aos ensaios a seco, o que, de acordo com a literatura, está associado ao efeito sinérgico entre corrosão e desgaste. / The present work had the objective perform abrasive wear tests on GMA welded joints in two high strength low alloy structural steels used in rail transport of ore, which presented di erent mechanical properties. The AWS ER70S-G and AWS ER80S-G wires were used in the lower strength steel (Steel A), whereas the AWS ER80S-G and AWS ER110S-G wires were used in the higher strength steel (Steel B). The welded joints were characterized in terms of their microstructures, hardness and abrasive wear resistance in dry sand/rubber wheel tests and sliding wear tests dry and with a corrosive medium (0.05M NaCl, arti cial water ). The results obtained from the dry sand/rubber wheel tests showed that for Steel A the welded joint with the wire AWS ER70S-G showed better wear resistance, while for Steel B the highest wear resistance was obtained with the AWS ER110S-G wire. From the results of dry sliding wear tests, the coarse grained heat a ected zone were the regions that presented the highest wear resistance. In Steel A, the base metal (BM) showed the least resistance, and between the weld metals (WM), the WM Steel A-80 showed the highest wear resistance. In Steel B, WM Steels B-80 showed the least wear resistance, while the WM Steels B-110 showed the wear resistance similar to BM. It was observed a higher wear for the samples that were submitted to the sliding wear test with solution, compared to the samples submitted to the dry wear test, which, according to the literature, is associated with the synergic e ect between corrosion and wear.

Orientador: Tomaz Manabu Hashimoto / Banca: Marcelo dos Santos Pereira / Banca: Antonio Jorge Abdalla / Banca: Pedro Paulo de Campos / Banca: José Rubens de Camargo / Resumo: Este trabalho analisa a transformação da austenita retida induzida por deformação, em um aço AISI 4340 com estruturas multifásicas. O aço foi submetido a duas rotas de tratamentos isotérmicos selecionadas e foi analisado quanto à fração volumétrica das fases e características mecânicas através da valiação dos limites de escoamento. Os corpos de prova foram submetidos a três diferentes níveis de tensões de tração, acima do limite de escoamento, para que se produzissem deformações plásticas determinadas. A influência destas deformações, na microestrutura e nos novos limites de escoamento, foram avaliadas. Mostrando variações significativas nas frações volumétricas das fases, com diminuição das quantidades de austenita, indicando uma transformação, por efeito TRIP (Transformation Induced Plasticity), em martensita o que provocou um aumento nos valores dos limites de escoamento e envelhecimento por deformação. / Abstract: The Transformation induced plasticity of austenite on the multiphase microstructure in a AISI 4340 steel was studied. Two sequences of isothermal treatments were performed. The volume fractions of the phases were analyzed and evaluation of mechanical characteristics through the yield point measurements. The samples were submitted at three different levels of traction above of the yield strength, so that definitive plastic deformations were produced. The influence of these deformations in the microstructure and in the new yield strength was evaluated. Significant variations were showed in the phase volume fraction, with reduction of the amounts of austenite indicating a martensitic transformation, by effect TRIP (Transformation Induced Plasticity) which results increases in the values of the yield strength and deformation ageing. / Doutor