Topology Optimization and Analysis of Thermal and Mechanical Metamaterials

To take advantage of multi-material additive manufacturing technology using mixtures of metal alloys, a topology optimization framework is developed to synthesize high-strength spatially periodic metamaterials possessing unique thermoelastic properties. A thermal and mechanical stress analysis formulation based on homogenization theory is developed and is used in a regional scaled aggregation stress constraint method, and a method of worst-case stress minimization is also included to efficiently address load uncertainty. It is shown that the two stress-based techniques lead to thermal expansion properties that are highly sensitive to small changes in material distribution and composition. To resolve this issue, a uniform manufacturing uncertainty method is utilized which considers variations in both geometry and material mixture. Test cases of high stiffness, zero thermal expansion, and negative thermal expansion microstructures are generated, and the stress-based and manufacturing uncertainty methods are applied to demonstrate how the techniques alter the optimal designs. Large reductions in stress are achieved while maintaining robust strength and thermal expansion properties. An extensive analysis is also performed on structures made from two-dimensional lattice materials. Numerical homogenization, finite element analysis, analytical methods, and experiments are used to investigate properties such as stiffness, yield strength, and buckling strength, leading to insights on the number of cells that must be included for optimal mechanical properties and for homogenization theory to be valid, how failure modes are influenced by relative density, and how the lattice unit cell can be used to build macrostructures with performance superior to structures generated by conventional topology optimization

Influence of mismatch on the defects in relaxed epitaxial InGaAs/GaAs(100) films grown by molecular beam epitaxy

Thick (∼3 μm) films of InxGa1−xAs grown on GaAs(100) substrates, across the whole composition range, have been examined by transmission electron microscopy and double‐crystal x‐ray diffraction. The results were compared with the observed growth mode of the material determined by in situ reflection high‐energy electron diffraction in the molecular beam epitaxy growth system. The quality of the material degraded noticeably for compositions up to x∼0.5 associated with an increased density of dislocations and stacking faults. In contrast, improvements in quality as x approached 1.0 were correlated with the introduction of an increasingly more regular array of edge dislocations

Bend transition temperature of arc-cast molybdenum and molybdenum - 0.5-percent- titanium sheet in worked, recrystallized, and welded conditions

Bend transition temperature of arc-cast worked, recrystallized, and welded molybdenum and molybdenum-titanium alloy shee

Master of Science

thesisSurface integrity plays a very important role in the life and functionality of machined surfaces used in a variety of engineering applications. Manufacturing processes and their sequence, along with the selection of cutting conditions and cutting tools, eventually dictate the type of surface that is being produced. Surface integrity is subdivided into several components, of which, some important components are surface roughness, residual stresses and subsurface microstructures. Enhanced understanding of all these factors and their interactions will potentially lead to advanced knowledge driven machining process planning. This thesis focuses on an experimental investigation of the effects of cutting tool coatings and cutting fluid application on surface integrity (residual stress, surface roughness and subsurface microstructure) in machined Ti6Al4V titanium alloy. For measuring residual stresses, the hole drilling method was used in this thesis research. The tools selected for the machining were uncoated flat-faced, uncoated grooved, multilayered (TiCN/Al2O3/TiN) CVD coated grooved, and single-layered (TiAlN) PVD coated grooved tools with tungsten carbide substrates. Three different cutting fluid application conditions were used namely: dry, minimal quantity lubrication (MQL) and flood. To illustrate the significance of this thesis, it was observed that a grooved multilayered CVD coated cutting tool under the influence of MQL condition, induced the highest near-surface residual stresses; on the other hand, the same tool, when machined under dry condition, produced the lowest residual stresses. Thus, it can be seen that a specific cutting tool material and/or geometry produce significantly different surface integrity when it is combined with different cutting fluid application conditions. Moreover, the microstructural analysis performed on these machined workpieces revealed significant changes in the subsurface microstructure with respect to the type of cutting tool-cutting fluid application combination used and correlated strongly with the measured residual stress profiles. The combined effect of the type of cutting tool along with the type of cutting fluid application condition on surface integrity is extremely significant. The results and findings o f this thesis have the potential to aid in choosing the combination of the cutting tool and the cutting fluid application that are best suited for machining. Apart from that, this thesis also provides several recommendations for future research on the fundamentals of the interactions between machining parameters such as tool coatings, tool geometry and cutting fluid applications and their significant effects on the generated surface integrity and the life of the component there after

Critical Life Prediction Research on Boron-enhanced Ti-6Al-4V

Research on boron-enhanced Ti-6-4 has demonstrated the following improvements to the Ti-6-4 alloy: up to 40% increase in ultimate tensile strength, up to 30% increase in modulus/stiffness, while maintaining greater than 10% ductility at RT conditions. The increased properties are attributed to small additions of boron (-or= wt%), which refine the microstructure and result in a small volume fraction (~6 vol%) of fine TiB whiskers. Previous research indicates potential for substantial improvements in fatigue, fatigue crack growth, and fracture toughness. However, uncertainty regarding these second-tier mechanical properties is currently limiting implementation of this class of titanium alloys. This study of fatigue variability of a powder-metallurgy, boron-enhanced Ti-6-4 alloy identifies the most prevalent damage mechanism and elucidates the impact on fatigue design limits. The alloy was produced via a unique prealloyed powder-metallurgy process. The powder mesh size used was -35, which equates to powder particles with a diameter of 500 µm and smaller. Specimens were ultimately machined from a rolled plate. The mean fatigue behavior compared favorably with available data on conventional Ti-6-4, both wrought and powder-metallurgy product forms. However, inclusions in the material were responsible for a few poor fatigue results, which ultimately govern the fatigue design limits. Variability assessment and fatigue crack growth analyses indicate that if the frequency and size of inclusions can be reduced, this material could become a more viable alternative for select turbine engine and aircraft applications

Effect of Microstructure on High-Temperature Mechanical Behavior of Nickel-Base Superalloys for Turbine Disc Applications

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.Ph.D.Committee Chair: Saxena, Ashok; Committee Member: Gokhale, Arun; Committee Member: Helmink, Randolph; Committee Member: Neu, Richard; Committee Member: Thadhani, Nares