• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 19
  • 12
  • 5
  • 4
  • 3
  • 1
  • Tagged with
  • 49
  • 49
  • 16
  • 14
  • 13
  • 12
  • 12
  • 11
  • 11
  • 9
  • 8
  • 8
  • 7
  • 7
  • 7
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

The Development and Analysis of Low Temperature Superplasticity in AZ91 Magnesium Alloy

Lin, Hsuan-Kai 16 July 2001 (has links)
none
2

Strain localization in extruded AZ31 Magnesium alloy

Huang, Chao-Chun 06 August 2009 (has links)
none
3

Hot working behavior of AZ31 Magnesium alloys

Suen, Der-Kai 12 August 2005 (has links)
none
4

Shearing on the Great Glen Fault: Kinematic and Microstructural Evidence Preserved at Different Crustal Levels

Becker, Cassandra 22 May 2023 (has links)
The NE-SW trending Great Glen Fault (GGF) is one of mainland Scotland's most significant crustal-scale faults, although our understanding of its early kinematics is in question. Previous studies generally agree that the GGF was initiated as a Silurian sinistral strike-slip fault displacing c. 425 Ma isotopically dated granitic plutons. Stewart et al. (2001) argued that dikes fed by these plutons were sinistrally sheared by the GGF while in the sub-magmatic state, suggesting continuous strike-slip motion on the GGF by 425 Ma. Strike-slip offset post-dating overlying Devonian sedimentary basins is likely only a few tens of kilometers, requiring substantial (100s of kms) Silurian-aged strike-slip movement on the GGF in most plate reconstruction models for the Caledonian mountain belt, now exposed in East Greenland, Scandinavia, and Scotland. In contrast, a recent study (Searle 2021) has argued that motion on the GGF may instead have initiated in the Upper Paleozoic and that off-set is therefore minimal, bringing current restoration models into question. Several papers report widespread field and microstructural evidence from crystalline bedrock and overlying Devonian sedimentary rocks for brittle upper-crustal shearing on the GGF. However, evidence for high-temperature crystal plastic shearing at deeper crustal levels on the GGF, potentially of Silurian to Early Devonian age, is limited. During summer 2022, suites of oriented and plastically deformed metasedimentary rock samples were collected from the NW side (Moine/Lewisian gneisses and quartzites), center (Moine quartzites), and SE side (Dalradian quartzites) of the GGF. Additional samples included plutonic rocks from locations adjacent to the GGF and the associated Strathconnon fault that were believed to have been intruded during strike-slip motion, but after regional metamorphism and deformation in the surrounding Moine rocks. Microstructures and quartz c-axis fabrics from samples on the NW side and in the center of the GGF indicate a NW side up to the SW sense of displacement about NE to E plunging slip vectors, and these results are compatible with oblique sinistral motion on the GGF below the brittle-ductile transition zone during Silurian - Early Devonian times. However, radiometric dating is needed to prove the absolute timing of this shearing. In contrast, on the SE side of the GGF, NW side up or NW side down senses of shearing are indicated at different locations. Brittle fracturing is observed in all collected samples, overprinting the earlier high-temperature (300 - 650 °C) crystal fabrics and microstructures developed below the brittle-ductile transition zone. No convincing microstructural evidence for sub-magmatic shearing during pluton emplacement was found in the samples collected. However, the local presence of high-low temperature (c. 650 - 300 °C) solid-state deformation microstructures in both quartz and feldspar grains in these 430 - 425 Ma plutons suggests that the plutons were deforming internally in response to far-field stresses generated by shearing on the adjacent GGF and Strathconnon fault during cooling to background regional temperatures. / Master of Science / The Great Glen Fault (GGF) is one of mainland Scotland's most significant large-scale faults, although our understanding of its early motion is debated. Most geologists agree that the GGF began displacing existing rocks during the Silurian (c. 444 - 419 Ma), including igneous bodies, known as plutons, of approximately the same age (c. 425 Ma). Stewart et al. (2001) argued that during shearing, dikes fed by these plutons were deformed before cooling to background temperatures, which may suggest that the GGF was continuously undergoing lateral strike-slip motion by 425 Ma and that post-Silurian offset was likely only a few tens of kilometers. Most plate reconstruction models for the Caledonian mountain belt, now exposed in East Greenland, Scandinavia, and Scotland, assume that significant lateral motion and shearing occurred on the GGF during the Silurian. However, new research has suggested that the GGF was initiated several million years later, bringing current restoration models into question. Several published papers have reported widespread evidence for upper-crustal brittle shearing of crystalline bedrock and overlying Early Devonian (c. 420 - 359 Ma) sedimentary basins within the GGF. However, evidence for lower-crustal shearing during the same time frame, resulting in plastic deformation, is limited. To address this knowledge gap, I collected suites of oriented bedrock samples and 430 - 425 Ma plutonic rocks from locations adjacent to the GGF and associated Strathconnon Fault believed to have been intruded during strike-slip motion. Samples from the NW side and center of the GGF suggest oblique left-lateral motion within the fault zone, with the rocks on the NW side of the GGF moving upward relative to the SE side, compatible with current generally accepted models for the Silurian-Early Devonian age on the GGF; however, these results must be verified with radiometric dating to constrain the absolute timing of shearing. On the SE side of the GGF, vertical offset is variable at different locations. Brittle upper-crustal shearing is observed in all samples, which overprints early high-temperature (300 - 650 °C) deformation. Early lower-crustal shearing on the GGF is recorded by these deformation indicators and was followed by uplift and fracturing within the GGF of these initially lower-crust rocks. The local presence of solid-state deformation microstructures in the plutons suggest internal deformation due to shearing on the adjacent Great Glen and Strathconnon Faults during their cooling to regional background temperatures.
5

Analysis on Cavitation in AZ-Series Mg Alloys during Superplastic Deformation

Lee, Ching-Jen 24 July 2003 (has links)
none
6

Microstructural Evolution of Aluminum Alloy 2219-T87 with Hot Torsion and Bobbin Tool Friction Stir Welding

Gilmore, Andrew Barrett 09 August 2022 (has links)
No description available.
7

Etude de la recristallisation au cours du laminage a chaud d’aciers a basse densite fer-aluminium / Study of recrystallization during hot rolling of low density iron aluminium steels

Castan, Christophe 25 October 2011 (has links)
Les directives de l'Union Européenne conditionnent la R&D du secteur automobile concernant l'utilisation de matériaux plus légers ayant pour but de réduire la consommation de carburant et une diminution de l’émission de gaz d’échappement. L’objectif est de mettre au point des aciers allégés d’au moins 10% (ρmax ≈ 7g/cm3). Les alliages fer-aluminium possèdent des propriétés physiques et mécaniques prometteuses mais présentent des défauts de surface appelés roping, apparaissant après l’emboutissage à froid. Cette étude a consisté à mieux comprendre les conditions de recristallisation au cours du laminage à chaud afin de contrôler la microstructure et ainsi limiter ces défauts. Il est généralement admis, lors d’une déformation à chaud, que les alliages ferritiques, à haute énergie de défaut d’empilement, donnent lieu aux processus de recristallisation dynamique géométrique (RDG) et de recristallisation dynamique continue (RDC). Dans cette étude, l’existence d’une transition entre les mécanismes de RDC et de recristallisation dynamique discontinue (RDD) a été mise en évidence pour des températures comprises entre 900 et 1100°C et des vitesses de déformation comprises entre 0,1 et 50s 1. La recristallisation post dynamique a aussi été étudiée afin d’observer l’évolution de la microstructure lors de maintiens en température. Un modèle développé antérieurement pour la RDC de l’aluminium a ensuite été utilisé afin de simuler une passe de laminage. Bien que la comparaison des résultats expérimentaux et simulés fasse apparaître un certain nombre de différences, ce modèle permet de reproduire qualitativement les évolutions de la microstructure. / The instructions of the European Union pilot the R&D in the automotive industry regarding the use of lightweight materials which aims at reducing fuel consumption and emission of exhaust gases.The objective is to develop steels of density reduced by at least 10% (ρmax ≈ 7g/cm3). Iron aluminum alloys display promising physical and mechanical properties but they often exhibit surface defects, referred to as roping, appearing after the deep drawing process. This study was carried out to better understand the conditions of recrystallization during hot rolling to control the microstructure and thereby limit these defects.During hot deformation, it is generally agreed that geometric dynamic recrystallization (GDRX) and continuous dynamic recrystallization (CDRX) operate in ferritic alloys with high stacking fault energy. In this study, the existence of a transition between CDRX and the mechanism of discontinuous dynamic recrystallization (DDRX) has been brought into evidence in the temperature range 900 1100°C and strain rate range 0.1-50s-1. Post-dynamic recrystallization was also studied to observe the evolution of microstructure during holding temperatures.A model formerly developed for the CDRX of aluminum was then used to simulate a rolling pass. Comparison of computed and experimental results shows some differences but this model can reproduce microstructural changes qualitatively.
8

Etude de la cinétique de recristallisation au cours du laminage à chaud d’aciers inoxydables ferritiques stabilisés / Study of recrystallization kinetics of stabilized ferritic stainless steels during hot rolling

Jacquet, Grégoire 28 October 2013 (has links)
Les aciers inoxydables ferritiques stabilisés, aussi performants dans de nombreux domaines et moins chers que les aciers inoxydables austénitiques, souffrent cependant d’une formabilité inférieure (mise en forme + défaut de chiffonnage / roping). Il convient donc d’optimiser les microstructures et textures finales de ces produits, ce qui passe entre autres par une meilleure connaissance de l’évolution du matériau durant le laminage à chaud (LAC).Des essais de bipoinçonnement effectués sur une machine Gleeble®, simulant la compression plane à cœur du matériau durant le LAC, ont permis de simuler des schémas de laminage mono et multipasses. Les effets de la déformation, de la température, de la vitesse de déformation, de la taille de grains initiale et de la composition chimique sur les évolutions dynamiques (durant une passe de laminage) et post-dynamiques (durant un temps interpasse) ont été investigués.Une passe de LAC fragmente la microstructure en cristallites par recristallisation dynamique continue (RDC). Une partie de ces cristallites deviendront les germes de la recristallisation post-dynamique (RPD) au cours du temps interpasse. Celui-ci se caractérise par la simultanéité d’activation de nouveaux germes, de la croissance de grains recristallisés au sein de zones écrouies mais également au détriment d’autres grains recristallisés.Le couplage d’un modèle de RDC existant avec un modèle de RPD créé à partir des observations expérimentales, permet de simuler des schémas de LAC multipasses et de retranscrire les effets de la majorité des paramètres opératoires. / Stabilized ferritic stainless steels are as efficient as austenitic stainless steels in many areas and less expensive. However, they suffer from a lower formability (forming + roping defect). It is therefore necessary to optimize the final microstructures and textures of these products, which requires in particular a better understanding of the evolution of the material during hot rolling.Plane strain compression tests carried out on a Gleeble® machine, reproducing the deformation during hot rolling in the center of the material, permitted to perform single- and multi-pass rolling schedules. The effects of deformation, temperature, strain rate, initial grain size and chemical composition on dynamic (during a rolling pass) and post- dynamic (during an inter-pass time) evolutions were investigated.A hot rolling pass fragments the microstructure and creates crystallites by continuous dynamic recrystallization (CDRX). A part of these crystallites becomes nuclei for the post-dynamic recrystallization (PDRX) during inter-pass time. The latter is characterized by the simultaneous activation of new nuclei and growth of recrystallized grains, not only within strain-hardened zones but also at the expense of other grains already recrystallized.The coupling of an existing CDRX model with a PDRX model based on experimental results allows to simulate multi-pass hot rolling schedules and to reproduce the effects of most of the operating parameters.
9

Análise multiescala da abrasão de aços austeníticos ao manganês aplicados em britadores de minério. / Multiscale abrasion analysis of austenitic manganese steels applied to ore crushers.

Machado, Paulo Cordeiro 02 October 2017 (has links)
O desgaste abrasivo de dois aços austeníticos ao manganês, materiais com grande utilização na mineração, foi estudado empregando metodologia multiescala (escalas: macro, meso e micro). Na macroescala foram estudados os mecanismos de dano e de desgaste de revestimento de britador utilizado em campo. Na mesoescala foram realizados ensaios de britador de mandíbula e de esclerometria linear. Na microescala o ensaio de esclerometria linear foi utilizado para avaliar os efeitos da camada encruada em campo e da orientação cristalográfica dos grãos austeníticos dos aços com 12 %Mn e 20 %Mn. As técnicas de caracterização utilizadas nesta pesquisa foram: macro e microdureza, nanodureza instrumentada, MO, MEV, DRX, EBSD, FIB e MET. A pesquisa foi dividida em três Capítulos, intitulados: \"Desgaste abrasivo dos aços austeníticos com 12 %Mn e 20 %Mn via ensaio de britador de mandíbula\"; \"Efeito do encruamento e da orientação cristalográfica no desgaste por riscamento dos aços austeníticos 12 %Mn e 20 %Mn\"; e \"Microestrutura da subsuperfície do aço austenítico com 12 %Mn deformado por desgaste abrasivo\". O primeiro Capítulo mostrou, a partir do ensaio de britador de mandíbulas (mesoescala), que o aço com 20 %Mn tem tendência de maior resistência ao desgaste que o aço com 12 %Mn. Este resultado foi obtido para a mandíbula fixa do britador, na qual a severidade de desgaste foi superior a mandíbula móvel, por apresentar microcorte e microsulcamento como micromecanismos predominantes, enquanto na mandíbula móvel o micromecanismo predominante foi a microendentação. No segundo Capítulo observou-se que o desgaste por riscamento (mesoescala e microescala) não depende do perfil de encruamento gerado em campo. Entretanto, foi identificado o efeito da orientação cristalográfica, planos (001), (111) e (101), no desgaste por riscamento dos aços com 12 %Mn e 20 %Mn. No último Capítulo a análise multiescala mostrou que a microestrutura deformada na subsuperfície sofre alterações semelhantes em diferentes intensidades. Nas três escalas de análise foram observadas uma camada com grãos ultrafinos (nanométricos), na subsuperfície, e uma de transição com maclas de deformação. A formação dos grãos ultrafinos foi associada à recristalização dinâmica por deformação plástica, na qual faz parte do mecanismo de auto reparação superficial. Além dos resultados apresentados, o desenvolvimento desta pesquisa de doutorado permitiu a elaboração de duas metodologias: i. análise do efeito da orientação cristalográfica no desgaste por microesclerometria; e ii. análise de microestrutura revelada por ataque iônico - FIB. / The abrasive wear of two manganese austenitic steels, materials broadly used in mining industry, was studied using multiscale methodology (scales: macro, meso and micro). In the macroscale the mechanisms of damage and wear of in-service crusher liner were studied. In the mesoscale, jaw crusher and linear scratch tests were performed. In the microscale the linear scratch test was used to evaluate the effects of the hardening layer and the crystallographic orientation of the austenitic grains of steels with 12 %Mn and 20 %Mn. The characterization techniques used in this research were: macro and microhardness, instrumented nanohardness, OM, MEV, DRX, EBSD, FIB and TEM. The research was divided into three chapters, entitled: \"Abrasive wear of steels with 12 %Mn and 20 %Mn via jaw crusher test\"; \"The effect of the in-service workhardening and crystallographic orientation on the micro-scratch wear of austenitic steels with 12 %Mn and 20 %Mn\"; and \"Subsurface microstructure of the deformed austenitic steel with 12 %Mn by abrasive wear\". The first chapter showed, from the jaw crusher tests (i.e. mesoscale), that the steel with 20 %Mn tends to be more wear resistant than the steel with 12 %Mn. This result was obtained to the fixed jaw crusher, in which the wear severity was superior to the movable jaw, since it presents microcutting and microploughing as predominant micromechanisms, whereas in the mobile jaw the predominant micromechanism was microendentation. In the second chapter, it was observed that scratch wear (i.e. meso and microscale) does not depend on the in-service work-hardening profile. However, it was identified the effect of crystallographic orientation, (001), (111) and (101) planes, on the scratch wear of the steels with 12% Mn and 20% Mn. In the last chapter, the multiscale analysis showed that the subsurface deformed microstructure changes with different intensities. At the three analysis scales, a layer with ultrafine grains was observed in the subsurface and mechanical twins. The formation of this layer, with nanometric grains, was associated with dynamic recrystallization by plastic deformation, in which it is part of the self healing effect. In addition to the results found, the development of this doctoral research allowed for the elaboration of two methodologies: i. Analysis of the effect of crystallographic orientation on the scratch wear; and ii. Microstructure analysis revealed by ion etching - FIB.
10

La recristallisation dynamique dans les matériaux anisotropes : caractérisation et modélisation dans la glace polycristalline / Dynamic recrystallization in anisotropic materials : characterization and modeling in polycrystalline ice

Chauve, Thomas 10 January 2017 (has links)
La glace est un matériau de structure cristallographique hexagonale ayant une anisotropie plastique très importante. La déformation est principalement accommodée par le glissement des dislocations dans le plan basal. Cette forte anisotropie du monocristal de glace conduit lors de la déformation d’un polycristal, à de fortes hétérogénéités de déformation et de contrainte. Lors de la déformation à haute température, les mécanismes de recristallisation dynamique permettent, par le développement d’une nouvelle microstructure et d’une nouvelle texture, d’augmenter la ductilité du matériau. L’objet de cette étude est de mieux caractériser les mécanismes de recristallisation et leurs liens avec les hétérogénéités de déformation afin de mieux comprendre le développement des nouvelles microstrutures et textures ainsi que leurs impacts sur le comportement du matériaux.Ce travail est basé sur des essais de type fluage dans les conditions où la recristallisation dynamique est importante (T > 0.95T f et 0.5 < σ < 1 M P a). Deux types de polycristaux sont étudiés : la glace granulaire, considérée comme un volume élémentaire représentatif, qui permet d’appréhender d’un point de vue statistique l’impact de la recristallisation sur le développement des microstructures et des textures ; et la glace colonnaire, qui permet d’étudier les différents mécanismes de germination et leurs liens avec les hétérogénéités de déformation à l’échelle inter et intra-granulaire. Les outils de caractérisation utilisés sont la mesure in situ de l’évolution du champ de déformation par corrélation d’images numériques et la caractérisation des microstructures et des textures pré- et post- déformation par mesure optique et Electron BackScatter Diffraction (EBSD).Étant donné les fortes hétérogénéités de déformation, les mécanismes de recristallisation dynamique continue et discontinue sont actifs au cours de la déformation. La formation de sous-joints de grains, la germination par gonflement (bulging), où la migration de joints de grains ont été mises en évidence. Des germes fortement désorientés par rapport aux grains parents ont également été observés. Ces observations impliquent un mécanisme de germi- nation différent de ceux mentionnés ci-dessus. De plus, les dislocations géométriquement nécessaires composants les sous-joints de grains ont été caractérisées à l’aide du tenseur de Nye extrait des mesures EBSD. Cela a permis d’observer des sous-structures de dislocations composées de dislocations c. Ces dislocations c étaient jusqu’alors très rarement observées et seulement dans des conditions très spécifiques.Une forte corrélation entre les hétérogénéités de déformation et les mécanismes de recristallisation a été mise en évidence grâce aux mesures de champs de déformation. Les mécanismes de recristallisation se concentrent dans les zones où la déformation est importante et ont tendance à réduire les hétérogénéités de déformation. De plus, ces bandes de déformation, où la recristallisation est active, sont orientées à environ ±45° et ont une dimension supérieure à la taille moyenne des grains, ce qui montre une organisation des hétérogénéités de déformation et de contrainte à grande distance.Un modèle théorique pouvant contraindre l’orientation des germes crées par la germination spontanée est proposé. Ce modèle, basé sur la relaxation de l’énergie élastique, du fait de l’anisotropie élastique du mono-cristal de glace, permettrait de favoriser les cristaux dont l’axe c est aligné avec la direction de la contrainte principale locale. Cette étude a été complétée par des expériences numériques, basées sur un modèle à champ complet et une loi de plasticité cristalline élasto-viscoplastique qui permet de reproduire les hétérogénéités de déformation et de contrainte. Ces expériences numériques ont montré que ce mécanisme de germination orienté pourrait permettre d’expliquer le développement des textures de recristallisation. / Ice is an hexagonal material in which deformation mainly occurs by dislocation glide along the basal plane conferring a strong viscoplastic anisotropy to the single crystal. Hence, during polycrystalline ice deformation the incompatibility between grains lead to highly heterogeneous strain. During ice creep at high temperature, dynamic recrystallisation occurs, leading to the development of a new microstructure and strong recrystallisation textures.These new microstructure and texture increase the ductility of the material. The aims of this study is to investigate dynamic recrystallisation mechanisms and their links with strain heterogeneities to better understand the development of these microstructure and texture.Creep experiments are carried out in conditions where dynamic recrystallisation is important (T > 0.95T f and 0.5 < σ < 1 M P a). Two kinds of polycrystalline samples are used: granular ice, which can be considered as a representative elementary volume and enable to understand the global impact of dynamic recrystallisation mechanisms on texture and microstructure; and columnar ice which enable to better constrain the dynamic recrystallisation mechanism such as nucleation and their link with the strain heterogeneities down to the intra and inter-granular scales. Strain field evolution is measured in situ using digital images correlation and pre- and post- deformation microstructures and textures are measured using optical imaging and Electron BackScatter Diffraction (EBSD).Due to the strong strain heterogeneities, both continuous and discontinuous recrystallisation mechanisms occured. Sub-grain boundary formation, nucleation by bulging and grain boundary migration are mechanisms very active during dynamic recrystallisation in ice. On top of that, we also observed new grains with orientations highly disoriented compared to the neighbouring grains. This observation implies a nucleation mechanism different compare to the one mentioned above. Using Nye theory on EBSD measurements constrains the geometrically necessary dislocations of the sub-grain boundaries. Tilt sub-grain boundaries made of non-basal c dislocations have been observed. Only few observations of c dislocations existed so far, all of them made in very specific conditions.A strong correlation between recrystallisation mechanisms and strain field heterogeneities have been observed. Recrystallisation mechanisms lead to a decrease and spread of strain heterogeneities. The strain localise into bands of deformation oriented at around ±45° from the compression axis and with a typical length higher than the mean grain size. These bands of high deformation localise most of the dynamic recrystallisation mechanisms. This observation shows that the long range interaction of the strain and stress heterogeneities.We propose a new model which could be able to constrain the nucleus orientation for spontaneous nucleation. This model, based on the elastic energy relaxation during nucleation tanks to the elastic anisotropy of ice single crystal, should constrain the c axis to be aligned with the locale principal stress direction. To investigate the impact of such oriented nucleation mechanism on the texture development, a numerical experiment has been proposed using full field simulation with an elasto-viscoplastic law able to simulate the strain and stress fields heterogeneities. These numerical experiments show that the oriented nucleation mechanism might be able to explain the development of recrystallisation texture.

Page generated in 0.1453 seconds