This thesis highlights the role of phase interfaces on phasetransformations in metallic materials. The deviation from localequilibrium at the moving phase interface has been analysed interms of solute drag theory and finite interface mobility. Inparticular the planar growth of proeutectoid ferrite fromaustenite in steel has been studied. The deviation from localequilibrium is caused by dissipation of Gibbs energy bydiffusion inside the phase interface and interface friction. Inthe analysis the interface is divided into three zones and thethermodynamic and kinetic properties are assumed to varycontinuously across the interface. A new model suitable formulticomponent alloys is developed. The model reduces to thefamiliar solute drag model by Cahn under simplifyingconditions. It was demonstrated how the interface model couldbe combined with a method for calculating the volume diffusionin both the growing and parent phases. With this combination ofprocedures the changes in local conditions at the interface, asthe growth rate changes due to long-range diffusion, could bedemonstrated for the case of continuous cooling in an Fe-Nialloy. The critical limit for massive transformation in the Fe-Niand Fe-C systems was calculated and found to lie well below theT0 line for both systems. The calculated limit for Fe-Ni wascompared with a recent experimental study and reasonableagreement was found. For the Fe-C system the limit calculatedwith the present model was compared with a phase-field model.The two approaches showed qualitatively the same behaviour andthe quantitative difference was due to different assumptions onhow properties vary across the interface. Finally, an attempt to simulate the partitionless growth offerrite in austenite in the Fe-Ni-C system was performed. Inthe applied model the dissipation of Gibbs energy inside theinterface and in the nickel spike ahead of the migratinginterface were accounted for. The long-range diffusion ofcarbon in austenite was treated with an approximate analyticalgrowth equation. A continuous change from paraequilibriumconditions and quasi-paraconditions could be shown in anisothermal section of the Fe-Ni-C phase diagram. Partitionlessgrowth starts in a parabolic fashion but slows down. For alloysoutside the limit for quasiparaconditions partitionless growthis predicted to stop abruptly while for alloys inside thatlimit growth approaches a second parabolic growth law. However,the latter case should not be expected in practise because ofimpingement effects.

<p>This thesis highlights the role of phase interfaces on phasetransformations in metallic materials. The deviation from localequilibrium at the moving phase interface has been analysed interms of solute drag theory and finite interface mobility. Inparticular the planar growth of proeutectoid ferrite fromaustenite in steel has been studied. The deviation from localequilibrium is caused by dissipation of Gibbs energy bydiffusion inside the phase interface and interface friction. Inthe analysis the interface is divided into three zones and thethermodynamic and kinetic properties are assumed to varycontinuously across the interface. A new model suitable formulticomponent alloys is developed. The model reduces to thefamiliar solute drag model by Cahn under simplifyingconditions. It was demonstrated how the interface model couldbe combined with a method for calculating the volume diffusionin both the growing and parent phases. With this combination ofprocedures the changes in local conditions at the interface, asthe growth rate changes due to long-range diffusion, could bedemonstrated for the case of continuous cooling in an Fe-Nialloy.</p><p>The critical limit for massive transformation in the Fe-Niand Fe-C systems was calculated and found to lie well below theT0 line for both systems. The calculated limit for Fe-Ni wascompared with a recent experimental study and reasonableagreement was found. For the Fe-C system the limit calculatedwith the present model was compared with a phase-field model.The two approaches showed qualitatively the same behaviour andthe quantitative difference was due to different assumptions onhow properties vary across the interface.</p><p>Finally, an attempt to simulate the partitionless growth offerrite in austenite in the Fe-Ni-C system was performed. Inthe applied model the dissipation of Gibbs energy inside theinterface and in the nickel spike ahead of the migratinginterface were accounted for. The long-range diffusion ofcarbon in austenite was treated with an approximate analyticalgrowth equation. A continuous change from paraequilibriumconditions and quasi-paraconditions could be shown in anisothermal section of the Fe-Ni-C phase diagram. Partitionlessgrowth starts in a parabolic fashion but slows down. For alloysoutside the limit for quasiparaconditions partitionless growthis predicted to stop abruptly while for alloys inside thatlimit growth approaches a second parabolic growth law. However,the latter case should not be expected in practise because ofimpingement effects.</p>

Energy needs are projected to continue to increase in the coming decades, and with the drive to use more clean energy to combat climate change, nuclear energy is poised to become an important player in the energy portfolio of the world. Due to the unique nature of nuclear energy, it is always vital to have safe and efficient generation of that energy. In current light water reactors, the most common fuel is uranium dioxide (UO2), an oxide ceramic. There is also ongoing research examining uranium-based based metallic fuels, such as uranium-molybdenum (U-Mo) fuels with low uranium (U) enrichment for research reactors as part of a broader effort to combat nuclear proliferation, and uranium-zirconium-based fuels for Generation IV fast reactors. Each nuclear fuel has weaknesses that need to be addressed for safer and more efficient use. Two major challenges of using UO¬2 are the fission gas (e.g. xenon) release and the decreasing thermal conductivity with increasing burnup. In UMo alloys, the major weakness is the breakaway swelling that occurs at high fission densities. The challenges presented by both fuel types are heavily impacted by microstructure, and several studies have identified that the initial microstructure of the fuel in particular (e.g. initial grain size and grain aspect ratio) plays a large role in determining when and how quickly these processes occur. Thus, knowledge of how such initial microstructures evolve is paramount in having stable and predictable fission gas release and thermal conductivity decrease (in UO2) and fuel swelling (in UMo alloys). Mobility is a critical grain boundary (GB) property that impacts microstructural evolution. Existing literature examines GB mobility for a few specific boundaries but does not (in general) identify the anisotropy relationships that this property has. This work first examined the anisotropy in GB mobility, specifically identifying the anisotropy trend for the low-index rotation axes for tilt GBs in BCC γ U, and fluorite UO2 via molecular dynamics simulation. GB mobility is calculated using the shrinking cylindrical grain method, which uses the capillary effect induced by the GB curvature to drive grain growth. The mobilities are calculated for different rotation axes, misorientation angles, and temperatures in these systems. The results indicated that the density of the atomic plane perpendicular to the (tilt) GB plane (which is also perpendicular to the rotation axis) significantly impacts which GB rotation axis has the fastest boundaries. Specifically, the atomic plane that has a higher density tends to have a faster mobility, because it is more efficient for atoms moving across the GB along such planes. For example, for body-centered cubic materials, the <110> tilt GBs are determined to have the fastest mobilities, while face-centered cubic (FCC) and FCC-like structures such as fluorite have <111> tilt GBs as the fastest. Knowledge of GB mobility and its anisotropy in pure materials is helpful as a baseline, but real materials have solutes or impurities (both intentionally and unintentionally) which are known to affect GB mobility by processes such as solute drag and Zener pinning. Additionally, in reactors, nuclear fission can produce many fission products, each of which acts as an additional impurity that will interact with the GB in some way. Because the initial microstructure and its subsequent evolution are vital for addressing the challenges of using nuclear fuel as described above, knowledge of the impacts of these impurities on GB mobility is required. Therefore, this work examined the impact of solutes and impurities on GB mobility and its anisotropy. In particular, the solute effect was examined using the UMo alloy system, while the impurity effect was examined using Xe (a very common fission product) in the γ U, UMo, and UO2 systems. It is found that both Mo and Xe can cause a solute drag effect on GB mobility in the γ U system, with the effect of Xe being stronger than Mo at the same solute/impurity concentration. Xe also causes a solute drag effect in UO2, though the magnitude of the effect is interatomic-potential-dependent. The mobility anisotropy trend was found to disappear at high solute and impurity concentrations in the metallic U and UMo systems but was largely unaffected in the UO2 system. These results not only increase our fundamental understanding of GB mobility, its anisotropy, and solute/impurity drag effects, but also can be used as inputs for mesoscale simulations to examine polycrystalline grain growth with anisotropic GB mobility and in turn examine how the fuel performance parameters change with these properties. / Doctor of Philosophy / Worldwide, energy needs continue to increase each year. Concerns related to climate change have led to an increased emphasis on renewable energies such as solar and wind, but the limitations of these resources prevent them from being the only energy sources. Nuclear energy is uniquely positioned to address several energy concerns: it is clean (no carbon emissions and air pollution), reliable (for example, 24/7 energy production, independent of weather), and energy-dense (one kilogram of fissile uranium provides roughly the same amount of energy as 3000 metric tons of coal). Currently, nuclear energy provides roughly 20% of the energy of the United States, but future predictions show a decrease in the total share of energy generation due to aging systems and a limited number of new reactors being built. The safety and efficacy of existing and future reactors are among the primary concerns for being able to allow nuclear energy to increase its energy share. To determine the safety and efficacy of new reactor designs, a computer simulation tool called fuel performance modeling has been used over the last few decades. This tool requires several material properties as input, one of which is how the nuclear reactor fuel microstructure changes based on a variety of conditions. A significant process contributing to microstructural change is grain growth. Grains (crystallites that make up the whole material) meet at interfaces called grain boundaries (GBs), and these GBs have two properties that largely determine how grain growth occurs: energy and mobility. Significant effort is being put into understanding these properties and their anisotropy, or how they change based on the GB character which is the relative mismatch between the two grains. This work contributes additional understanding of GB mobility anisotropy in two nuclear fuels: uranium dioxide (UO2, the primary fuel in current reactors) and a uranium-molybdenum (UMo) alloy (the primary fuel for newer research reactors). In particular, computer simulation is used to determine GB mobility for several unique GB systems. It is found that for pure nuclear fuels, GB mobility anisotropy is largely determined by which atomic plane has the highest density perpendicular to the GB. When the fuel is no longer pure (through the addition of alloying elements or other impurities) the anisotropy changes significantly in UMo fuels, such that at high concentrations of solute or impurities there is little to no anisotropy, while very little change is observed in the anisotropy in UO2.

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This thesis concerns computer simulation of diffusional processes in alloys. The main focus is on the development of simulation techniques for diffusion in single-phase domains, but also diffusion controlled phase-transformations and interfacial processes are discussed. Different one-dimensional simulation techniques for studying the Kirkendall effect are developed and analyzed. Comparisons with experimentally observed marker migration show good agreement for small shifts and comparisons with observed Kirkendall porosity show reasonable agreement under the assumption that a certain supersaturation is needed before the vacancies coalesce into pores. A convenient approach in simulations of kinetics is to use thermodynamic software, e.g. Thermo-Calc, to calculate thermodynamic quantities, e.g. chemical potentials, required in the simulation. The main drawback with such an approach is that it will generate a large amount of additional computational work. To overcome this problem a method that decreases the amount of computational work has been developed. The new method is based on artificial neural networks (ANN). By training the ANN to estimate thermodynamic quantities a significant increase in computational speed was obtained. By calculating the dissipation of available driving force due to diffusion inside migrating interfaces an approach for including the effect of solute drag in computer simulations of grain growth and phase transformations has been developed. The new method is based on an effective interfacial mobility and simulations of grain growth have been performed in binary and ternary systems using experimentally assessed model parameters. / QC 20100930

Surface energy plays a major role in many phenomena that are important in technological and industrial processes, for example in wetting, grain growth and sintering. In this thesis, such surface-energy driven processes are studied by means of the phase-field method. The phase-field method is often used to model mesoscale microstructural evolution in materials. It is a diffuse interface method, i.e., it considers the surface or phase boundary between two bulk phases to have a non-zero width with a gradual variation in physical properties such as energy density, composition and crystalline structure. Neck formation and coarsening are two important diffusion-controlled features in solid-state sintering and are studied using our multiphase phase-field method. Inclusion of Navier-Stokes equation with surface-tension forces and convective phase-field equations into the model, enables simulation of reactive wetting and liquid-phase sintering. Analysis of a spreading liquid on a surface is investigated and is shown to follow the dynamics of a known hydrodynamic theory. Analysis of important capillary phenomena with wetting and motion of two particles connected by a liquid bridge are studied in view of important parameters such as contact angles and volume ratios between the liquid and solid particles. The interaction between solute atoms and migrating grain boundaries affects the rate of recrystallization and grain growth. The phenomena is studied using a phase-field method with a concentration dependent double-well potential over the phase boundary. We will show that with a simple phase-field model it is possible to model the dynamics of grain-boundary segregation to a stationary boundary as well as solute drag on a moving boundary. Another important issue in phase-field modeling has been to develop an effective coupling of the phase-field and CALPHAD methods. Such coulping makes use of CALPHAD's thermodynamic information with Gibbs energy function in the phase-field method. With the appropriate thermodynamic and kinetic information from CALPHAD databases, the phase-field method can predict mictrostructural evolution in multicomponent multiphase alloys. A phase-field model coupled with a TQ-interface available from Thermo-Calc is developed to study spinodal decomposition in FeCr, FeCrNi and TiC-ZrC alloys. / QC 20100622

<p>A self‐consistent model for non‐partitioning planar ferrite growth from alloyed austenite is developed. The model captures the evolution with time of interfacial contact conditions for substitutional and interstitial solutes. Substitutional element solute drag is evaluated in terms of the dissipation of free energy within the interface, and an estimate is provided for the rate of buildup of the alloying element ‘‘spike’’ in austenite. The transport of the alloying elements within the interface region is modeled using a discrete‐jump model, while the bulk diffusion of C is treated using a standard continuum treatment.</p> <p>The model is validated against ferrite precipitation and decarburization kinetics in the Fe‐Ni‐C, Fe‐Mn‐C, Fe‐Mo‐C, Fe‐Si‐C, Fe‐Cr‐C and Fe‐Cu‐C systems.</p> / Doctor of Philosophy (PhD)