Case hardening distortions are a major problem for gear manufacturers. The aim of the current work is to create a simulation model, able to predict how and when case hardening distortions arise. The results presented in this thesis form a basis for such a model. Two case hardening steels, with base carbon contents of 0.20 and 0.21 % C were studied using dilatometer experiments. One of them was carburized to 0.36, 0.52 and 0.65 % C in order to investigate the influence of carbon content. Experiments were performed during both isothermal and continuous heating and cooling conditions. The results were used to evaluate phase transformations, heat expansion behaviors and phase transformation strains. The expansion behavior of the material was modeled as a function of temperature, carbon content and phase fractions. The phase transformations to martensite and bainite were modeled, using the Koistinen-Marburger equation and a transformation rate equation based on Austin-Rickett kinetics, respectively. Experiments were simulated using the COMSOL Multiphysics software, to verify the model with respect to martensite and bainite transformations, heat expansion behavior and phase transformation strains.

Heat treatment for the purpose of material strengthening is accompanied by residual stresses and distortion. During these processing steps, steel alloys experience a phase change that in turn modify their overall mechanical response. To properly account for the cumulative composite behavior, the mechanical response, transformation kinetics and subsequent interaction of each phase have to be properly accounted for. Of interest to material designers and fabricators is modeling and simulating the evolutionary process a part undergoes for the sake of capturing the observable residual stress states and geometric distortion accumulated after processing. In an attempt to capture the aforementioned physical phenomena, this investigation is premised upon a consistent thermodynamic framework. Following this, the single phase Evolving Microstructural Model of Inelasticity state variable model is extended to accommodate the occurrence of multiphases, affirming that the interaction between coexisting phases is through an interfacial stress. Since the efficacy of a multiphase model is dependent on its ability to capture the behavior of constituents phases and their subsequent interaction, we introduce a physically based self-consistent strain partitioning algorithm. With synthesis of the aforementioned ideas, the additional transformation induced plasticity is numerically accounted for by modifying each phase’s flowrule to accommodate an interfacial stress. In addition, for simulating the cohabitation of two phases, the mechanical multiphase model equations is coupled with a previously developed non-diffusional phase transformation kinetics model. A qualitative assessment of the material response based on a Taylor, Sachs and self-consistent polycrystalline approximation is carried out. Further analysis of the multiphase model and its interaction with transformation kinetics is evaluated.

No description available.

The use of aluminiferous coatings profoundly improves the service life of superalloys but leads to the microstructural degradation of superalloys and thus the loss of mechanical properties. To solve this trade-off, two strategies were employed in this research. At first, we modified MCrAlY coatings by inducing Ta to reduce the interdiffusion effect on substrate alloys. This strategy was verified by 2000 h/1100 °C oxidation tests in two Ta-containing MCrAlY-IN792 systems. The system with 3.3 wt.% Ta MCrAlY displays an outstanding resistance to γ′ depletion in the substrate and comparable oxidation property in comparison with a reference system of Ta-free MCrAlY-IN792. Increasing Ta to 7.4 wt.% results in reduced oxidation resistance. Thermodynamic simulations revealed the phase-transformation mechanism induced by initial interdiffusion, uncovering the cause of γ′ depletion in the substrate and the mechanism behind improving resistance to γ′ depletion by Ta addition. In addition, we developed novel Y-doped AlCoCrFeNi high-entropy alloys by tuning Al/Cr ratio ACR. After a long-term isothermal test in air at 1100 °C up to 1000 h, the higher ACR alloy displayed a stronger oxidation resistance at the early oxidation stage, whereas a contrary result could be detected in the later stage. The microstructural analysis confirmed that the fast growth of spinel dominated the early oxidation process, leading to higher oxidation rate of the lower ACR alloys. The later stage was governed by the growth of Al2O3. Lager size Al2O3 gain formed on the lower ACR alloy impeded the inward diffusion of oxygen and thus reduced the oxidation rate, which was further verified by our thermodynamic calculations.

No description available.

Shape memory ceramics (SMCs) are burgeoning functional materials based on zirconia with a reversible, stress-inducible martensitic phase transformation. Compared to metallic shape memory alloys, SMCs have broader operating temperatures, higher critical stresses, and larger mechanical hysteresis loops. These advantages make SMCs attractive for high-output actuation and sensing in extreme environments or energy dissipation applications; however, the key phase transformation generates large stresses and strains that accumulate at grain boundaries and result in fracture of monolithic SMCs. This means that material forms with decreased mechanical constraint are necessary. Transformation without fracture has been previously demonstrated with SMC micropillars and individual microparticles, but these material forms lack useful applications. By utilizing easily scalable granular packings of discrete free particles, the transformation can be triggered in bulk without fracture in much the same way. The granular packing material form introduces significant complexity as the internal stress distributions responsible for the phase transformation are highly heterogeneous on the macro-, meso-, and micro-scales. Moreover, the unconstrained phase transformation behaves differently than the constrained transformation, which is more studied in zirconia. The interactions of these various factors are explored from a fundamental perspective in this work, notably including (1) a unique 'continuous mode' of both forward and reverse transformation in granular packings, (2) the dependence of transformation behavior on macro-, meso-, and microstructure, and (3) the evolution of the granular packings' structure and energy dissipation capacity over 10,000 loading cycles. Diverse experimental techniques are employed, ranging from mechanical testing and calorimetry to in situ neutron diffraction, to support theory based on the martensitic phase transformation in zirconia, the shape memory and superelastic effects, and granular material physics. / Doctor of Philosophy / Shape memory materials are capable of remembering their original shape even when they are deformed, and can return to that shape when they are heated. This unique property stems from a phenomenon called martensitic phase transformation which bridges the gap between microscopic structural changes and macroscopic shape changes as a response to specific environmental changes. Most of the common shape memory materials are metallic, like nitinol (NiTi), which has uses in orthodontic wires and cardiological stents, but there are also ceramic materials that can display the shape memory effect. These shape memory ceramics are based on zirconia (ZrO2), and are distinct from metallic shape memory materials because of their brittle behavior and high temperature stability owing to their chemical structure. The work presented in this thesis concerns the behavior of shape memory ceramics in granular form (i.e., loose powders) over a range of external conditions. Diverse experimental techniques are employed to investigate differences between granular and non-granular shape memory ceramics. This work shows how the unique structure of a granular material, which is dominated by highly uneven force distributions and microscopic effects, interacts with the martensitic phase transformation in shape memory ceramics to produce a 'continuous' mode of transformation that differs from non-granular shape memory materials. This continuous mode is itself dependent on the granular material's macro-, meso-, and micro-structure, and on the shape memory material's composition and history. In the future, shape memory ceramics might leverage the insights gained from this work for applications including energy dissipation or on-demand shape changes (i.e., actuation).

The growing need for high-performance electrode materials in electrochemical conversion and storage applications requires further fundamental investigation on the working and degradation mechanisms of these materials. Among various functional materials, transition metal oxides are still one of the main choices due to their tunable chemical compositions and diverse crystal structures in most aqueous and organic electrolytes. The charge transfer process mainly occurs at the electrode-electrolyte interface, and controlling the electrochemical interfacial stability represents a key challenge in developing sustainable and cost-effective electrochromic materials. The present thesis focuses on classical tungsten trioxide (WO3) materials as the platform to uncover the previously unknown interrelationship between phase transformation, morphological evolution, nanoscale color heterogeneity, and performance degradation in these materials during 3,000 cyclic voltammetry cycles. Through the application of novel cell design, synchrotron/electron spectroscopic, and imaging analyses, we observe that the interface between the WO3 electrode and 0.5 M sulfuric acid electrolyte undergoes constant changes due to the tungsten oxide dissolution and redeposition. The redeposition of dissolved tungsten species provokes in situ crystal growth, which ultimately leads to phase transformation from the semicrystalline WO3 to a nanoflake-shaped, proton-trapped tungsten trioxide dihydrate (HxWO3·2H2O). The multidimensional (surface and bulk) quantification of the electronic structure with X-ray measurements reveals that the tungsten reduction caused by proton trapping is heterogeneous at the nanometric scale and is responsible for the nanoscale color heterogeneity. The Coulombic efficiency, optical modulation, apparent diffusion coefficients, and switching kinetics are gradually diminished during 3,000 cyclic voltammetry cycles, resulting from the structural and chemical changes of the WO3 electrode. We hypothesize that the high interfacial reactivity in the electrode-electrolyte interfacial region could be the universal underlying mechanism leading to undesired bulk structural changes of inorganic electrochromic materials. / M.S. / With the rapid development of human society, the research of new energy-saving materials has become a focus of attention. Among them, electrochromic devices can effectively adjust their color through a controllable electrochemical reaction and have a wide range of uses in our daily life. For example, smart windows can reduce glare and heat without blocking the natural light, thereby providing buildings and vehicles with better thermal and visual comfort. Electrochromic optical displays can lower energy consumption. Variable reflectance mirrors such as anti-glare car rear-view mirrors can ensure the safety of driving. Lastly, wearable apparel such as electrochromic lenses for spectacles and sunglasses can protect users from ultraviolet radiation. Although electrochromic materials and devices have not expanded from the niche market, the enormous potential that they hold cannot be ignored and wide-scale commercialization should be sought after. Tungsten oxides electrochromic devices have proved to utilize the full spectrum of the incident light through structure design. These devices can also be configured with solar cells as a state-of-art integrated self-powered system with satisfactory optical modulation that can be obtained without any external electrical energy input. Moreover, WO3-based devices have also been combined with electrodeposition technology to achieve fast color-switching kinetics. However, the long-term durability in the acidic electrolyte under electrochemical cycling conditions needs to be further improved, and the road of full commercialization is still unpaved. To design high-performance electrochromic materials, it is imperative to study the degradation mechanism under long-term electrochemical cycling conditions. In the present thesis, the performance degradation of the WO3 electrode in acid electrolytes involves chemical changes. Through a better understanding of the fundamental degradation process, the design of high- performance electrochromic metal oxides can be developed.

This dissertation presents an assortment of research aimed at understanding the composition-dependence of deformation behavior and the response to thermomechanical processing, to enable efficient design and processing of low stacking fault energy (SFE) high entropy alloy (HEAs). The deformation behavior and SFE of four low SFE HEAs were predicted and experimentally verified using electron microscopy and in-situ neutron diffraction. A new approach of employing a minimization function to refine and improve the accuracy of a semi-empirically derived expression relating composition with SFE is demonstrated. Ultimately, by employing the minimization function, the average difference between experimental and predicted SFE was found to be 2.64 mJ m-2. Benchmarking with currently available approaches suggests that integrating minimization functions can substantially improve prediction accuracy and promote efficient HEA design with expansion of databases. Additionally, in-situ neutron diffraction was used to present the first in-situ measurement of the interspacing between stacking faults (SFs) which were correlated with work hardening behavior. Electron transparent specimens (< ~100 nm thick) were used in order to resolve nanoscale planar faults instead of the thicker sub-sized specimens (on the order of millimeters in thickness) which exhibit the classical stages III work hardening behavior characteristic of low SFE metals and alloys. The present study demonstrates these characteristic dimensions of SFs can be tracked in real-time using neutrons or high-energy x-rays. SFs have also been shown to act as barriers to dislocation motion and thus contribute to strengthening and sustained work hardening during deformation.

Metastable β titanium alloys stand out as prominent candidates for structure materials in aerospace industries due to their light weight and exceptional high strength. This dissertation systematically investigates the microstructural evolutions in the metastable β Ti-5Al-5Mo-5V-3Cr (wt. %, Ti5553) alloy induced by various defects including grain boundary, twin boundary, and dual-phase interface using advanced characterization techniques such as transmission Kikuchi diffraction (TKD), 3D FIB-SEM tomography, and 4D STEM. Firstly, the morphology of grain boundary α precipitates was characterized using quantitative 3D FIB-SEM tomography combined with 3D phase field simulation. Our findings highlighted the critical role of the inclination angle between habit plane of α and grain boundary plane in determining the morphology of grain boundary α precipitates. Secondly, the nanoscale substructures of a novel high-indexed {10 9 3} twin and its influence on the formation of hierarchical α microstructure were studied, employing conventional TEM and aberration-corrected STEM. Thirdly, the early stage α nucleation in Ti-5553 was studied utilizing interrupted heat treatments and ex-situ characterizations via TEM and aberrationcorrected STEM. Our findings indicated that the preformed β/ω interface can act as nucleation sites for α precipitates. Lastly, the microstructure and defects in the direct energy deposited (DEDed) Ti-5553 alloy were investigated. The results demonstrate that the addition of stainless steel 316L can significantly refine the grain size while also introducing different defects.

The direct chill (DC) casting technique to produce billets for extrusion and ingots for rollingwas developed in the 1930s. The principle, which is still valid, is a two-stage cooling with a primary cooling at a mould surface followed by water spraying directly on the surface. Improvements of this technique have mainly focused on changes to the primary cooling, where a water-cooled metal mould has been replaced by different techniques to minimize cooling at this stage. The drive for development comes from the extrusion industry, which can increase the productivity and quality of extruded profiles by improving the billet surface appearance and structure. Hot top casting supported by airflow against the casting surface during the primary cooling is currently the standard procedure to achieve acceptable billet surfaces. The goal is to minimize the depth of the surface segregation zone, which is the governing factor for the appearance of different phases in the surface region. Billet surface quality is evaluated by quantifying surface appearance, segregation zone thickness, and  occurrence of large Mg2Si and β-particles near the surface. The β-Al5FeSi intermetallic phase and coarse Mg2Si particles have negative effects on extrudability and workability of 6xxx Al alloys billets. To achieve extruded products with a high surface quality the as-cast billets are  heat-treated before extrusion. During heat treatment the undesired intermetallic particles, i.e., β-AlFeSi platelets are transformed to rounded α-Al(FeMn)Si intermetallic phases. In this  research the formation of the surface segregation for smooth defect-free surfaces in both as-cast and homogenized billets was studied. In addition, the surfaces with defects such as wavy, spot and vertical drag defects were investigated and possible mechanisms for initiation of those defects were explained. Moreover, for a better understanding of the homogenization process in-situ studies of the heat treatment of 6082, 6005, 6060 and 6063 Al alloys were carried out by using a transmission electron microscope (TEM). Based on the observations, an explanation of the probable mechanisms taking place during transformation from β-to α-phase was presented. / <p>Vid tidpunkten för disputationen var följande delarbeten opublicerade: delarbete 5 manuskript, delarbete 6 inskickat och delarbete 7 inskickat.</p><p>At the time of the doctoral defence the following papers were unpublished: paper 5 manuscript, paper 6 submitted, paper 7 submitted.</p>