Interest in titanium for mass-scale applications has driven an exploration for rapid, cost-effective consolidation of titanium powders. The effects of electric current and pressure on binary diffusion in beta phase titanium niobium are studied to enhance understanding of Spark Plasma Sintering (SPS), an advanced powder consolidation technology. Binary diffusion couples were annealed in the SPS system, as well as a custom-fabricated load-free furnace, for one hour to elucidate the influences of pressure and current at 1000, 1100, and 1150 C. The results show Ti-Nb interdiffusion coefficient dependence on composition, temperature, current, and pressure. Compared to published results, the activation energy for low concentration Nb, 10-22 at%, has shown to be reduced in the SPS by an average 38kJ/mol at 15MPa and 70kJ/mol at 80MPa. The effect on activation energy of direct current without pressure, at a similar current density as the SPS, shows an average decrease of 105kJ/mol. The possible mechanisms for these changes are discussed, and concepts for subsequent studies are provided.

Magnesium (Mg) is of interest as a lightweight structural material, given its potential for energy-efficient and eco-friendly applications. However, Mg and its alloys have a hexagonal close packed structure that is characterized by an inherent plastic anisotropy which leads to poor formability and low strength, thereby limiting their use in many important engineering applications. Deformation twinning, an important plastic deformation mode in Mg, critically influences the strength and ductility of Mg. Consequently, a fundamental understanding of twin behavior under different stress and strain conditions is critical to optimize the performance of Mg and its alloys and thereby broaden their applicability in engineering systems.

In this dissertation research, experimental studies, using electron backscatter diffraction (EBSD) and scanning transmission electron microscopy, were used in combination with atomistic simulations using molecular dynamics and the nudged elastic band method to provide insight into twin behavior and investigate how the stress and strain fields modulate deformation twinning in Mg. The results from this dissertation research established multiple correlations between grain-scale microstructural characteristics and twin behavior by collecting and analyzing twin evolution information from large EBSD datasets and established the preferred conditions for twin nucleation, propagation, and growth. We observed room temperature, deformation-induced solute segregation in a Mg-Y alloy at faceted {10-12} twin boundaries. The segregated Y atoms exert a pinning effect and lead to anisotropy on the mobility of twin boundaries. We calculated the stress-strain field modulated energy barrier for coherent twin boundary migration and found that there is a power-law relationship between the rate of CTB migration and the ratio between the twin resolved shear stress and the critical resolved shear stress for CTB migration. We confirmed the presence of a pure-shuffle twin nucleation and early-stage growth mechanism at disconnection-dense sites between stacking faults in a random defect network under deformation. The twin variant selection was found to be correlated with the line direction of the disconnection at the nucleation site. These key findings deepen our understanding of the fundamental mechanisms that govern twin behavior and provide alloy design pathways to control the mechanical response of Mg alloys by engineering the microstructure of deformation twins.

Laser Directed Energy Deposition (L-DED) Additive Manufacturing (AM) offers unprecedented flexibility in direct fabrication of metallic components in a way that can be readily integrated with existing CNC subtractive machining technologies. The core building block of the technology is the melt pool, the dynamic bead of molten material established by the energy equilibrium between incident laser energy and thermal dissipation. While the unique solidification microstructure of the melt pool has attracted intense scrutiny, the mechanisms determining how mass is originally incorporated into the melt pool have been less well studied. In this work, three new tools are applied to the task of broadening the understanding mass capture behaviors. First, over long time scales it was observed that mass capture efficiency evolves over the course of depositing many layers as machine conditions change; a non-empirical model constructed to track this revealed self-stabilizing behavior in working distance in open-loop control systems. Second, on very short time scales, high speed videography was employed to understand what happens at the moment of impact between a feedstock powder particle and the melt pool. It was revealed that particles are captured by surface tension before fully melting. Third, this particle retention time was investigated with numeric simulation to highlight its relationship to particle size, impact velocity, thermal distributions and wettability.

As global transportation, space exploration, and energy consumption demand grows, safer and more efficient turbine engines, rocket engines, and power generators are needed. To meet the rapidly growing demand for transportation and energy consumption worldwide, more efficient engines and generators necessitate higher operating temperatures which in turn requires alloys with retained strength levels at ever increasing temperatures. In the past decade, refractory complex concentrated alloys (RCCAs) have gained prominence through numerous reports of superior strengths at higher homologous temperatures than conventional refractory and super alloys. However, these RCCAs, comprised of transition metals from subgroups IV, V, and VI, tend to be brittle at room temperature, hindering their broad applicability. Recent findings reveal that interstitial impurities may significantly contribute to, and convolute observations of, the ductility and strength of RCCAs at room temperature. The studies presented in this dissertation investigate the role of interstitial impurities such as O and N in RCCAs and provide an understanding and insight into their role in the design and processing of RCCAs. In the first investigation, a literature review examines and discusses the field’s current understanding of the role of interstitial impurities, in the microstructure and mechanical behavior of RCCAs. Moreover, context is provided from the binary interactions of interstitial impurities with refractory metals and their contribution to developing and processing conventional refractory alloys as a framework to gain insight into interstitial impurity mechanisms in RCCAs. With the understanding of interstitial impurity interactions with RCCA constituents, more holistic approaches to the design of RCCAs are suggested to engineer the mechanisms of intended and unintended interstitial impurities through alloy design and processing. The second investigation quantified the origins of interstitial impurities from the feedstocks and residual gases during plasma arc melting (PAM) in the MoNbTaW RCCA. In the PAM synthesized MoNbTaW RCCAs, the thermodynamic drive for grain boundary segregation and oxide formation was characterized using atom probe tomography (APT) and analyzed using density functional theory (DFT) calculations. With this understanding, alloy design and processing techniques necessary to control interstitial impurities in RCCAs were then described. In the third study, the role of O in the NbTaTiHf RCCA, a previously developed RCCA with room temperature ductility, was investigated. The role of O in the microstructural evolution of the NbTaTiHf alloy and the alloy’s sensitivity to O were analyzed and an additional design criterion for future RCCA design was provided. In the fourth and fifth studies, advanced mechanical properties, specifically tensile creep and fracture toughness, of the NbTaTiHf RCCA were investigated, providing further insight to the design and processing of ductile RCCAs. Together, these studies represent systematic and meticulous approaches to investigate how interstitial impurities can influence the design and processing of RCCAs. From this insight, criteria and techniques for designing and processing RCCAs with high strength and ductility could be recommended.