Thesis (Ph. D.)--University of Wisconsin--Madison, 1966. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin, 1967. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Th.--Sci. phys.--Nancy--I.N.P.L., 1976.

Th.--Sci. phys.--Lille 1, 1977. N°: 375.

Th. 3e cycle--Phys. des matér.--Lille 1, 1979. N°: 770.

abstract: The goal of this research was to reduce dislocations and strain in high indium content bulk InGaN to improve quality for optical devices. In an attempt to achieve this goal, InGaN pillars were grown with compositions that matched the composition of the bulk InGaN grown on top. Pillar height and density were optimized to facilitate coalescence on top of the pillars. It was expected that dislocations within the pillars would bend to side facets, thereby reducing the dislocation density in the bulk overgrowth, however this was not observed. It was also expected that pillars would be completely relaxed at the interface with the substrate. It was shown that pillars are mostly relaxed, but not completely. Mechanisms are proposed to explain why threading dislocations did not bend and how complete relaxation may have been achieved by mechanisms outside of interfacial misfit dislocation formation. Phase separation was not observed by TEM but may be related to the limitations of the sample or measurements. High indium observed at facets and stacking faults could be related to the extra photoluminescence peaks measured. This research focused on the InGaN pillars and first stages of coalescence on top of the pillars, saving bulk growth and device optimization for future research. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2011

No description available.

Dans ce travail, nous avons étudié la relaxation de couches d’hétérostructures InGaN/GaN obtenue par épitaxie en phase vapeur aux organométalliques (EPVOM) et épitaxie aux jets moléculaires (EJM) principalement par microscopie électronique en transmission (MET). Pour ce faire, nous avons fait varier la composition de l'indium de 4.1% au nitrure d'indium pur, ce qui correspond lors de la croissance sur GaN à un décalage paramétrique allant de 1% à 11.3%. Le travail a porté sur des couches dont l’épaisseur allait de 7 nm à 500 nm. A partir d’une composition en indium voisine de 10%, nous mettons en évidence la formation d’un réseau de dislocations vis dont la ligne se promène dans l’interface, avec de très longues sections droites le long des directions <11-20>. Ces dislocations coexistent avec un réseau de dislocations coins qui commence à se former vers 13%, il disparait complétement autour d’une composition en indium de 18%. Le réseau de dislocation vis se densifie de plus en plus au-delà. Outre ces dislocations de décalage paramétrique, d'autres mécanismes qui contribuent à la relaxation de la contrainte dans ces hétérostructures InGaN/GaN ont été mis en évidence. Ainsi, au-dessus d'une composition d'indium supérieure à 25%, de nombreux phénomènes se produisent simultanément. (1) Formation des dislocations de décalage paramétrique à l'hétérointerface; (2) une composition de la couche qui s’enrichit en indium vers la surface; (3) des fortes perturbations de la séquence hexagonale conduisant à un empilement aléatoire; (4) croissance à trois dimensions (3D) pouvant même conduire à des couches poreuses lorsque la composition en indium est comprise entre 40% et 85%. Cependant, on met en évidence qu’il est possible de faire croître de l’InN pur de bonne qualité cristalline s'améliore grâce à la formation systématique d'une couche 3D. / In this work, we have investigated the strain relaxation of InGaN layers grown on GaN templates by MOVPE and PAMBE using TEM. To this end we varied the indium composition from 4.1% to pure indium nitride and the corresponding mismatch was changing from less than 1% to 11.3%, the thickness of the InGaN layers was from 7 nm to 500 nm. When the indium composition is around 10%, one would expect mostly elastically strained layers with no misfit dislocations. However, we found that screw dislocations form systematically at the InGaN/GaN interface. Moreover, below 18% indium composition, screw and edge dislocations coexist, whereas starting at 18%, only edge dislocations were observed in these interfaces. Apart from the edge dislocations (misfit dislocations), other mechanisms have been pointed out for the strain relaxation. It is found that above an indium composition beyond 25%, many phenomena take place simultaneously. (1) Formation of the misfit dislocations at the heterointerface; (2) composition pulling with the surface layer being richer in indium in comparison to the interfacial layer; (3) disruption of the growth sequence through the formation of a random stacking sequence; (4) three dimentional (3D) growth which can even lead to porous layers when the indium composition is between 40% and 85%. However, pure InN is grown, the crystalline quality improves through a systematic formation of a 3D layer.

The intermetallic compound TiAl possesses a unique complex of properties that include sufficiently low material density, high values of the strength-to-ductility ratio, high elastic moduli, high oxidation resistance, low creep rate, and improved fatigue characteristics. These properties make TiAl alloys very attractive, particularly for structural applications for aerospace and aeronautic industries, where, at certain temperatures, they might be capable of replacing heavy nickel-based superalloys. However, so far applications of TiAl alloys have been limited by their poor ductility. Many of the recent studies have focused on the source of this limited ductility and on methods to improve this property. It has been found out experimentally that the strength and ductility of $gamma$-TiAl alloys can be affected by many different parameters, including alloy stoichiometry, heat treatment, deformation temperature, impurity content, grain size, and ternary element additions. In this thesis we present the results of our computer simulations of deformation and fracture in TiAl. In contrast to many previous studies our simulations include the interaction of the crack with point defects in the lattice. We use the molecular statics technique with atomic interactions described in terms of the embedded atom method. We simulate the crack propagation along (100), (001), (110) and (111) planes in TiAl. The cleavage along (100) and (001) planes shows purely brittle behavior, whereas the cleavage along (110) and (111) planes is accompanied by extensive dislocation emission. Our studies of the crack interaction with point defects reveal that vacancies and antisites near the crack tip can influence the amount of plastic deformation. Another important observation is that the antisite formation energy near the crack tip is generally lower than in the perfect lattice. This observation suggests the formation of relatively disordered zones near the crack tip at high temperatures, and leads us to a formulation of a new mechanism of a brittle-to-ductile transition in TiAl. / Ph. D.

Improving the formability and crashworthiness of wrought magnesium alloys are the two biggest challenges in current magnesium technology. Magnesium is the best material candidate for enabling required improvements in fuel economy of combustion engines and increases in ranges of electric vehicles. In hexagonal closed-packed (HCP) structures, effects of grain size/morphology and crystallographic texture are particularly important. Prior research has established a general understanding of the dependences of strength and strain anisotropy on grain morphology and texture. Unfortunately, deformation, recrystallization, and grain growth strategies that control the microstructures and textures of cubic metals and alloys have not generally worked for HCPs. For example, in Magnesium, the deformation texture induced by primary forming operations (rolling, extrusion, etc.) is not randomized by recrystallization and may strengthen during grain growth. A strong texture reduces formability during secondary forming (stamping, bending, hemming etc.) Thus, the inability to randomize texture has impeded the implementation of magnesium alloys in engineering applications. When rare earth solutes are added to magnesium alloys, distinct new textures are derived. However, rare earth texture derivation remains insufficiently explained. Currently, it is hypothesized that unknown mechanisms of alloy processing are at work, arising from the effects of grain boundary intrinsic defect structures on microstructural evolution. This dissertation is a comprehensive attempt to identify formal methodologies of analyzing the behavior of grain boundaries in magnesium. We focus particularly on twin boundaries and asymmetric tilt grain boundaries using molecular dynamics. We begin by exploring twin nucleation in magnesium single crystals, elucidating effects of heterogeneities on twin nucleation and their relationships with concurrent slip. These efforts highlighted the necessity of imperfections to nucleate {10-12} twins. Subsequent studies encountered the importance of deformation faceting on the high mobility of {10- 12} and stabilization of observed twin mode boundaries. Implementation of interfacial defect theory was necessary to decipher the complex mechanisms observed which govern the development of defects in grain boundaries, disconnection pile-up, facet nucleation, interfacial disclination nucleation, disconnection movements, disconnection transformation across interfacial disclinations, crossaceting, and byproducts of interactions between lattice dislocations and grain boundaries.