Efficient energy utilization has been central to human progress, with each new energy source driving societal and technological advancements. While fossil fuels like coal, oil, and natural gas currently dominate, their environmental impact and finite availability underscore the urgent need for sustainable alternatives. Developing environmentally friendly and efficient energy conversion processes is vital to addressing the challenges of resource scarcity and climate change. Magnetocaloric materials (MCMs) are at the forefront of research in advanced cooling technologies due to their ability to exhibit a magnetocaloric effect (MCE). This area of study holds both significant fundamental scientific interest and critical application potential. From a scientific point of view, MCMs are highly intriguing due to their ability to exhibit a giant magnetocaloric effect (GMCE). Uncovering the underlying mechanisms of first-order magnetic transitions (FOMT) plays a pivotal role in advancing research and development in this field [1–6]. From an application perspective, related research primarily focuses on developing 3D-printed bulk materials to achieve gradient composites manufacturing of MCMs with enhanced heat transfer coefficients [7]. This approach is particularly relevant given the narrow operating range of FOMTs, which necessitates multiple material layers to achieve optimal efficiency [8]. However, for the successful commercialization of magnetic cooling and heat pump technologies, the primary future challenge lies in ensuring mechanical stability of GMCMs, which is a critical prerequisite. This thesis explores the unique structure and thermomagnetic properties of Fe2Pbased and potential novel MCM systems using techniques such as magnetization, heat capacity, Mössbauer spectroscopy, Density Functional Theory (DFT), X-ray and neutron diffraction.

In this thesis, an algorithm to model the magnetic perturbation field caused by ships is designed and implemented. A systematic description of methods used for modelling the magnetic signature of ships is given. The algorithm fits coefficients of a prolate spheroidal harmonic expansion of the scalar potential of the magnetic field using a least angle regression method (LARS) modified to implement Lasso regularisation. A Monte Carlo method with model selection based on Akaike's information criterion (AIC) is used to select optimal parameters specifying the prolate spheroidal coordinate system centred on the ship. Furthermore, a method to restrict the degree and order of the harmonic expansion is presented and an extension of the scikit-learn module in Python is given. The predictive power of the model was verified using simulated test data, which showed that the designed model is able to make adequate predictions, but improvements are needed. Different analyses on the inputs of the model showed that the model is succesful for low levels of noise, but is susceptible to overfitting for higher levels of noise. Several recommendations for further research are made.

Self-healing materials have recently drawn the attention of industry as a new solution to the degradation of materials. Recent progress on the field of autonomous self-healing metals has been in the line of self-healing creep-resistant steels. This is challenging due to the nanometer scale of the creep cavities and the fact that only 3D partially filled cavities can be examined to understand the phenomenon. As a consequence, valuable information is hard and expensive to acquire. This thesis proposes to study the surface precipitation phenomenon as an easier alternative to understand creep-cavity precipitation. Surface precipitation have only been studied in binary steels, therefore, the thesis focuses on the influence of the addition of a second self-healing element by studying the time evolution of gold tungsten containing iron based steels. The results provide new insights into the theory of self-healing steels, where tungsten has been found to decelerate the gold precipitation kinetics.

The present thesis discusses two modelling endeavours that serve to provide insights into both the magnetic and structural dynamics associated with a first order magnetic phase transition.
Firstly, density functional theory has been used to model the lithiation of a supercell of 2x2x2 the conventional unit cell of the ferromagnet Fe2P. Subsequently, the magnetic moments in the resulting structure were set to proxy a paramagnetic state and compute the energy difference between the ferromagnetic and paramagnetic state. It has been found that non-magnetic Li-atoms substitute for magnetic Fe-atoms at 3g-positions in the a,b-plane. Moreover, since these are the positions of Fe-atoms that carry a high magnetic moment and induce a magnetic moment on the Fe-atoms in the 3f-layer, the lithiation of the respective positions leads to a reduction of the magnetic moment per formula unit. The energy difference between the ferromagnetic state and the selected proxy of a paramagnetic state showed to decrease linearly in the fraction of lithiated 3g-positions. This deviates from experimental findings suggesting an increase of Tc of the structure upon lithiation. This difference can stem from the fact that the lithiation fractions considered here were higher than those achieved experimentally or that the selected proxy of a paramagnetic state was incorrect.
Secondly, the finite element method has been applied to approximate the displacement field of a 2D structure consisting of grains distributed over two structural phases with different associated lattice parameters. The development of shear stress discontinuities across grain boundaries has been studied in relation to the degree of porosity in a domain. The replacement of grain boundaries by voids appears, after correction for the orientation of edges at these grain boundaries, to lead to a reduction in the values for these discontinuities. More delicate development of the grid to correspond to physically realistic orientations of grain boundaries is needed to gain more robust quantitative measures for this relation. Moreover, the relation between the orientation of grain boundaries and shearing stresses at these boundaries forms an interesting venue for further research to which the present FEM-model can be readily applied.

Solid-state phase transformations in steels cover a broad range of aspects. The underlying physics behind these phase transformations usually include nucleation, diffusion, lattice reconstruction and interactions between solutes and grain boundaries and interfaces. These features and the fact that the events take place at high temperatures, in the bulk, at time scales ranging from milliseconds to hours and at length scales ranging from atomic dimensions to millimeters make even the most widely studied phase transformation in steel, the austenite-ferrite phase transformation, only approximately understood and therefore an attractive topic for investigation. Quantitative data obtained by sophisticated physical characterization techniques in combination with supporting physical microstructural models addressing the relevant length and time scale are required to bring the field of ferrous physical metallurgy further.
This thesis focusses on two new approaches to orchestrate phase transformations in steels such that more physical insight is obtained or that new properties can be reached: (1) cyclic partial austenite-ferrite phase transformations that are designed to unravel the grain growth, and more specifically the interface mobility by avoiding concurrent nucleation of new phases. This topic is studied by computational studies and 3D neutron depolarization studies that are capable to in-situ monitor the ferrite grain size and fraction. (2) Self healing of creep damage by site selective precipitation of supersaturated iron-based alloys. A strong preference for precipitation at free creep cavity surfaces compared to that in the bulk can result in a filling of creep cavities and a significant extension of the creep life time. To make this self-healing mechanism applicable for creep-resistant steels, a search for an alternative healing agent for Au in Fe is to be executed and new design recipes need to be extracted on the basis of the experimental input from advanced characterization techniques such as electron microscopy and X-ray nanotomography.

A time of flight MIEZE spectrometer, which employs radio frequency spin flippers with square pole shoes and a magnetic yoke is presented. These flippers can achieve higher fields than conventional resonant RF flippers, which employ an air core. High fields are crucial for the construction of a high resolution and compact MIEZE spectrometer. Setups using conventional and novel flippers are constructed for comparison and a variety of experiments to characterize MIEZE instruments. Evidence is presented which indicates that high field flippers are capable of generating a 100kHz MIEZE signal comparable to that obtained with a conventional setup. Furthermore the need for a fast and thin detector is demonstrated. In addition the shape of the MIEZE focal spot is determined to be Gaussian. Finally the importance of stable timing for time of flight MIEZE is demonstrated. This research is relevant for the implementation of MIEZE on the Larmor instrument at ISIS pulsed neutron source in the UK.

The autonomous filling of creep-induced grain-boundary cavities by tungsten-rich precipitates has been studied for Fe-W alloys at various temperatures, loading conditions and creep rupture times. To this aim, scanning electron microscopy, energy-dispersive spectroscopy and X-ray nano-tomography were used. Additionally, the effect of local plastic deformation, followed by ageing processes, on the surface precipitation in these alloys has been studied by means of scanning electron microscopy, together with energy-dispersive spectroscopy. Finally, a diffusion-based finite element analysis has been performed to study cavity filling for different combination of diffusion mechanisms. From scanning electron micrographs it can be confirmed, that Fe-W alloys show damage-selective precipitation of the Laves phase (Fe2W). Furthermore, the filling ratios of individual cavities are determined and it is found, that the autonomous healing strongly depends on the duration of the loading at of elevated temperature. From the quantitative analysis of the size and shape of the precipitates and cavities it is found, that the orientation of grain boundaries to the stress direction has an influence on the morphology of the induced-cavities, as well as, the filling ratio. The grain-boundary cavities orientated favourably for the cavity growth show low filling ratios, due to the fact, that their propagation proceeds with a higher rate than the solute transport. The critical volume of a cavity, that can be fully filled with solute atoms, is found to be of the order of 10 μm3, while the average filling ratio for Fe-W alloys is found to be around 50% for the sample loaded to 100% of its lifetime. Moreover, the scanning electron micrographs of the indentation-deformed samples subjected to ageing revealed surface precipitates (Fe2W), which existence has to be further researched. The hardness measurements of Fe-W samples showed that after 200 hours of ageing at 580oC, the strength of the alloy is only reduced if the material has been pre-strained, but it remains constant for the solution heat treated specimens. This result shows, that the process of dislocation recovery dominates over work-hardening only in the case of a high dislocation density. Finally, the finite element analysis revealed, that during the diffusive cavity filling of an equilibrium-shaped void, three different diffusion profile regimes are plausible. Namely, a diffusive filling dominated by volume diffusion, which occurs for a relatively small spacing between neighbouring cavities and is independent of the ratio of bulk and grain-boundary diffusion. Secondly, a diffusive filling governed by volume diffusion with contributing grain-boundary diffusion, which occurs for relatively low ratios of bulk and grain-boundary diffusion. And ultimately, a diffusive filling governed by grain-boundary diffusion, which occurs for relatively high ratios of bulk and grain-boundary diffusion.

In this thesis, the boundary conditions for the development of giant magnetocaloric materials are investigated. The magnetocaloric effect is found in magnetic materials, when they are subjected to an external magnetic field. This leads to abrupt magnetization changes that cause a temperature change in the material. Materials based on Fe2P show giant temperature changes around room temperature and are especially suited for cooling applications. This is due to the large magnetization change that can be realized with the application of a relatively small magnetic field around the ferro- to paramagnetic phase transition. This transition is of a first order, giving rise to latent heat and rise to
‘‘giant’’ effects. After earlier studies investigated the relation between microscopic and macroscopic properties of these materials, the focus of this thesis is on the electronic factors that play a role in the stability and phase transitions of these compounds. After all, when the mechanism behind these phase transitions is clear, is it easier to search for new materials that show similar phase transitions. Two strategies are possible: elucidating the mechanism of Fe2P-based materials or investigating materials that show similar phase transitions. The latter is described in the next paragraph...

Mn-Fe-P-Si compounds are, to date, the most promising materials for refrigeration and energy conversion applications due to the combination of giant magnetocaloric effect (GMCE) and low material cost. The nature of magneto-elastic transition was studied on both length- and time-scales using neutron diffraction, muSR and Mӧssbauer spectroscopy techniques for Mn-Fe-P-Si compounds. The revealed strong spin-lattice coupling is responsible for the different magnetic orders, the tunability of the phase transition, and the excellent magnetocaloric properties in these compounds. This thesis contributes to a better understanding of the spin-lattice coupling and phase-transition behavior for Mn-Fe-P-Si compounds.