Student Number : 9800022T - PhD thesis - School of Mechanical, Industrial and Aeronautical Engineering - Faculty of Engineering and the Built Environment / For many applications, it may be useful to be able to estimate creep properties of a material from simpler testing procedures such as tensile tests than the conventional creep testing procedures. Most alloys used for creep service conditions are in a hardened condition and thus tertiary creep, controlled by micro structural degradation, is dominant. The object of the study was to investigate a reasonably simple method for estimating the creep behavior of a low alloy 1% Cr, 0.25 % Mo steel from tensile yield data. The study involved performing of series of investigations, including age hardening, tensile and creep tests. Microstructural degradation was monitored from specimens held in a furnace for different times and temperatures, which were then tested in tension at room temperatures. Tensile tests were carried out at different temperatures and strain rates and the data used to determine material parameters for use in kinetic equations describing deformation. For comparison, creep curves were obtained from both creep tests and tensile tests results. Tests on furnace aged specimens were used to quantify softening due to material degradation and formulate a structure evolution and kinetic expressions used to determine creep curves. The modified equation by Dorn was used to determine the material parameters and to predict flow characteristics. Two sets of mechanisms were observed. At low temperature and high stress (above 550MPa) dislocation by glide mechanism was investigated. At higher temperatures and low stress (below 550MPa), some form of power law creep was observed. Glide mechanism was investigated and material parameters σ ) , n and activation volume v, were calculated. The calculated value of σ ) was assumed for both plastic deformation and the softening kinetics. A reasonably good estimate of the creep behavior of the low alloy steel used in this investigation in which tertiary creep dominates can be calculated from tensile yield stress values. Furthermore, the creep rate and recovery have similar stress dependences, with the stress and temperature dependence similar to that predicted by recovery theory. The value of activation energy observed for creep for this alloy is in line with the processes which could be related to self diffusion. In order to justify the significance of this study, four existing empirical models are discussed, highlighting their merits and demerits with respect to the models used in this study. These are θ-Projection, Damage Mechanics, Estrin-Mecking and the Internal Stress Methods. Generally, in this class of alloys, recovery process occurs under an effective stress (i.e. an applied stress less the internal stress). Thus the possibility of using tensile data obtained in this study in the internals stress model was explored. The model could replicate the one used in this study if the change in internal stress value o σ is assumed to be negligible. This could be assumed to be true for tensile data at high stresses and low temperature especially during secondary creep rate when the internal stress approximates to the applied stress and at short test durations.

Cellular metals made from aluminum, titanium, or other metals are becoming increasingly popular for use in structural components of automobiles, aircraft, and orthopaedic implants. Civil engineering applications remain largely absent, primarily due to poor understanding of the material and its structural properties. However, the material features a high stiffness to weight ratio, excellent energy dissipation, and low thermal conductivity, suggesting that it could become a highly valuable new material in structural engineering. Previous attempts to characterize the mechanical properties of steel foam have focused almost exclusively upon uniaxial compression tests, both in experimental research and in computational simulations. Further, computational simulations have rarely taken the randomness of the material’s microstructure into account and have instead simplified the material to a regular structure. Experimental tests have therefore been performed upon both hollow spheres and PCM steel foams to determine compressive, tensile, and shear properties. Computational simulations which accurately represent the randomness within the microstructure have been validated against these experimental results and then used to simulate other material scale tests. Simulated test matrices have determined macroscopic system sensitivity to various material and geometrical parameters.

<p>Through thousands of years of evolution, natural biological systems have optimized their structures to thrive in diverse ecological conditions. Extracting and leveraging the inherent design principles of these biological systems can provide inspiration for the development of advanced lightweight structural materials. To effectively facilitate this transition, it is crucial to understand the specific mechanisms by which the microstructure of biological materials influences their mechanical properties. This dissertation focuses on understanding microstructure-property correlations in three biological systems: Venus flower basket, Cholla cactus, and Organ pipe coral.</p> <p>The Venus flower basket exhibits a cylindrical cage-like structure made from a complex network of silica fibers which exhibit a core-shell like layered architectures. A novel multimodal approach involving nanoindentation, ex situ and in situ fiber testing, and post-failure fractography was utilized to precisely understand the impact of the layered structure on the tensile and fracture behavior of fibers. The observation of fibers in real-time revealed, for the first time, that the initiation of failure occurs at the fiber's surface and progressively advances towards the core, traversing multiple layers. The concentric layers encompassing the central core act sacrificially, employing various toughening mechanisms to protect the core. Furthermore, nanoindentation experiments performed in situ in water shed light on the significance of the layered fiber structure in a marine environment. Another interesting system is the Cholla cactus. In arid environments, Cholla cactus produces porous wood with a mesh-like structure. To comprehensively understand the structure, properties, and designs of Cholla cactus wood, various techniques such as x-ray tomography, scanning electron microscopy, nanoindentation, and finite element simulations were employed. The structure and function of different wood components was investigated from both biological and mechanical behavior perspectives. The impact of the unique structure of wood components on the design of engineering materials is discussed. Finally, the dissertation focuses on the Organ pipe coral, which exhibits a hierarchical structure comprising vertical tubes and horizontal platforms at the macrostructure level. At the microstructure level, cells are formed through a unique arrangement of micrometer-sized plates made of calcium carbonate. Nanoindentation was used to assess the impact of this hierarchical structure on micromechanical properties. The results unveiled distinct toughening mechanisms operating at different length scales within the coral.</p> <p>17</p> <p>By gaining a precise understanding of the correlations between microstructure and properties in various biological materials, this research provides valuable insights for the design of advanced architected structural materials. The unique interplay between microstructure, function, and properties is discussed.</p>

No description available.

The Reduced Enrichment for Research and Test Reactor (RERTR) now known as the Material Minimization and Management Reactor Control program (MMMRC) seeks to replace the use of highly enriched uranium (HEU) fuels used in research and test nuclear reactors around the world. The low enriched uranium (LEU) fuels must have fissionable uranium densities comparable to the HEU fuels. After extensive investigation by various researchers around the world, the U-Mo alloys were selected as a promising candidate. The Mo alloyed with U allows for the stabilization of the face-centered cubic ?-U phase, which demonstrated favorable irradiation behavior. However, deleterious diffusional interaction between the fuel and the cladding, typically Al-base alloy, remain a challenge to overcome for application of U-Mo alloys as the LEU fuel. Zr has been identified as a potential diffusion barrier between monolithic U-10 wt.% Mo (U10Mo) metallic fuel and AA6061 cladding alloys for the development of a LEU fuel system. However, interdiffusion and reaction between the Zr barrier and U10Mo fuel can produce phases such as Mo2Zr, and promote the destabilization of ?-U phase into ?'-U (U2Mo) and ?-U. In order to better understand this phenomenon, this study examined the phases that are present in the U10Mo alloys with varying Zr concentration, 0, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0 wt.% at room temperature after heat treatment at 900°C for 168 hours and 650°C for 3 hours. These two temperatures are relevant to fuel plate fabrication process of homogenization and hot-rolling, respectively. Scanning electron microscopy and X-ray diffraction were employed to identify and quantitatively document the constituent phases and microstructure to elucidate the nature of phase transformations. For U10Mo alloys containing less than 1.0 wt.% Zr, there was no significant formation of Mo2Zr after 900?C homogenization and subsequent heat treatment at 650?C for 3 hours. The ?-U phase also remained stable correspondingly for these alloys containing less than 1.0 wt.% Zr. For U10Mo alloys containing 2 wt.% or more Zr, a significant amount of Mo2Zr formation was observed after 900?C homogenization and subsequent heat treatment at 650?C for 3 hours. For these alloys, destabilization of ?-U into ?'-U (U2Mo), UZr2 and ?-U was observed. The alloy containing 20 wt.% Zr, however, did not demonstrate ?-U decomposition even though Mo2Zr was observed after heat treatments. The formation of Mo2Zr effectively reduced the stability of the metastable ?-U phase by depleting the ?-stabilizing Mo. The destabilization of ?-U phase into the ?-U phase is not favorable due to anisotropic and poor irradiation behavior of ?-U phase. Therefore the formation of Mo2Zr at the interface between U10Mo fuel and Zr diffusion barrier must be carefully controlled during the fabrication of monolithic LEU fuel system for successful implementation.

Yes / Titanium alloys are known for their excellent corrosion resistance; however, low surface hardness results in poor wear resistance, which limits its potential application. This study employs a novel two-step process to embed a hard Ni coating containing a mixture of nanosized particles (Al2O3 and TiO2) into the surface of the Ti-6Al-4V alloy using an electric arc produced during the inert tungsten gas welding process. The surface of the sample was evaluated using Vickers Microhardness, Scanning electron microscopy, Energy dispersive spectroscopy and pin-on-plate wear testing. Microstructural analysis showed that impregnating the titanium surface with Ni/(Al2O3 and TiO2) nanomaterials resulted in the formation of a hard martensitic structure to a depth of approximately 2 mm below the surface. The changes observed are driven by modification of the surface chemistry and the presence of nickel, causing grain size reduction, solid solution strengthening and dispersion strengthening of the treated layer by the nanoparticles. The hardness of the treated layer increased by more than 180% when 40 nm Al2O3 and 30 nm TiO2 particles were embedded into the surface. Similarly, the wear resistance of the treated surface improved by 100%.

<p> The stress relieving mechanisms in two different batches of thermal-shock resistant calcia-PSZ have been investigated. The nature of the stress relief in the two materials appears to result from the transformation of the pure ZrO₂ component of the microstructure at temperatures below, within, and above the normal transformation temperature range. In the batch #1 material, which contains a larger volume fraction of monoclinic phase, the cubic matrix material behaves in a "brittle" fashion resulting in the production of a high density of microcracks in the body. The density of these cracks is such that the level of energy that can be stored in the body is limited and thermal shock resistance results. The batch #2 material contains considerably less monoclinic material and the evidence suggests that the cubic matrix within it can act in a ductile fashion. This ductility together with the twinning of the monoclinic component of the microstructure possibly relieves the stresses developed in the material on thermal shock. </p> <p> In the batch #2 material, large platelets were observed to develop following thermal cycling from temperatures above those of the normal transformation. It has been demonstrated that stress plays a major role in the development of these features. In view of the possible ductility of the cubic matrix in this material it is suggested that the thermal cycling "works" the material, texturing the pure zirconia component in it, so leading to the development of the observed platelets. </p> / Thesis / Master of Engineering (ME)

No description available.

Currently, there are two major driving forces for considering alternative materials to lead- based products, specifically interconnections, in electronics applications, including the impending legislation or regulations which may tax, restrict, or eliminate the use of lead and the trend toward advanced interconnection technology, which may challenge the limits of present soldering technology. The reliability of solder joints is a concern because fracture failures in solder joints accounts for 70% of failures in electronic components. Lead-free solders are being investigated as replacements for lead solders currently used in electronics. Thermo-mechanical properties describe the stresses accumulated due to thermal fatigue as a result of CTE mismatch within the system. By understanding the failure mechanisms related to lead-free solders, the application of lead- free solders could be more strategically designed for specific applications. The objective of this thesis is to observe microstructural change in large-area solder joints caused by thermal cycling and relate these changes to reliability issues in large-area lead and lead-free solder constructed semiconductor power devices. This study focused on the microstructural changes within the solder alloy of a large-area solder joint under thermal cycling conditions. Two major primary observations were made from this research, they are: 1) due to a combination of testing conditions and material properties, the lead-free solders, Sn/3.5Ag and Sn/Ag/0.7Cu, sustained the most severe damage as compared to Sn/37Pb, and 2) due to elevated stresses at the solder/substrate interface in a simulated power semiconductor device sample damage was found to be most severe. / Master of Science

La fusion Sélective Laser (SLM), en tant que procédé de fabrication additive, permet la conception de formes complexes par une méthode de construction couche par couche, à partir d’un lit de poudre. L’interaction entre la poudre et l’onde électromagnétique est encore trop peu maîtrisée pour stabiliser efficacement la zone de fusion, et optimiser l’énergie utilisée lors du procédé. Cette étude a pour objectif : (a) la compréhension et la maîtrise des mécanismes de mise en couche par rouleau, propre aux machines SLM Phenix; (b) l’analyse et la quantification des évolutions morphologiques et microstructurales de cordons de fusion en fonction des paramètres du procédé SLM et des paramètres du lit de poudre définis précédemment ; (c) l’élaboration d’un modèle thermique et microstructurale représentatif de l’édification de cordons unitaires par le procédé SLM. Lors de cette étude, le procédé de mise en couche par rouleau a été étudié, et modélisé, afin de contrôler les épaisseurs et taux de compacités du lit de poudre. Les caractéristiques des cordons unitaires de Ti6Al4V produits par SLM ont pu alors être analysées en fonction des paramètres du lit de poudre et des paramètres énergétiques. Cette démarche a pour but de quantifier l’impact du conditionnement du lit de poudre sur le mécanisme de fusion SLM. Par la suite, une analyse fine et une reconstitution microstructurale a pu en être dégagée. Enfin, un modèle thermique radiatif couplé à une prédiction microstructurale des cordons unitaires a pu être élaboré, permettant ainsi une compréhension approfondie du mécanisme de fusion. / Selective Laser Melting (SLM), through additive manufacturing process, allows the conception of specific shapes through a layer-by-layer building method from a powder bed. The emphasis between processing parameters as, laser power, scan speed, scan strategy… has already been well investigated for a wide panel of material. Nevertheless, the powder interaction with electromagnetic waves remains a topical issue to handle the stabilization of the melting pool, and optimize the amount of energy used within the process.The purpose of this survey is : (a) the understanding and handling of powder bed layering mechanism through SLM Phenix rolling blade ; (b) the analysis and quantification of morphological and microstructural evolutions single tracks according to SLM process and powder bed parameters ; (c) development of a thermal and microstructural model standing for post SLM single tracks edification.First of all, the process of powder spreading by rolling blade has been investigated in order to tame and modelize the porosity and effective thickness of the powder bed. Thereafter, characteristics of Ti6Al4V single tracks produced by SLM were analyzed according to process and bed powder parameters. This approach tends to quantify the impact of the powder bed packing on the SLM melting mechanism. Accordingly, fine microstructural analysis and reconstruction have been extracted. Finally, a radiative thermal model linked to a microstructural prediction of single tracks has been settled, leading to a deeper understanding of the melting mechanism.