<p>AI-Mg sheets (5xxx series) for body-in-white (BIW) application are mostly<br />used for automotive structural parts due to their specific combination of<br />formability and strength. The limiting behavior for the wide application of AA5754<br />sheets is strain localization. The effect of microstructure inhomogeneties on<br />strain localization have recently been attracting a great deal of interest but not<br />fully understood. In this present work, the effect of grain-level microstructure<br />inhomogeneties in AA 5754 sheets is investigated.<br />Uniaxial tensile experiments combined with two 2-dimensional Digital<br />image correlation (DIG) techniques have been performed on coarse-grained<br />specimens to evaluate the deformations of individual grains. Grain orientations<br />and their evolution were measured by the electron backscattered diffraction<br />(EBSD) technique, and surface features such as slip traces were observed by<br />optical microscopy.<br />The regions of high local strain ('hot spots') within coarse-grained samples<br />nucleate at a very early stage of deformation and most of them continuously<br />grow throughout most deformation stages. 'Hot spots' are correlated with 'soft'<br />grains (i.e. grains with high Schmid factors) and soft-evolution grains (i.e. grains<br />with the <101> direction close to tensile axis).</p> / Master of Applied Science (MASc)

<p> A new paradigm in biomedical engineering calls for biologically active implants that are absorbed by the body over time. One popular application for this concept is in the engineering of endovascular stents that are delivered concurrently with balloon angioplasty. These devices enable the injured vessels to remain patent during healing, but are not needed for more than a few months after the procedure. Early studies of iron- and magnesium-based stents have concluded that magnesium is a potentially suitable base material for such a device; alloys can achieve acceptable mechanical properties and do not seem to harm the artery during degradation.</p><p> Research done up to the onset of research contained in this dissertation, for the most part, failed to define realistic physiological corrosion mechanisms, and failed to correlate degradation rates between <i>in vitro</i> and <i>in vivo</i> environments. Six previously published works form the basis of this dissertation. The topics of these papers include (1) a method by which tensile testing may be applied to evaluate biomaterial degradation; (2) a suite of approaches that can be used to screen candidate absorbable magnesium biomaterials; (3) <i>in vivo-in vitro</i> environmental correlations based on mechanical behavior; (4) a similar correlation on the basis of penetration rate; (5) a mid-to-late stage physiological corrosion mechanism for magnesium in an arterial environment; and (6) the identification of corrosion products in degradable magnesium using transmission electron microscopy.</p>

Hypersonic vehicles require material systems that can withstand the extreme environment they experience during flight. Carbon-based materials and ultra-high temperature ceramics are candidates for materials systems that will protect hypersonic vehicles. In order to study the material response, an oxyacetylene torch facility and thermal gravimetric analysis are used to investigate the gas-solid interactions under conditions that simulate aspects of flight. The oxyacetylene torch facility is characterized as a function of position from the tip for heat flux and oxygen content. By understanding the local heat flux and oxygen conditions, experiments are designed so that graphite ablation rates can be measured as a function of heat flux and partial pressure of oxygen. Further investigation shows that composition of the material influences the temperature response where ultra-high temperature ceramics exhibit the lowest surface temperatures. Using thermal gravimetric analysis, the isothermal oxidation behavior of ultra-high temperature ceramics from 1000-1600°C is investigated using a Dynamic Non- Equilibrium method in order to understand the reaction kinetics of ZrB₂-SiC where parabolic rate constants are determined. Isothermal oxidation behavior is compared to non-isothermal mass gain and oxide scale formation where specimens oxidized isothermally gain 3 times more mass and have oxide scales 4 times as thick. Finally, the effect of SiC content in ZrB₂ on temperature during oxyacetylene torch testing is determined. Increasing the amount of SiC results in lower front face temperatures because more heat is absorbed due to the endothermic reactions of evaporation of SiO₂.

<p> The objective of this research was to characterize the seawater transport and its effect on the transverse tensile strength of a carbon/vinylester composite. The moisture contents of neat vinylester and unidirectional carbon/vinylester composite panels immersed in seawater were monitored until saturation. A model for moisture up-take was developed based on superposition of Fickian diffusion, and Darcy&rsquo;s law for capillary transport of water. Both the predicted and measured saturation times increased with increasing panel size, however the diffusion model predicts much longer times while the capillary model predicts shorter time than observed experimentally. It was also found that the saturation moisture content decreased with increasing panel size. Testing of macroscopic and miniature composite transverse tensile specimens, and SEM failure inspection revealed more fiber/matrix debonding in the seawater saturated composite than the dry composite, consistent with a slightly reduced transverse tensile strength. </p>

<p> The tensile strength of the fiber/matrix interface was determined through the development of an innovative test procedure. A miniature tensile coupon with a through-thickness oriented, embedded single fiber was designed. Tensile testing was conducted in a scanning electron microscope (SEM) while the failure process could be observed. Finite element stress analysis was conducted to determine the state of stress at the fiber/matrix interface in the tensile loaded specimen, and the strength of the interface. Test specimens consisting of dry E-glass/epoxy and dry and seawater saturated carbon/vinylester 510A were prepared and tested. The load at the onset of debonding was combined with the radial stress distribution near the free surface of the specimen to reduce the interfacial tensile strength (&sigma;<i>_i</i>). For glass/epoxy, &sigma;<i>_i</i> was 36.7&#129;&plusmn;8.8 MPa. For the dry and seawater saturated carbon/vinylester specimens the tensile strengths of the interface were 23.0&#129;&plusmn;6.6 and 25.2&#129;&plusmn;4.1 MPa, respectively. The difference is not significant.</p>

<p> For galvanized or galvannealed steels to be commercially successful, they must exhibit several attributes: (i) easy and inexpensive processing in the hot mill, cold mill and on the coating line, (ii) high strength with good formability and spot weldability, and (iii) good corrosion resistance. At the beginning of this thesis, compositions with a common base but containing various additions of V or Nb with or without high N were designed and subjected to Gleeble simulations of different galvanizing(GI), galvannealing(GA) and supercooling processing. The results revealed the phase balance was strongly influenced by the different microalloying additions, while the strengths of each phase were somewhat less affected. Our research revealed that the amount of austenite formed during intercritical annealing can be strongly influenced by the annealing temperature and the pre-annealing conditions of the hot band (coiling temperature) and cold band (% cold reduction). In the late part of this thesis, the base composition was a low carbon steel which would exhibit good spot weldability. To this steel were added two levels of Cr and Mo for strengthening the ferrite and increasing the hardenability of intercritically formed austenite. Also, these steels were produced with and without the addition of vanadium in an effort to further increase the strength. Since earlier studies revealed a relationship between the nature of the starting cold rolled microstructure and the response to CGL processing, the variables of hot band coiling temperature and level of cold reduction prior to annealing were also studied. Finally, in an effort to increase strength and ductility of both the final sheet (general formability) and the sheared edges of cold punched holes (local formability), a new thermal path was developed that replaced the conventional GI ferrite-martensite microstructure with a new ferrite-martensite-tempered martensite and retained austenite microstructure. The new microstructure exhibited a somewhat lower strength but much high general and local formabilities. In this thesis, both the physical and mechanical metallurgy of these steels and processes will be discussed. This research has shown that simple compositions and processes can result in DP steels with so-called Generation III properties. </p>

Iron-cobalt (FeCo) and its various alloys have many applications where soft magnetic materials are needed, especially in high temperature applications. Recent research has looked into the nanocrystallization of amorphous alloys of FeCo and very briefly into the oxidation of FeCo nanoparticles and bulk materials. Attempts will be made to more carefully investigate the oxidation of FeCo and its alloys utilizing nanoparticles, and thin films with (100), (110), and (211) texture to observe the kinetics of oxidation. Thin film epitaxial relationships between the substrate and thin films have been determined, and this will be extended to the oxide and thin film. The role of alloying has been discussed, especially in the context of oxidation of FeCo. The composition of the oxide at different oxidizing temperatures is also proposed. FeCo-based nanoparticles have been analyzed to understand their change in magnetization and oxide phase as a function of temperature. The oxide thickness has been measured at various temperatures, along with the observation of a voided core. This research has been coupled with thin film work to show that the core gets richer in cobalt as oxidation progresses, with Fe acting as the mobile species. Oxygen may diffuse early in the oxidation, but only until a certain oxide thickness has been established. The oxidation kinetics seen in the nanoparticles is slower than that seen in thin films, and it has currently been analyzed to follow a logarithmic rate law at lower temperatures. To understand the formation of faceted nanoparticles, nucleation and growth has been modeled for both BCC and FCC systems showing the surface energy ratios necessary to produce different faceting of nanoparticles. It has been shown that the critical nuclei are the same as the growth shapes. To extend the basic science research into the applications field, thin film work on CoCrPt has been performed to achieve out-of-plane anisotropy in thicker films for use in a portable AGFM. While this has been achieved, further study is necessary to improve the remnant magnetization and make it more comparable to SmCo, which is the current standard. The magnetic properties have been measured as a function of temperature and film thickness to begin understanding the system better to produce the desired thin film properties for a biomedical sensor.

<p> Formation of a high density of Mn-Ni-Si nanoscale precipitates in irradiated reactor pressure vessel steels could lead to severe, unexpected embrittlement, which may limit the lifetimes of our nation&rsquo;s light water reactors. While the existence of these precipitates was hypothesized over 20 years ago, they are currently not included in embrittlement prediction models used by the Nuclear Regulatory Commission. This work aims to investigate the mechanisms and variables that control Mn-Ni-Si precipitate (MNSP) formation as well as correlate their formation with hardening and embrittlement. </p><p> A series of RPV model steels with systematic variations in Cu and Ni contents, two variables that have been shown to have a dominant effect on hardening, were irradiated in a series of test reactor and power reactor surveillance irradiations. Atom probe tomography (APT) measurements show that large volume fractions (f_v) of MNSPs form in all the steels irradiated at high fluence, even those containing no added Cu, which were previously believed to have low sensitivity to embrittlement. It is demonstrated that while Cu enhances the rate of MNSP formation, it does not appear to significantly alter their saturation f_v or composition. The high fluence MNSPs have compositions consistent with known intermetallic phases in the Mn-Ni-Si system and have f_v very near those predicted by equilibrium thermodynamic models. In addition, X-ray diffraction experiments by collaborators shows that these precipitates also have the expected crystal structure of the predicted Mn-Ni-Si phases. </p><p> Post irradiation annealing experiments are used to measure the hardness recovery at various temperatures as well as to determine if the large f_v of MNSPs that form under high fluence neutron irradiation are thermodynamically stable phases or non-equilibrium solute clusters, enhanced or induced by irradiation, respectively. Notably, while post irradiation annealing of a Cu-free, high Ni steel at 425&deg;C results in dissolution of most precipitates, a few larger MNSPs appear to remain stable and may begin to coarsen after long times. A cluster dynamics model rationalizes the dissolution and reduction in precipitate number density, since most are less than the critical radius at the annealing temperature and decomposed matrix composition. The stability of larger precipitates suggests that they are an equilibrium phase, consistent with thermodynamic models. </p><p> Charged particle irradiations using Fe³⁺ ions are also used to investigate the precipitates which form under irradiation. Two steels irradiated to a dose of 0.2 dpa using both neutrons and ions show precipitates with very similar compositions. The ion irradiation shows a smaller f_v, likely due to the much higher dose rate, which has been previously shown to delay precipitation to higher fluences. While the precipitates in the ion irradiated condition are slightly deficient in Mn and enriched in Ni and Si compared to neutron irradiated condition, the overall similarities between the two conditions suggest that ion irradiations can be a very useful tool to study the susceptibility of a given steel to irradiation embrittlement. </p><p> Finally, the large f_v of MNSPs that are shown to form in all steels, including those low in Cu, at high fluence, even those without added Cu, result in large amounts of hardening and embrittlement. A preliminary embrittlement prediction model, which incorporates MNSPs at high fluence, is presented, along with results from a recent test reactor irradiation to fluences representative of extended lifetimes. This model shows very good agreement with the data.</p>

The mechanical properties of the extracellular matrix are dynamic and change during biological processes such as disease progression and wound healing. Most synthetic (or man-made) tissue scaffolds have static properties. Therefore it is necessary to replate cells in order to determine the effects that different matrix mechanical properties have on cells, and virtually impossible to study the effects of a dynamically changing modulus on cell growth. There have been several scaffolds recently developed with tunable mechanical properties, but few exhibit any reversibility which is important for simulating repeated wounding and healing cycles. In this work, we develop a gelatin based hydrogel with azodianiline (ADA) as a secondary crosslinking unit. Upon irradiation with 365 nm light the gel softens as the ADA undergoes a photoisomerization. These changes can be reversed upon exposure to visible light. With applications in mechanobiology in mind, contraction at the cellular scale was measured, as well as the macroscopic changes in the shear elastic modulus and compressive modulus in response to exposure to UV and visible light.

Environmental microorganisms are implicated as the causative agents in a significant portion of healthcare associated infections (HAI) and antimicrobial resistant infections (AMR), which result in increased costs and suffering around the world. Furthermore, common environmental microorganisms participate in microbiological degradation of materials and the bio-fouling of various systems. This also results in a tremendous amount of damage in many different materials and many different sectors. The focus of this dissertation was the development of an additive that could be easily added to common materials to make them self-disinfecting and to protect them from microbial damage. The ultimate goal was to develop an additive that could be added using standard techniques and without adversely affecting the final material. Cuprous iodide (CuI) was determined to be an ideal starting material for the development of improved antimicrobial materials because of its neutral appearance and high antimicrobial activity as compared to other silver and copper materials. It was found that the antimicrobial efficacy of CuI could be amplified if prepared as a small particle and especially in the presence of vinylpyrrolidone polymers. A comminution process was then developed to produce these small particles. By using select copolymers, various CuI small particles formulation were developed to be compatible with a variety of different matrices. The efficacy of these CuI containing matrices was dependent on the compatibility of the CuI formulation with the matrix. A variety of applications were demonstrated with good antimicrobial efficacy where the particles were easily added to the finished material with minimal or no change in appearance.