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<p> The fracture behaviour of a medium strength bainitic steel (SAE 4340 in the 11 as transformed and in the "warm rolled" condition) . and four carbon-manganese structural steels (in the hot rolled ferritepearlite condition) was investigated. The purpose was to isolate those features of the microstructure which exert control over the fracture properties. </p> <p> The detailed nature of the microstructure of the steels was studied with transmission and scanning electron microscopy, qualitative x-ray analysis and quantitative metallography. An attempt was made to correlate the fracture behaviour with the microstructure through models which relate to the structure properties to the unnotched tensile properties. </p> <p> In the case of the bainitic steels it was found that the carbide morphology, dislocation substructure and prior austentite grain size have the major influence on fracture properties. In contrast, the fracture properties of the structural steels were controlled by the volume fraction of inclusions and to some extent by the shape of the inclusions. </p> / Thesis / Master of Engineering (MEngr)

The objective of this study is to characterize critical component structure-properties on a Dodge Neon for material response refinement in crashworthiness simulations. Crashworthiness simulations using full-scale finite element (FE) vehicle models are an important part of vehicle design. According to the National Highway Traffic Safety Administration (NHTSA), there were over six million vehicle crashes in the United States during 2004, claming lives of more than 40,000 people. Crashworthiness simulations on a detailed FE model of a 1996 Plymouth/Dodge Neon were conducted on the NHTSA for different impact crash scenarios. The top-ten energy-absorbing components of the vehicle were determined. Material was extracted from the as-built vehicle and microstructural analyses were conducted. Tension tests at different temperatures and strain rates were performed as well as microhardness tests. Different microstructural spatial clustering and mechanical properties were found for diverse vehicle components. A plasticity model based on microstructure was used to predict the material response of the front bumper.

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Near stoichiometric compositions of Ba3+3xB1+yNb20g (B=Co or Zn) perovskites were studied by microstructure analysis and optical techniques. Materials considered in the present research belong to the family of perovskites exhibiting disorder-1:2 order phase transitions that are important for microwave applications. It was found that deviation from stoichiometry involving cation deficiencies on Ba-or B-positions facilitates formation of an ordered structure for small values of cation deficiencies. Excessive deviation from the nominal values as well as introduction of extra cations destabilizes the perovskite structure leading to the precipitation of secondary phases. Formation of a Ba-deficient Bs6BNb9030 (B = Co or Zn) phase influences the grain growth rate through reduction in the surface energy of grains. In combination with large strain in precursor materials caused by applied pressure during fabrication and high sintering temperature this results in increased porosity and lower density. Appearance of Raman active modes in the considered Ba3+3xBl+yNbz0g materials was attributed to the formation of a 1:2 cation ordered structure. It was shown that microwave losses are influenced by the degree of 1:2 cation ordering that depends on the formation of secondary phases as well as a densification process. The appearance of an "extra" peak in Raman spectra was attributed to the formation of 1:1 cation order described based on the "space-charge" model. Changes in the position of the mode, attributed to "breathing-type" vibrations of oxygen anions from materials having "partially" ordered 1:1 structure to those having 1:2 ordered structure, indicate formation of more rigid oxygen octahedra associated with lower microwave losses. Structural distortion caused by 1:2 cation ordering results in changes in the mutual orientation of transition metal-ligand molecular orbitals and the appearance of two emission bands signifying formation of two different Nb06 octahedra. The first octahedron, present in the 1:2 ordered structure, gives origin to the lower energy photoluminescence band, while the second one, forming a disordered cubic structure, produces an emission peak at higher energies with the variation in the position of the maximum depending on the type of cation on the B-site. Changes in the maximum position of the high-energy peak were attributed to different structure distortions caused by off-center motion of Nb^5+ and stabilization by neighboring B06 octahedra. The stabilization power of B06 octahedra depends on the covalency of B-0 bonds and is larger for cobalt containing perovskites. / Thesis / Doctor of Philosophy (PhD)

The corrosion-wear resistance tradeoff in conventional metals imposes a great challenge to their reliable long-term performance under extreme conditions where surface stress and corrosive environment coexist (i.e., tribocorrosion). In this work, strategies to introduce chemical and structural heterogeneity with controlled length-scale at nanometers were proposed and studied in three metallic systems (i.e., Zr-based, Al-based and Mg-based), in order to enhance their tribocorrosion resistance. In the first study, ZrCuNiAl thin film metallic glasses (TFMG) with either homogeneous or heterogeneous local composition were deposited by magnetron sputtering through controlling processing conditions (i.e., argon (Ar) pressure). It was found that the mechanical properties, wear, corrosion and tribocorrosion resistance of ZrCuNiAl TFMG were significantly affected by nanoscale chemical heterogeneity. As a result, nanoscale chemical heterogeneity promoted ductility but reduced hardness, which in turn weakened wear resistance. While, in the 0.6 M NaCl solution, the resistance to pitting corrosion and tribocorrosion was improved because the presence of nanoscale chemical heterogeneity facilitates to generate more protective passive layer with lower defect density and faster repassivated capability, compared to their homogenous counterparts. In the second study, nanoscale chemical and structural heterogeneity were introduced in Al by forming Al/X nanostructured metallic multilayers (NMMs), where X=Mg, Cu, and Ti. Compared to the respective monolithic films, the alternating nanolayer configuration not only increased strength due to the presence of abundant interfaces but also reduced surface activity and pitting susceptibility. The electrochemical performance was significantly affected by the interaction, i.e., galvanic effect, between Al layer and underlayer constituents, which in turn led to different tribocorrosion behaviors, Specifically, transmission electron microscopy revealed that the materials loss in Al/Mg and Al/Cu NMMs primarily resulted from corrosion, while Al/Ti was dominated by severe plastic deformation during tribocorrosion as a result of sustained surface passivity. Lastly, in the bulk biodegradable Mg alloys system, the surface was treated by femtosecond laser shock peening (fs-LSP) technique with ultra-low pulse energy to introduce structural heterogeneity. Treatment conditions (e.g., power density, direct ablation and confined ablation) significantly affected the ultimate peening effect and further surface performance. In this work, the optimized peening effect was obtained at 28 GW/cm2 laser power density in the confined ablation with the assistance of the adsorption layer and confining medium. Combined with transmission electron microscopy and finite element analysis, the improvement of surface performance was attributed to high dislocation density near the surface, rather than compressive residual stress. The existence of structural heterogeneity not only reduced corrosion kinetics but simultaneously improved the self-repassivation in the blood bank buffered saline solution at body temperature. / Doctor of Philosophy / In various industrial applications such as marine infrastructure, nuclear power plants, and biomedical devices, the synergistic effect of wear and corrosion, known as tribocorrosion, is an inevitable material degradation phenomenon. To resist such aggressive degradation and prolong the service life of metals in complex environments, it is crucial to simultaneously enhance the wear and corrosion resistance, i.e., tribocorrosion resistance of metals. Unfortunately, the corrosion-wear resistance tradeoff in conventional metals imposes a great challenge. For example, most precipitation-hardened Al alloys impart high strength and wear but exhibit low resistance against localized corrosion as a sacrifice owing to the micro-galvanic coupling between the matrix and precipitates. Several previous works pointed out that compositional and structural heterogeneity, even at the nanoscale, could simultaneously affect the mechanical properties and corrosion resistance of metals. However, few works have been performed to understand the effects of such heterogeneity and their length-scale during tribocorrosion of metals. In this dissertation, by combining materials processing, advanced characterization, and tribocorrosion testing, the effects of chemical and structural heterogeneity, as well as their length-scale, on the deformation and degradation mechanisms of metals were studied using model systems of Zr-, Al- and Mg-based alloys, where the chemical and/or structural heterogeneity were introduced by tuning the materials processing conditions. Firstly, the nanoscale chemical heterogeneity was introduced into ZrCuNiAl thin film metallic glasses (TFMG) by adjusting argon (Ar) pressure during magnetron sputtering. Compared with the homogeneous composition, heterogenous local composition in ZrCuNiAl TFMG improved ductility but sacrificed hardness and wear resistance. In 0.6 M NaCl solution, higher pitting corrosion and tribocorroison resistance can be observed due to the generation of low defect density protective passive film with low defect density and with fast repassivation rates in heterogeneous ZrCuNiAl TFMG. Secondly, the architecture of nanostructured metallic multilayer in Al-based with different constituents, from noble to active metals (e.g., Cu, Ti and Mg), were studied the effects of chemical and structural heterogeneity on wear, corrosion and tribocorrosion performance. The results showed that the deformation and corrosion behaviors significantly depended on the distinct interfaces and chemical modulation at the nanoscale, caused by different constituents, which ultimately resulted in various tribocorrosion resistance in 0.6 M NaCl solution at room temperature. Transmission electron microscopy of deformed and degraded sample surfaces showed characteristic different deformation and degradation modes of all samples, governed by the synergistic effects of the mechanical and corrosion properties of the constituting materials. Specifically, severe plastic deformation mainly led to material loss in Al/Ti NMMs owing to the noble surface reactivity, while corrosion was the dominant factor for material loss in Al/Mg and Al/Cu NMMs during tribocorroison. Lastly, the ultra-low pulse energy femtosecond laser shock peening technique was successfully applied to introduce structural heterogeneity in the bulk biodegradable Mg alloys since in some cases the deposition is not feasible for bulk metals. The optimizing peening effect was firstly investigated and was achieved at confined ablation conditions under 28 GW/cm2 laser power density. Results show that the high dislocation density near the surface was contributing to the surface strengthening effect, high corrosion and tribocorrosion resistance in a simulated body environment via transmission electron microscopy observation. The finite element analysis method investigated the compressive residual stress in current work that did not significantly affect the surface performance of Mg alloys. In summary, the study of this dissertation contributes to a good basis and design strategy of conventional metals for applications under complex environments.

Experimental methods were developed and used to quantify the behavior of composite materials during heating to support development of heat and mass transfer pyrolysis models. Methods were developed to measure specific heat capacity, kinetic parameters, microstructure changes, porosity, and permeability. Specific heat and gravimetric data for kinetic parameters were measured with a simultaneous differential scanning calorimeter (DSC) / thermogravimetric analyzer (TGA). Experimental techniques were developed for quantitative specific heat measurement based on ASTM standards with modifications for accurate measurements of decomposing materials. An environmental scanning electron microscope (ESEM) was used in conjunction with a heating platform to record real-time video of microstructural changes of materials during decomposition and cooling following decomposition. A gas infusion technique was devised to measure porosity, in which nitrogen was infused into the pores of permeable material samples and used to determine the open-pore porosity of the material. Permeability was measured using a standard pressure differential gas flow technique with improvements over past sealing techniques and modifications to allow for potential high temperature use. Experimental techniques were used to measure the properties of composite construction materials commonly used in naval applications: E-glass vinyl ester laminates and end-grain balsa wood core. The simultaneous DSC/TGA was used to measure the apparent specific heat required to heat the decomposing sample. ESEM experiments captured microstructural changes during decomposition for both E-glass vinyl ester laminate and balsa wood samples. Permeability and porosity changes during decomposition appeared to depend on microstructural changes in addition to mass fraction. / Master of Science

Master of Science / Department of Grain Science and Industry / Hulya Dogan / Viscoelastic wheat flour doughs are renowned for their ability to produce high quality aerated bread products. Dough exhibits extremely complex rheological properties which makes it capable of occluding and retaining gas cells. The ability of these bubbles to resist failure and remain stable throughout the proofing and baking process is critical to final bread structure and volume. Understanding these factors is important when creating the distinct structural and textural characteristics that consumers desire in baked products. In this study, a method was established for using X-ray microtomography (XMT) to study the microstructure of proving dough as well as bread made from three very different wheat flours. Doughs were prepared according to AACC Method 10-10B optimized straight-dough bread-making method. Sections from unproofed (0 min), underproofed (20 min) and optimally proofed (40 min) doughs were carefully cut and frozen at –80°C. Baked loaves were also prepared following standard test bake procedures. Small specimens were cut from two locations of both the proofed and baked loaves prior to microstructural analysis. A total of 96 dough and bread samples were scanned using a high resolution desktop X-ray micro-CT system Skyscan1072 (Skyscan, Belgium) consisting of an X-ray tube, an X-ray detector and a CCD-camera. X-ray images were obtained from 137 rotation views through 180° of rotation. Hundreds of reconstructed cross sectional images were analyzed using CTAn (v.1.7) software. 3-D analysis of the bubbles indicated that average dough void fractions increased dramatically over proof time from 30.9% for the unproofed dough (0 min) to 62.0% and 74.5 % for the underproofed (20 min) and optimally proofed (40 min) doughs respectively. Oven spring caused further expansion in the baked loaves which increased average void fraction to 84.3%. Gas cell size distributions were largely skewed to the right and shifted in that same direction as processing time increased. Differences in gas cell size seen among flour varieties were largely due to variations in the size of the largest cells caused by coalescence.

Engineers constantly seek advancements in the performance of aircraft and power generation engines, including, lower costs and emissions, and improved fuel efficiency. Nickel-base superalloys are the material of choice for turbine discs, which experience some of the highest temperatures and stresses in the engine. Engine performance is proportional to operating temperatures. Consequently, the high-temperature capabilities of disc materials limit the performance of gas-turbine engines. Therefore, any improvements to engine performance necessitate improved alloy performance. In order to take advantage of improvements in high-temperature capabilities through tailoring of alloy microstructure, the overall objectives of this work were to establish relationships between alloy processing and microstructure, and between microstructure and mechanical properties. In addition, the project aimed to demonstrate the applicability of neural network modeling to the field of Ni-base disc alloy development and behavior. A full program of heat-treatment, microstructural quantification, mechanical testing, and neural network modeling was successfully applied to next generation Ni-base disc alloys. Mechanical testing included hot tensile, hot hardness, creep deformation, creep crack growth, and fatigue crack growth. From this work the mechanisms of processing-structure and structure-property relationships were studied. Further, testing results were used to demonstrate the applicability of machine-learning techniques to the development and optimization of this family of superalloys.