<p>Aluminum alloys have broad applications in aerospace, automotive, and defense industries as structural material due to the low density, high-specific strength, good castability and formability. However, aluminum alloys commonly suffer from problems such as low yield strength, low stiffness, and poor wear and tear resistance, and therefore are restricted to certain advanced industrial applications. To overcome the problems, one promising method is the fabrication of aluminum matrix composites (AMCs) by introducing ceramic reinforcements (fibers, whiskers or particles) in the metal matrix. AMCs typically possess advanced properties than the matrix alloys such as high specific modulus, strength, wear resistance, thermal stability, while remain the low density. Among the AMCs, particulate reinforced aluminum matrix composites (PRAMCs) are advantageous for their isotropic properties, ease of fabrication, and low costs. Particularly, the PRAMCs with in-situ particulate reinforcements have received great interest recent years. The in-situ fabricated particles are synthesized in an aluminum matrix via chemical reactions. They are more stable and finer in size, and have a more uniform distribution in the aluminum matrix and stronger interface bonding with aluminum matrix, compared to the ex-situ particulate reinforcements. As a consequence, the in-situ PRAMCs have superior strength and mechanical properties as advanced engineering materials for a broad range of industrial applications.</p> <p>This dissertation focuses on the investigation of high strength aluminum matrix composites reinforced with in-situ particulates. The first chapter provides a brief introduction for the studied materials in the dissertation, including the background, the scope, the significance and the research questions of the study. The second chapter presents the literature review on the basic knowledge, the fabrication methods, the mechanical properties of in-situ PRAMCs. The strengthening mechanisms and strategies of in-situ PRAMCs are summarized. Besides, the micromechanical simulation is introduced as a complementary methodology for the investigation of the microstructure-properties relationship of the in-situ PRAMCs. The third chapter shows the framework and methodology of this dissertation, including material preparation and material characterization methods, phase diagram method and finite element modelling. </p> <p>In Chapter 4, the microstructures and mechanical properties of in-situ Al₃Ti particulate reinforced A356 composites are investigated. The microstructure and mechanical properties of in-situ 5 vol. % Al₃Ti/A356 composites are studied either taking account of the effects of T6 heat treatment and strontium (Sr) addition or not. Chapter 5 studies the evolution of intermetallic phases in the Al-Si-Ti alloy during solution treatment, based on the work of Chapter 4. The as-cast Al-Si-Ti alloy is solution treated at 540 °C for different periods between 0 to 72 h to understand the evolution of intermetallic phases. In Chapter 6, a three-dimensional (3D) micromechanical simulation is conducted to study the effects of particle size, fraction and distribution on the mechanical behavior of the in-situ Al₃Ti/A356 composite. The mechanical behavior of the in-situ Al₃Ti/A356 composite is studied by three-dimensional (3D) micromechanical simulation with microstructure-based Representative Volume Element (RVE) models. The effects of hot rolling and heat treatment on the microstructure and mechanical properties of an in-situ TiB₂/Al2618 composite with minor Sc addition are investigated in Chapter 7. TiB₂/Al2618 composites ingots were fabricated <i>in-situ</i> via salt-melt reactions and subjected to hot rolling. The microstructure and mechanical properties of the TiB₂/Al2618 composite are investigated by considering the effects of particle volume fraction, hot rolling thickness reduction, and heat treatment. </p>

<p> The automotive industry is using advanced high strength steels (AHSS) to improve the fuel efficiency of passenger vehicles by lightweighting strategy. The higher strength of AHSS allows vehicle manufacturers to implement thinner and lighter components while still meet the safety requirements. Press hardened steels (PHS) exhibit the highest tensile strength among AHSS and are widely used for manufacturing crash relevant automotive parts. Boron-containing steel with enhanced hardenability is the most commonly used grade of steel for press hardening. The addition of a small amount of boron, 0.002 – 0.005 wt.%, can effectively increase hardenability. However, the boron addition also causes problems in commercially production of steel slabs by continuous casting. Defects including transverse corner cracks, surface cracks, and internal halfway cracks are sometimes found in continuously-cast boron steel slabs during or after the final cooling process. These problems can arise during the post-casting cooling process because boron addition changes the phase transformation behavior of steel.</p> <p>The cooling of slabs during and after continuous casting is a multiphysics process including coupled heat transfer, solidification, solid-solid phase transformations, and deformation. Numerical models are helpful for a better understanding of the cooling process and the interaction of different physical phenomena in it. In this work, a 3-D thermomechanical finite volume model (FVM) with coupled heat transfer, stress, and phase transformation calculations is developed to investigate the temperature history, phase evolution, and stress development during cooling.</p> <p>The model is used to simulate the cooling process of continuous cast steel slabs at different post-casting stages. The effect of boron addition on stress development and phase evolution during cooling of a single slab is investigated via simulation of both boron-containing and non-boron steels. The results show the slab with boron consists of mostly bainite, in contrast to the non-boron grade which is mostly ferrite and pearlite after cooling. Higher tensile stresses, both peak and residual, and plastic strains, which could lead to cracking, are observed at the edge of slab in the boron-containing grade. The effect of slowing the cooling rate by using a radiation shield is studied for the boron-containing steel. The reduced thermal gradient and the increased ferrite formation reduce the stresses in the slab. The cooling process of a stack of multiple slabs is also simulated to study the influence of slabs stacking on cooling rate and slab deformation. A slower cooling rate can be achieved in stacked slabs and the compressive load provided by slabs above the slab can prevent large deformation and flatten the slab during cooling. The combination of slab stacking and radiation shield is modeled to study the stress development under a slow cooling rate that is feasible in practice. Boron addition also affects the water quenching process of steel strips on the runout table after hot rolling. Simulations of strips with and without boron show different cooling curves, residual stress and phase distributions as austenite decomposition does not occur for boron-containing steel due to the fast cooling rate. Therefore, the cooling strategy on the runout table should be adjusted accordingly to control the coiling temperature and improve strip quality.</p>

<p>  </p> <p>Stress corrosion cracking (SCC) is a longstanding critical materials challenge in austenitic stainless steels (AuSS). Recently, there has been mounting concern regarding the potential for Chloride-induced stress corrosion cracking (CISCC) along arc weld seams on austenitic stainless-steel canisters used as spent nuclear fuel (SNF) dry storage containers, due to the residual stress from the welding process and exposure to chloride-rich coastal air at storage sites. To ensure the safety of the SNF storage, fundamental understanding and mitigation methods of CISCC are critical in both engineering design and maintenance of the storage canisters before and after their deployment. With the recent development of high-resolution characterization and analysis techniques, a more robust and comprehensive understanding of the fundamental TGCISCC mechanism starts to be more accessible. In this thesis, comprehensive state-of-the-art techniques, including SEM, EBSD, HREBSD, FIB, ATEM, TKD, potential dynamic measurement, XRD, and nanoindentation will be used to further understand the mechanistic mechanism of TGCISCC in AuSS from macroscopic scale down to atomistic scale. </p>

<p>  </p> <p>Aluminum (Al) possesses a plenitude of remarkable properties, such as strong corrosion resistance, high thermal and electrical conductivity, and high specific strength. However, Al and its alloys are still remarkably weaker than most high strength steels and susceptible to drastic softening at high temperatures, preventing many applications where its low density would be beneficial. Severe plastic deformation can yield ultra-fine grained Al alloys with similar strengths as steels, although they are highly unstable even at room temperature. Nanotwinned (NT) metals have demonstrated concomitant strength and ductility, enabled by twin boundaries which simultaneously act to inhibit dislocation motion and generate partial dislocations that aid in plasticity. In spite of having a high stacking fault energy, nanotwins have been introduced into Al alloys using transition metal solutes during magnetron sputtering. This thesis aims to explore the impact Zr has on the microstructure, deformation, and thermal stability of nanotwins in NT Al.</p> <p>Our studies identify how Zirconium (Zr) aids in the formation of a significant volume fraction of 9R phase and an abundance of finely spaced incoherent twin boundaries, leading to a maximum hardness of 4.2GPa. They further uncover through <em>in-situ</em> micropillar compression that NT Al-Zr alloys are highly deformable and reach a flow stress of ~1.1GPa. Constant strain rate nanoindentation tests demonstrate the enhanced strain rate sensitivity in NT Al-Zr alloys. Zr is also identified to be a remarkable thermal stabilizer when incorporated into NT Al-Co alloys, with no apparent softening up to 450 °C (0.78 T­m). The influence of substrate texture on nanotwinned Al-Zr alloys microstructure was also thoroughly explored.</p>

<p><a>Low reflow temperature solder interconnect technology based on Sn-Bi alloys is currently being considered as an alternative for Sn-Ag-Cu solder alloys to form solder interconnects at significantly lower melting temperatures than required for Sn-Ag-Cu alloys. </a></p> <p>A new low temperature interconnect technology based on Sn-Bi alloys is being considered for attaching Sn-Ag-Cu (SAC) solder BGAs to circuit boards at temperatures significantly lower than for homogeneous SAC joints. Microstructure development studies of reflow and annealing, including Bi diffusion and precipitation, are important in understanding mechanical reliability and failures paths in the resulting heterogeneous joints. Experiments in several SAC-SnBi geometries revealed that Bi concentration profiles deviate from local equilibrium expected from the phase diagram, with much higher local concentrations and lower volume fractions of liquid than expected during short-time high temperature anneals in the two-phase region. As annealing time increased and Sn grain coarsening occurred, the compositions and fractions revert to the phase diagram, suggesting an “anti-Scheil” effect. A Bi interface segregation model based on Bi segregation at Sn grain boundaries was developed to explain the Bi distribution characteristics in Sn during two-phase annealing process. </p> <p>Besides hybrid joints, microstructural evolution after reflow and aging, especially of intermetallic compound (IMC) growth at solder/pad surface finish interfaces in homogeneous SnBi LTS joints, is also important to understanding fatigue life and crack paths in the solder joints. This study describes intermetallic growth in homogeneous solder joints of Sn-Bi eutectic alloy and Sn-Bi-Ag alloys formed with electroless nickel-immersion gold (ENIG) and Cu-organic surface protection (Cu-OSP) surface finishes. Experimental observations revealed that, during solid state annealing following reflow, the 50nm Au from the ENIG surface finish catalyzed rapid (Au,Ni)Sn₄ intermetallic growth at the Ni-solder interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to significant solder joint embrittlement during fatigue testing. Further study found that the growth rate of (Au,Ni)Sn₄ intermetallic could be reduced by In and Sb alloying of SnBi solders and is totally eliminated with Cu addition. Fatigue testing revealed Au embrittlement is always present in solder joints without Cu, even with In and Sb additions due to (Au,Ni)Sn₄ formation. The fatigue reliability of Cu-containing alloys is better on ENIG due to the formation of (Ni,Cu,Au)₆Sn₅ at the solder-surface finish interface instead of (Au,Ni)Sn₄.</p> <p>With the development of SnBi LTSs, a new generation alloy called HRL1 stands out for its outstanding reliability during thermal cycling and drop shock testing. This study focused on microstructure evolution in SnBi eutectic, SnBiAg eutectic and HRL1 solders (MacDermid Alpha) homogeneous joints and hybrid joints with SAC305 formed with ENIG and Cu-OSP surface finishes. Experimental results revealed that with more microalloying elements, HRL1 has significantly refined microstructure and slower Sn grain growth rate during solid-state aging compared with SnBi and SnBiAg eutectic alloys. Intermetallic compound growth study showed that during solid state annealing following reflow, the (50nm) Au from the ENIG finish catalyzed rapid (Au,Ni)Sn₄ intermetallic growth at the Ni-solder interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to significant solder joint embrittlement during creep and fatigue loading. However, (Au,Ni)Sn₄ growth and gold embrittlement was completely eliminated for HRL1 due to Cu additions in it, and HRL1 has significantly better fatigue reliability than SnBi and SnBiAg eutectic alloys on both OSP and ENIG surface finishes.</p>

<p>Ni-base superalloys are among the highest temperature capable alloys and are used pervasively throughout the transportation, energy, and nuclear industries. However, their microstructures have been largely limited to containing the γ´ (cubic) and γ´´ (tetragonal) phases to enable high strength at elevated temperatures, and this fixation has restricted alloy development opportunities. In the past three decades, a new set of alloys, strengthened by the γ´´´ (orthorhombic) phase, was developed by Haynes International. The alloys exhibit comparable strength to existing Ni-based superalloys and show a 25% decrease in the thermal expansion coefficient, designed for tighter clearances (thus improving engine efficiency) and help to reduce thermally induced fatigue from engine cycling. </p> <p>The newest iteration of such alloys, HAYNES^® 244^®, has a nominal composition of Ni-22.5Mo-8Cr-6W (wt.%), and each alloying element is used to help precipitate the γ´´´-Ni₂(Cr, Mo, W) phase. The deformation mechanisms of this material are currently unknown. Previous studies investigating the predecessor alloy, HAYNES^® 242^® alloy, showed deformation twinning to be the dominant deformation mechanism during mechanical testing, but the physical phenomena responsible for this mode of deformation were not clearly elucidated. As a result, the primary motivation of this project is to understand the deformation behavior of the 244 alloy from the atomistic level and upwards. </p> <p>This work details efforts to elucidate these deformation mechanisms using an integrated computational and experimental approach. First-principles calculations were performed to determine the entire generalized stacking fault energy (GSFE) surface and slip pathways of the γ´´´ phase for dislocation slip. The various planar defects that could form from dislocation slip were predicted to provide significant barriers for dislocation motion due to their very high planar defect energies (~1000 mJ/m²), likely precluding shearing of the precipitates. We incorporated these results into phase field dislocation dynamics (PFDD) to simulate dislocation-precipitate interactions of finite size. These results showed that the planar defect energies of the γ´´´ phase largely govern the deformation behavior and critical resolved shear stress for precipitate shearing, regardless of precipitate shape, size, or orientation. Extensive mechanical testing conducted from room temperature up to 760 ºC over strain rates ranging from 10^-9 s^-1 to 10^-4 s^-1 combined with transmission electron microscopy validated the predicted deformation structures of creep and tensile samples. Shearing of individual precipitates by intrinsic and extrinsic stacking faults, as well as extensive deforming twinning, was observed. The integrated GSFE and PFDD simulations showed that the precipitates would resist dislocation shearing and favor twinning as the preferred deformation mechanism at all temperatures and strain rates investigated. These results provide pathways for microstructural and composition modification to further increase the strength of γ´´´ strengthened alloys in the future.</p> <p><br></p>

<p>  </p> <p>Betavoltaics are self-contained  radioisotope power sources where radioisotopes irradiate a semiconductor and  generate electricity similar to a photovoltaic cell. Betavoltaics differ from  other power sources as it is ideal for long-lasting (>20 years), low,  continuous power applications where battery replacement is not feasible. Ideal  functions for betavoltaics include sensors in hard to reach places such as  underwater and deep space applications, as well as cardiac pacemakers where  power source replacement is undesirable or impossible. However, betavoltaics  are limited in application by its power output since it only produces power in  the nanowatt range. Betavoltaic performance can be improved by two methods:  Increasing the amount of activity of the radioisotope or increasing the  performance of the semiconductor. Currently, commercial betavoltaics utilize a  titanium tritide film to irradiate a gallium arsenide semiconductor. The  objective of this dissertation is to identify a tritium storage medium that  can produce more power in the betavoltaic than the currently used titanium  tritide. This was done in three steps: First, metal film options were  simulated in MCNP to evaluate tritium substrate self-shielding, semiconductor  beta irradiation and determine ideal thicknesses. Second, metal film options  at ideal thicknesses were manufactured and evaluated during the hydrogen  loading process to determine the viability of materials fully absorbing  hydrogen. Lastly, the loading kinetics would be evaluated to further investigate  hydride/tritide formation in the storage medium if full loading is not  realized to determine the ideal thickness required, or if other factors during  the loading process need to be considered.</p> <p>Metallic films were evaluated to  maximize tritium packing and optimized for minimizing self-shielding to  improve performance for betavoltaic cells beyond the titanium tritide films  currently used. Ideal, fully loaded tritium metallic films, such as lithium,  aluminum, titanium, magnesium and palladium tritides, were simulated in MCNP6  (Monte Carlo N-Particle 6) to evaluate power deposition into a gallium  arsenide semiconductor by varying the thickness of the films. Lithium was  identified as the best storage option with an optimal thickness of 4 μm and a  theoretical betavoltaic current output of 644 nA for a gallium arsenide  semiconductor, tripling the current output emitted by an ideal titanium-loaded  film. </p> <p>The viability of lithium and  aluminum film loading were evaluated in the hydrogen loading system while comparing  to titanium as a benchmark. Unlike titanium and aluminum films where films  were in a solid state through the loading process, lithium has to be melted  into a liquid state to be loaded. The uptake of hydrogen by the films was  determined by Sievert's method, where the pressure drop recorded by the  Hydrogen Loading System was the measured pressure of hydrogen absorbed by the  film. All film loadings showed a pressure drop that corresponded to the  expected pressure drop from loading. The films were characterized after  loading to confirm hydrogen absorption and formation of hydride. Both lithium  and titanium demonstrated hydride formation while the aluminum did not.</p> <p>The pressure drops during loading  were compared to the Mintz-Bloch model. For some loadings in all materials,  there was good correlation between experimental loadings and Mintz-Bloch  models, primarily due to the hydride formation happening quickly. Differences  can be explained from the speed of the hydride reaction and thermal  decomposition of the hydride during loading. The Mintz-Bloch model further  confirmed that the aluminum did not form a hydride during loading.</p> <p>Lithium was demonstrated to be a  viable hydrogen loading substrate. The film was characterized to be lithium  hydride after hydrogen loading and its loading kinetics matched very well with  the Mintz-Bloch model. Aluminum was demonstrated to not be viable as a  hydrogen loading substrate as it requires significantly higher pressures,  beyond the allowed limits for tritium handling, to form a hydride and  permanently hold when exposed to atmosphere.</p>

<p> This thesis demonstrates the combination of virtual reality (VR) and artificial intelligence (AI) specifically exploring the practical application of Natural Language Processing (NLP) and GPT-based models in educational VR laboratories. The objective is to design a comprehensive learning environment where users can independently engage in laboratory experiments, deriving similar educational outcomes as they would from a traditional, physical laboratory setup, particularly within the realms of Science, Technology, Engineering, and Mathematics (STEM) disciplines.</p> <p>Using machine learning techniques and authentic virtual reality simulating educational experiments, we propose an advanced learning platform—Virtual Reality Instructional Laboratory Environment (VRILE). A key feature of the VRILE is an AI-powered instructor capable of not only guiding the learners through the tasks but also responding intelligently to their actions in real time.</p> <p>The AI constituent of the VRILE uses the GPT-2 model for text generation in the field of Natural Language Processing (NLP). To ensure the generated instructions were contextually relevant and meaningful to lab participants, the model was trained on a dataset derived from an augmentation over user interactions within the VR environment.</p> <p>By pushing the boundaries of how AI can be utilized in educational VR environments, this research paves the way for broader adoption across other domains of engineering education. Furthermore, it provides a solid foundation for future research in this interdisciplinary field. It marks a significant stride in the integration of technology and education, encouraging more ventures into this promising frontier.</p>

<p>An investigation of the influence of three alloying elements Chromium, Phosphorus, and Nitrogen with the solute types of oversized substitutional, undersized substitutional, and interstitial on the irradiation induced microstructural evolution and hardening</p>

<p dir="ltr">The process of metal additive manufacturing is characterized by the layer-by-layer construction of components, where each individual layer may be subjected to distinct thermal variations, resulting in differences in cooling rates and thermal gradients. These variations can impact the microstructure and, subsequently, mechanical properties of the final product, especially as the height of the build increases. In the present investigation, an evaluation was undertaken to ascertain the impact of build height on the structural integrity of Ti-6Al-4V samples produced using the laser powder bed fusion (LPBF) technique. The study encompassed a comprehensive examination of microstructural features, the microhardness measurement, as well as an evaluation of defect characteristics including size, location, and distribution, with respect to the build height. Tensile and fatigue tests were conducted to elucidate the potential dependence of fatigue and tensile failures on the build height. Two groups of specimens were fabricated: the first, underwent continuous fabrication, while the second involved a pause at the half height, with the process resuming after a 24-hour interval. The results of this investigation unveiled a discernible influence of the height of the build on the structural integrity of components under cyclic loading. Most fatigue specimens were observed to exhibit failure in the upper portion of the gage section with respect to the build direction. Analyses of microstructure revealed a consistent grain morphology in alignment with the build direction, and a uniform distribution of hardness throughout the build height was noted. However, for the specimens in the first group, more process-induced defects were detected within the top half of the gage section in comparison to the bottom half, while there was no noticeable difference in the distribution of defects in the second group. The results suggest that in LPBF process, as the build height is increased, there is a higher likelihood of process-induced defect formation, ultimately resulting in a reduction in structural integrity at greater build heights.</p>