Studies on the effect of the prior austenite grain size on the phase transformation kinetics of bainite are contradictory. Literature explains these contradictions by the presence of two different types of nucleation sites, the prior austenite grain boundaries and the tips of previously formed bainitic ferrite sub-units. The difference in their activation energies for nucleation, ΔQ, is known to determine whether the phase transformation kinetics of bainite are accelerated by prior austenite grain refinement or coarsening. However, the factors that influence ΔQ are not entirely understood, which is the reason why the contradictory results regarding the effect of the prior austenite grain size on the phase transformation kinetics of bainite observed in the different studies cannot be explained yet.

This master thesis investigates the effect of the prior austenite grain size on the phase transformation kinetics of bainite at different transformation temperatures in a low-carbon high-silicon steel. The experiments were divided into two groups, with one group consisting of specimens with finer prior austenite grains and one with coarser prior austenite grains. Specimens were transformed to bainite at three different isothermal transformation temperatures, 440 °C, 410 °C and 380 °C. Bainite formation was investigated by in-situ synchrotron XRD experiments performed at DESY to study the evolution of the phase fractions, lattice parameters and microstrains of bainitic ferrite and austenite. Furthermore, microstructure investigations on the specimens transformed at DESY and additional interrupted quenching experiments were conducted to understand the effect of the prior austenite grain size and the isothermal transformation temperature on the microstructure and the sheaf morphology formed in the early stages of the phase transformation. Finally, simulations were performed to determine the effect of the prior austenite grain size and the transformation temperature on ΔQ.

The experiments showed that, by decreasing the transformation temperature, the phase transformation kinetics of the group with fine prior austenite grains were decelerated, whereas the phase transformation kinetics of the group with coarse prior austenite grains were accelerated. The simulations exhibited an increase in ΔQ as the isothermal transformation temperature was decreased, indicating that sheaf growth by successive nucleation events at the tips of previously formed sub-units becomes increasingly prevalent. While the specimens with coarse prior austenite grains provide more potential nucleation sites at the tips of previously formed sub-units, the specimens with fine prior austenite grains provide more nucleation sites for grain boundary nucleation, which explains the reverse effect of the transformation temperature on the phase transformation kinetics of the two experiment groups. Microstructure observations have shown that the effect of the transformation temperature on processes, such as carbon partitioning and the transition from upper to lower bainite, could play an important role in explaining the observed effect of the prior austenite grain size on the phase transformation kinetics of bainite.

Self-healing of creep-induced damage in newly designed 12Cr self-healing ferritic steels is studied. The damage healing is achieved by the segregation of supersaturated solute atoms at the free surface of the creep-induced cavities, and in this research, the healing phase is the W-riched Laves phase. Two kinds of self-healing steel with different precipitation driving forces of Laves phase are investigated, namely fast self-healing steel (FSHS) and slow self-healing steel (SSHS), in which the FSHS is designed to show the healing effect after 280 hours, while SSHS is designed to show healing effect after 27000 hours. Although both systems are self-healing systems, only FSHS is expected to show the self-healing phenomenon under the testing condition of this study, while SSHS not. Creep tests were performed on bone-shaped samples with constant stresses ranging from 100 to 260 MPa at 550 °C. The creep behavior is compared with traditional 9-12Cr ferritic steels, the relationship between the minimum creep rate and stress obeys Norton's power law, and the relationship between the lifetime and stress also obeys a power law. The total strain and stress exponents in power law of both SSHS and FSHS are relatively high compared to traditional 12Cr ferritic steel with similar grain size. The microstructure of the creep-failed samples, including the fracture surfaces, the uniformly-strained region (i.e., the stress-affected region), and the stress-free region are investigated with scanning electron microscopy (SEM). By comparing the feature of the stress-affected region and stress-free region exposed to the same thermal history, the effect of high-temperature exposure could be isolated from this. The effect of stress on the Laves phase precipitation behavior and the damage evolution could be understood more clearly. For both the SSHS and FSHS, a ductile and transgranular fracture mode is confirmed by analyzing the fracture surfaces. In the stress-affected region and stress-free region of the creep-failed samples, M₂₃C₆ precipitates, Si/C-rich impurities, and pores were found in all observed samples. The Laves phase was only found in the samples with a longer lifetime (> 300 h). A TEM observation and EDX scanning was conducted on the FSHS sample with a lifetime of 2487.2 hours. Laves phase precipitates were found on the grain boundaries and in the matrix, while no grain boundary cavities were found. Statistics based on pores observed on samples tested under different stress showed that the pores were present prior to creep tests and are generally not due to creep. A combined analysis of creep behavior and microstructure showed that dislocation climbing is likely the dominant creep mechanism. The absence of typical creep grain-boundary cavities was allocated to the relatively ductile matrix of 12Cr self-healing steel or the unfavorable dominating creep mechanism for creep cavity forming. With the prerequisite of self-healing not found in all tested samples, self-healing was considered not found under current testing conditions. The statistics based on the Laves phase showed that the precipitation depends on creep time, and the stress is indeed lowering the nucleation barrier, thus promoting the Laves phase to precipitate in the stress-affected areas.

In this report, a model is presented to alleviate some of the computational work that goes into the effort of finding the magnetic properties of magnetocaloric materials. The model utilizes an interior point optimization routine to solve for the minimal exchange energy configuration of a system, given the exchange interactions of the material. The model is tested against four materials (Ni, MnO, Fe\textsubscript{2}P and Mn\textsubscript{2}Sb). For Ni and MnO, the exchange interactions are also computed. Three iterations of the model are compared. The base model, which only considers exchange interactions inside a chosen supercell, the base model with the inclusion of boundary conditions, and the base model with boundary conditions and the addition of an algorithm to find optimal solutions. \\ The algorithm analyzes the found results by the optimization routine, and if the result is considered not properly symmetric, runs the optimization routine another time, from a symmetrical starting point obtained from the outcome of the previous run. \\ In all versions of the model, effectiveness (percent of runs that resulted in the optimal configuration) and average run times were recorded. Three initialization methods for the model were used, and also tested for their effectiveness. For the algorithm, a parameter $\gamma$ is introduced that changes the size of some of the moments for the new starting points. Six different values for $\gamma$ were tested for their effectiveness against a test set of suboptimal solutions.
The model with the addition of boundary conditions and the algorithm performed the best out of the three iterations of the model, with an effectiveness of 99.895\%, and an average run time ranging from 0.62 s for 2$\times$2$\times$2 Ni, to 94.64 s for 3$\times$3$\times$3 Fe\textsubscript{2}P, in the case of $\gamma = 0.3$. To conclude, the model with the inclusion of the boundary conditions and the algorithm proves to be a robust method to evaluate the magnetic configuration of a material, especially for smaller systems.

Plasticity accompanied by dislocation motion is the essential property of metals. A large number of crystal plasticity models have been developed to predict the mechanical response in the work hardening regime while little attention is paid to pre-yield behavior. In this work, we propose a novel model both pre-yield and post-yield from a discrete dislocation dynamics (DDD) database of 55 DDD realizations. Dislocation density and Frank-Read source height are chosen as microstructural state variables to represent the material state. Flow rule and dislocation multiplication model are in good agreement with DDD results. Discrepancies result from the onset of micro-plasticity in pre-yield. The axial stress-strain response predicted by the novel model is consistent with current DDD data. The evolution equation of source height fails in capturing the evolution in accordance with DDD data, which originates from the inaccuracy brought by indirect extraction of source shape from DDD dislocation networks. Future research recommendations include enlargement of the database, extraction of source shape in situ, and evolution equations for link-based microstructural variables.

With green energy well on its way to becoming a necessity rather than a commodity it is vital to not only focus on improving the source of energy, but also the efficiency of the eventual energy usage. Vapor-compression based household refrigerators are one of the most used electrical appliances in the common household and consume electricity throughout the day, so finding a more efficient alternative would be beneficial. One such alternative uses the magnetocaloric effect as cooling mechanism and could theoretically reach a much higher efficiency. However, the search for the right material with the right characteristics for the commercial realization of this technology is still ongoing and this thesis aims to assist in making progress in said search. The focus of this thesis is on the substitution of manganese with nickel in Mn-Fe-P-Si compounds and how the magnetocaloric properties of these compounds hold up against similar compounds that had manganese substituted with cobalt as this lowered the Curie temperature and decreased the thermal hysteresis, which are favourable results. Unfortunately, there are a few factors such as price and criticality that motivated replacing cobalt with a close relative such as nickel in hope of achieving similar results. Measurements done with a superconducting quantum interference device (SQUID) indicated that the synthesized Mn-Fe-Ni-P-Si compounds have very similar magnetic properties to the Mn-Fe-Co-P-Si compounds. Differential scanning calorimetry (DSC) measurements showed that both compounds reduce thermal hysteresis similarly, but the nickel compounds are capable of lowering the curie temperature much more with increased doping compared to the cobalt compounds. X-ray powder diffraction (XRD) analysis proved that nickel is substituted in the proper place and that the decreasing Curie temperature is the result of a change in the ratio of the unit cell parameters a and c. Nickel thus showed to be an excellent replacement of cobalt to be used to substitute manganese with to lower the Curie temperature of the initial Mn-Fe-P-Si compound and reduce thermal hysteresis. More research could be done on further tuning the composition by for example increasing the silicon doping as this has shown to increase the Curie temperature rather than decrease it whilst also reducing the thermal hysteresis. Combined with the results from this thesis it could lead to a composition with more favourable magnetocaloric characteristics.