Use of load-bearing materials whose functionality arises from architectured microstructures, so called architectured materials, has been hindered by the challenge of connecting them. A solution in nature is found at the tendon enthesis, a tissue that connects tendon and bone, two vastly different natural architectured materials. The tendon enthesis provides stability and allows for mobility of a joint though effective transfer of muscle forces from tendon to bone, while exhibiting toughness across a wide range of loadings. Unfortunately, many painful and physically debilitating conditions occur at or near this interface when the enthesis architecture is compromised due to injury or degeneration. Surgical and natural repairs do not reconstitute the natural toughening mechanisms of the enthesis and often fail. Hence, understanding the architectural mechanisms by which healthy and pathologic tendon entheses achieve strength and toughness would inform the development of both biological and engineered attachments.Integrating biomechanical analyses, failure characterizations, numerical simulations, and novel imaging, this thesis presents architectural mechanisms of enthesis toughening in a mouse model. Imaging uncovered fibrous architecture within the enthesis, which controlled trade-offs between strength and toughness. Ex vivo enthesis failure modes exhibited nanoscale differences in damage, milliscale differences in fiber load-sharing, and macroscale differences in energy absorption that depended on structure, composition, and the nature of loading. The elastic and failure responses of the tendon enthesis also varied with the direction of loading. This variation was due to the fibrous nature of the tendon enthesis, with a clear role for bony anatomy and fiber recruitment in enthesis toughening behavior. In vivo, , the loss of toughening mechanisms at the enthesis due to pathologic loading was evaluated by either increased (i.e., overuse) loading via downhill treadmill running or decreased (i.e., underuse) loading via botulinum toxin A induced paralysis. These loading environments led to changes in the mineralization and architecture at the tendon enthesis. These micro-architectural adaptations compromised the trade-offs between strength and toughness; overuse loading prompted active reinforcement and stiffening of the underlying trabeculae, leading a maintenance of strength and a compromise in overall toughness, whereas underloading prompted active resorption of the underlying trabecular architecture, leading to a compromise in both strength and toughness. The mouse models of the tendon enthesis failure revealed a correlation between tendon enthesis architecture and injury prevention (i.e., toughening) mechanisms. To test this concept in a clinical setting, an injury classification system was developed for patellar tendinopathy and partial patellar tendon tears. This classification system identified the stages of tear progression and prognosis by tracking changes to patellar tendon architecture. Results revealed a relationship between patellar tendon thickness and likelihood of improvement with nonoperative treatment. Taken together, this dissertation revealed how fibrous architecture can be tailored to toughen attachments between vastly different materials. This understanding can have prognostic value: tracking changes to enthesis architecture can be used as a tool for identifying candidates for various treatment options, as we showed for the patellar tendon clinical example. Furthermore, the toughening mechanisms identified here provide guidance for enhancing enthesis surgical repair and designing enthesis tissue engineered scaffolds, as well as motivating biomimetic approaches for attachment of architectured engineering material systems.

PhD - Science / The relationships between densification and microstructure, and between microstructure and mechanical and electrical properties of liquid phase sintered silicon carbide were studied in detail using hot pressing, gas pressure sintering and ultra–high pressure sintering techniques. Silicon carbide was sintered with 10 mass-% addition of the Y2O3-Al2O3 system, with various molar ratios. Hot pressing was carried out at 1925oC under 30 MPa, in argon, for half an hour. Materials were gas pressure sintered at 1925oC, under a final gas pressure of 80 bars (8MPa), in argon, for an hour. Ultra-high pressure sintering was done at ca. 1550oC, under 5.5 GPa pressure, for 15 minutes. The hot pressed and gas pressure sintered materials were subsequently heat treated at 1925oC and 1975oC. Most of the silicon carbide materials were sintered to a density around 99% of theoretical density. The heat treatment of the hot pressed materials resulted in an increase in density not changing the porosity. The densities of the heat treated hot pressed materials corresponded to the density of the gas pressure sintered materials. This resulted from the difference in composition of grain boundary phases – yttrium silicates in the hot pressed materials and yttrium aluminates in the gas pressure sintered and heat treated materials. The average silicon carbide grain size in the materials strongly depended on the densification method. In gas pressure sintered and heat treated materials the mean grain size was up to three times higher than that in the hot pressed materials. Grain growth appeared to be higher in the highest alumina-content materials. The heat treatment at 1975 °C resulted in more pronounced anisotropic grain growth. The ratio of the silicon carbide polytypes of sintered materials and materials heat treated materials at 1925oC, did not change significantly. In the materials heat treated at 1975oC Rietveld analysis revealed a decrease in SiC-6H polytype and an increase in amount of 4H and 15R polytypes, compared to the materials heat treated at 1925oC. This can be attributed to the increase in diffusion rates of aluminium into the SiC lattice at 1975oC. Segregation patterns were observed in the high yttria content materials, with Y2O3:Al2O3 molar ratios greater than or equal to two, after gas pressure sintering and heat treatments. This was suggested to be due to he poor wetting of the silicon carbide grains by the yttria-rich grain boundary phase. On heat treatment, the Vickers hardness of hot pressed materials was found to be increased from 20 to 26 GPa and elastic modulus from 318 to 338 GPa. In addition, the log of the electrical conductivity of liquid phase sintered silicon carbide (measured at 330oC) ranged from 10-8 to 10-3 with the changes in grain boundary phases observed after the heat treatments. The grain boundary phase composition also influenced the strength of the materials, The highest strength, 657 + 50 MPa, was measured for the hot pressed material containing the YAG phase. Indentation fracture toughness was mostly influenced by the SiC grain growth during heat treatments. The most significant increase in fracture toughness, the largest being from 3.7 MPa.m1/2 up to 5.6 MPa.m1/2, was observed in the higher alumina content materials after heat treatment at 1975oC. The increase in fracture toughness was attributed to the presence of a higher amount of platelet-like SiC grains within a broader grain size distribution. These elongated grains increased fracture toughness by increasing crack path deflection and crack bridging. The electrical properties were evaluated by Impedance Spectroscopy measurements between room temperature and 330oC. The LPS SiC materials can be classified into three groups with different electrical properties. This classification could be related to the grain boundary phases present in the materials. The materials with the lowest conductivity were all hot pressed materials, containing crystalline silicates and amorphous grain boundary phases. The materials with intermediate conductivity include gas pressure sintered materials and a hot pressed material, which contained crystalline aluminates (Y3Al5O12, YAlO3 and Y4Al2O9) in their grain boundaries. The materials with the highest conductivity only contained the aluminates, YAlO3 and Y4Al2O9. A pseudopercolation model of conduction was proposed, in which electrons move along a path which goes through the thinner intergranular layers, with largest nearest neighbour contact. The temperature dependence of the log of the conductivity of hot pressed and gas pressure sintered materials showed that the conduction mechanism in these liquid-phase sintered silicon carbide materials was variable range hopping conduction of electrons between defect sites. The non-Arrhenius behaviour, together with the observed wide range of peak frequencies, led to the conclusion that the effect of silicon carbide itself was not observed in the impedance spectra. The 1/T0.25 log conductivity dependence showed that the Cole-Cole arcs are due to insulating grain boundary phases rather than semiconducting SiC. In the Cole-Cole plots of the hot pressed and heat treated hot pressed materials only the effect of one phase could be observed. In the gas pressure sintered materials and the hot pressed material containing mainly YAG phase, the effects of two phases were seen in the frequency range measured. Ultra-high pressure liquid-phase sintered silicon carbide materials showed ultra-fine SiC grains, which were highly inter-grown. Segregated grain boundary “core-rim” structures, consisting of an inner core of nonequilibrium yttria and outer rim of equilibrium yttrium silicate were observed in materials containing 4 mass-% to 15 mass-% sintering additives. The hardness of ultra-high pressure sintered 10 mass-% materials increased with alumina-content, from 20 GPa – 22 GPa, and increased with decrease in sintering additive, up to 23 GPa (for the 4 mass-% material).

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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.

No description available.

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