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Abstract: Advanced physics-based computational models have been developed in this thesis to study dynamic failure in novel light-weight materials used in impact applications, focusing on applying different computational techniques to study three material systems: 1. novel self-propagating high-temperature synthesized (γ + α2)-TiAl/Ti3Al- Al2O3 ceramic-metal composites, 2. single crystal magnesium, and 3. nano-grained boron carbide. For the (γ + α2)-TiAl/Ti3Al- Al2O3 cermet, a progressive study has been carried out by developing a three-dimensional microstructure-based finite element model. The models have been developed to investigate the rate-dependent mechanical response features (e.g., compressive strength, flow stress hardening, and energy absorbing efficiency) and predominant failure mechanisms (e.g., void deformation and growth, particle cracking, and interface decohesion), and have been validated with high fidelity experimental data. Validating the numerical model has enabled predicting the material response outside of the experimentally-accessible conditions. To date, few studies have been made to bridge ceramic-metal material models under different loading conditions with experimental inputs, such as porosity, particle volume fraction, elastic modulus of the constitutions, and void clusters. To address this, a modified variational formulation of the Gurson model has been used to allow for the damage caused by the voids. The strain hardening components, as one of the most unknown material properties, have been calibrated by matching the modeling results with experimental data. Following validation, the effects and implications of different parameters such as particle volume fraction, porosity, unit cell size ratio, and the variability of the inclusions have been presented and discussed. After that, the model was extended to explore the response of the α2(Ti3Al)+γ(TiAl)-submicron grained alumina cermet by considering high particle volume fraction and under high strain-rate loading applications. In this part of the study, the energy absorbing efficiency, as one of the important factors in using the ceramic-metal composites in high-rate applications, has been studied. The results have indicated that the particle shapes and void volume fraction play an important role on energy absorption capabilities of the (γ + α2)-TiAl/Ti3Al-Al2O3 cermets' damage tolerant design. These new understandings have informed the fabrication and refinement of new generations of the cermet variant for commercial use in protection products by industry partners. In the second thrust of my research involving magnesium and boron carbide, I have developed an advanced physics-based time-dependent phase-field model to predict deformation and failure mechanisms (e.g., fracture and twinning) in these anisotropic materials. Computationally, a monolithic scheme has been used as a powerful technique to increase the accuracy of the solver and high-performance computer clusters have been employed to solve large-scale problems associated with the research. The coupled differential equations have been implemented in an open-source high-level Python interface, FEniCS. First, the model is extensively validated for an intrinsically brittle material, magnesium. Comparing with molecular dynamics simulation, the twin interface velocity is explored in order to obtain the kinetic coefficient. Finding this velocity-related parameter is important as one of the main factors for determining the driving force of twinning for magnesium. In addition, the spatial distribution of the shear stress field in the parent and twinned phases is investigated. The result provides insights into the effect of twin's thickness on further twin nucleation and growth. Next, the critical strain and initial twin embryo size required for propagation and growth of a single twin embryo in magnesium are predicted by the current phase-field approach. After that, the effect of twin-twin and twin-defect interactions is explored because this may increase the likelihood for crack and failure leading to reduction of material lifetime. Then, the phase-field approach is extended to predict various deformation mechanisms in nano-grained B4C (e.g., fracture and twinning). In addition, the crack propagation under compressive loading is treated by using a new decomposition for the strain energy density, which represents a valuable contribution to the literature. Overall, this thesis consists mainly of three parts adapted from the two published and the two submitted journal articles: 1- A physics-based model to capture the mechanical response of (γ+α2)-TiAl/Ti3Al- Al2O3 cermet under quasi-static and dynamic loading, 2- An advanced phase-field approach to evolution and interaction of twins to unravel time-evolved twinning behavior in magnesium at nanoscale, and 3- A comprehensive calibrated and validated phase-field model for studying the deformation mechanisms of nanocrystalline Mg and B4C in order to provide guidance for material refinement via tailoring their mechanical properties and microstructure.

Electrified aircraft propulsion systems are sought for increased efficiency, reduced emissions, and noise abatement in next-generation aerospace concepts. For hybrid-electric and all-electric systems, the operational range is limited by the energy density of state-of-the-art electrochemical energy storage devices. Specifically, lithium-ion batteries have too low of a theoretical energy density, and higher energy density devices based on lithium-metal anodes introduce safety concerns with the formation of lithium dendrites leading to cell shorting and thermal runaway. To overcome these limitations, multifunctional energy storage devices provide simultaneous energy storage and load-bearing performance to achieve systems-level mass savings by minimizing parasitic weight of the structure. Recent development of structural batteries has leveraged carbon fiber composites as active anodes and structural materials. However, few investigations have explored the load-bearing capabilities of sintered lithium-ion battery active materials. In this work, processing-microstructure-property relationships were investigated to improve multifunctional performance of a strain-free anode composite based on Li4Ti5O12 containing nickel current collector particles. The influence of processing controls of nickel volume fraction, sintering temperature, and dwell time were analyzed with respect to bulk electrical conductivity and current collector particle size coarsening. Conductivity was modeled by power law percolation theory and increased as a function of nickel volume fraction and sample densification. Conductivities greater than 1 S/cm were achieved at relatively low volume fractions with tuning of sintering conditions. Analysis of particle size distributions revealed the impact of nickel fraction on particle interaction, and sintering temperature and dwell time on increase in average particle size. Coarsening was modeled according to Ostwald ripening theories and an activation energy of 1.04 eV was measured, indicating transport through matrix grain boundaries. In addition to bulk conductivity, the fracture stresses of samples were analyzed as a function of processing controls. Fracture stress was significantly improved at higher nickel fractions, up to 200 MPa, and fracture stress was strongly dependent on sample densification. Samples processed for long dwell times that showed very large average particle sizes showed toughening in the load-displacement curves, and crack bridging and plastic deformation of nickel was observed at fracture surfaces. To more clearly capture trends in microstructural development controlling variation in multifunctional properties, additional microstructural descriptors were analyzed including counts per area, relative particle spacing, and particle shape. Comparing trends in each descriptor allowed for the deconvolution of particle network formation from particle coarsening that would be impossible through analysis of particle size evolution alone. Finally, the compilation of microstructural descriptors was leveraged to train machine learning regression models. Random forest regression models allowed for the prediction of conductivity and fracture stress from observations of the microstructure with high accuracy, with up to 0.98 adjusted R2. Model analysis indicated the relative importance of each descriptor, with the nickel area coverage, nickel particle size, bulk density, and nickel particle network complexity metrics the most important for predicting properties. These studies of processing-microstructure-property relationships inform future design of load-bearing lithium-ion electrodes. This work correlated relationships between processing conditions and conductivity and strength, and developed methods to characterize and identify the relative importance of microstructural parameters to design for multifunctionality. Future research on composites that incorporate additional phases and alternative processing routes can build upon the findings of this study.

Abstract: Detailed characterization and experimental studies have been carried out on a novel self-propagating high-temperature synthesized (γ + α2) – TiAl/Ti3Al-Al2O3 cermet to investigate the microstructure, rate-dependent mechanical response, and rate-dependent failure mechanisms under uniaxial compressive loading. Cermet materials have gained significant research attentions recently in structural applications because they process greater ductility and toughness over most advanced ceramics while maintaining moderate strength and hardness. To apply a material in full-scale industrial applications, a comprehensive understanding of the material microstructure and its rate-dependent responses is critical. For the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet studied in this thesis, limited information is provided in the literature regarding the mechanical properties, stress-strain response, failure mechanisms, and their rate-dependency. This thesis seeks to address this gap and provide an in-depth investigation of the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet. This thesis consists mainly of two parts, which are adapted from the two published journal articles written by the author. A progressive study has been carried out in this thesis by first examining the as-received material microstructure, followed by a series of mechanical testing and post-mortem analysis. In the first part of this thesis, the micro/nanostructural features and mechanical responses of the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet were explored. The material composition, phase distribution, and elemental concentration were characterized. Three phases were identified in the cermet, including γ-TiAl, α2-TiAl, and Al2O3. The material exhibited ultrafine microstructure, with the size of alumina particles between 0.5 microns to 1.5 microns and occupying 65 ± 1% areal fraction of the material. Some alumina particles are connected to form clusters in the material, where a heterogeneous and complex microstructure is observed. Transmission electron microscopy investigation found iron/nickel-based nano-precipitates, and these were believed to be contributing to the mechanical properties of the material. The rate-dependency on the compressive strength, stress-time history profile, and failure mechanisms were studied by quasi-static and dynamic uniaxial compression. Surface texturing behavior was observed under dynamic loading condition using high-speed imaging, which was further explored in the second part of this thesis. In the second part of this thesis, the rate-dependent mechanical properties and failure of the cermet were investigated using mechanical testing and advanced characterization tools. Quasi-static and dynamic uniaxial compression tests coupled with high-speed imaging and digital image correlation were used to determine the rate-dependency of stress-strain behavior, compressive strength, and failure strain. The stress-strain curves in the dynamic experiments exhibited a series of alternating stress relaxation and strain hardening cycles, where a 1.3 times increase in compressive strength from 2780 ± 60 MPa to 3410 ± 247 MPa, and a 1.4 times increase in failure strain from 0.0166 ± 0.0017 to 0.0264 ± 0.0032 were determined with a seven order increase in strain rates from ~ 10-4 s-1 to ~ 103 s-1. The scatter in strength and failure strain measurements indicate large variability in the material, especially for such composites with complex microstructures. Advanced characterization tools, including scanning electron microscopy, high-resolution (scanning) transmission electron microscopy, and two-dimensional x-ray diffraction were used to map out the failure mechanisms activated under the different loading rates, specifically focused on identifying causalities of the texturing and soften/hardening cycling. Globally distributed dislocations and twinning were observed as a consequence of dynamic loading, and extensive cleavage in the titanium aluminide phase, void growth, transgranular cracking, and particle fracture were identified as dominant failure mechanisms activated under high strain rate loading. Crystalline texturing with profound microstructural evolution in the titanium aluminide phase was also found under dynamic loading, and this was correlated with the macroscopic surface texturing observed using high-speed imaging and the cyclical stress relaxation and strain hardening behavior in the stress-strain curves. The crystalline texturing, which manifested globally as the surface texturing behavior, is thought to be the consequence of dynamic recrystallization and grain reorientation, although additional studies are needed. Overall, this thesis presents: 1. A preliminary data set of the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet for material design, manufacturing, and modeling of advanced cermets; and 2. A thorough understanding of the rate-dependency of the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet and provides insights in cermet material micromechanical modeling and improvement.

Binder jet additive manufacturing effectively replaces preform preparation by traditional powder metallurgy methods and allows more complex geometries to be potentially fabricated. The use of a pressure-less melt infiltration technique provides a method of achieving a fully dense composite in a near-net-shape fashion, cost effective and scalable. TiC preforms (15 x 15 x 10/7.5 mm) were fabricated via Binder Jet Additive Manufacturing and then infiltrated by NiAl3 and Ni3Al using a pressure-less melt infiltration technique. Two compositions, Ni3Al and NiAl3, were used as infiltrant materials in order to compare wetting behavior and infiltration kinetics.A stark difference in shape retention between Ni3Al and NiAl3 infiltrated preforms was observed after infiltration. It was found that the TiC particles in the as printed preform were arranged in an interconnected network structure as a result of the binder jet process and/or sintering step and is responsible for maintaining structural integrity of the printed preform. TiC dissolution by liquid Ni3Al was significant enough to disband this network structure and force particle rearrangement and lead to poor shape retention. Conversely, TiC particle rearrangement did not occur during infiltration of the Al-rich NiAl3 alloy and thus the network structure remained intact leading to excellent shape retention of the infiltrated preform.It was found NiAl3 exhibits a complex melting and solidification behavior where an Al-rich phase segregates heavily from the melted material. The presence of an Alrich inter-particle matrix phase indicates the possibility of a “metered” infiltration process where multiple liquid phases of variant compositions infiltrate the “macro” and “micro” capillaries of the TiC preform in a staggered fashion. The composition and time of infiltration remains unknown and a topic of future work. Additionally, Thermodynamic simulation predicts a reaction at the TiC interface by liquid NiAl3 to form Al4C3, with an interfacial reaction product potentially explaining the lack of TiC dissolution and volumetric shrinkage during and after infiltration by NiAl3.

Inexpensive miniaturized gas sensing elements detecting various gases with high sensitivity, good thermal stability, and short response time are in high demand. Compatibility of thin films with the silicon technology is the key towards realizing such sensors. High purity Ni-ZrO2 precursor solutions were synthesized and spin deposited by MOD technique on Si substrate and reduced sintered (700-750C/20-30 mins) achieving required surface resistivity. The results show divalent nickel to be 4 coordinated (presence being ¼ of the organic solvents), forming a planar tetragonal polymeric network. Increase/decrease in solvent/solute ratio has shown network instability, increasing the surface resistivity suggestive of +2 metal coordination. Current envelopes and Resistance Vs Temperature responses were measured in the presence of gases like N2, CH4, and O2 for its applications in gas sensing. Due to the high TCR and higher resistance sensitivity to the environment, these films have shown to be potential candidates for gas sensing applications.