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In this thesis multiferroic materials are investigated through a number of different experimental techniques, particularly p,+SR, neutron scattering and impedance spec- troscopy. The magnetic and dielectric properties of materials, which form multiferroic states through different mechanisms, are explored. Inelastic neutron scattering studies have been made of LuMn03 and LuFe204. Large reciprocal space maps have been made of LuFe204 and an initial spin wave dispersion found. The exchange parameters have been estimated from the data and are shown to support ab initio calculations. The spin wave dispersion of LuMn03 has been mapped throughout the Brillouin zone and the exchange parameters extracted by comparison with a spin-wave model. Com- parisons have then been made with other members of the hexagonal manganite family. The analysis is supported by thermal expansion and magnetization studies. The first experiment on a multiferroic combining the p,+SR technique and applied electric fields was performed as a proof of principle experiment on HoMn03. The effect of the elec- tric field on the magnetism has been detected and calculations performed to try and identify muon stopping sites. The family of rare earth chromites have been studied through magnetization and heat capacity experiments as well as p,+SR. The vary- ing entropy and energy level splitting within the family is examined and the debate as to whether they are multiferroic is addressed. An instrument to study dielectric properties at cryogenic temperatures through impedance spectroscopy was designed and constructed. Multiferroics investigated in this thesis using neutron scattering and p,+SR were then studied with this apparatus to look for coupling between magnetic and ferroelectric order parameters. A dielectric anomaly associated with the magnetic transition temperature was observed in LuFe204 which is evidence of the coupling between the order parameters.

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When the structural integrity of the reactor pressure vessel (RPV) in a pressurised water re- actor (PWR) is analysed, certain types of postulated accidents have to be considered. The conditions during a loss-of-coolant accident (LOCA) resemble a warm prestressing (WPS) sce- nario. Warm prestressing refers to a prior overload sequence which in certain circumstances improves the fracture resistance of a ferritic steel structure. Furthermore, a RPV is a welded structure. The welding process gives rise to residual stresses (RS) that can have a significant influence on the integrity of the RPV. Tensile residual stresses are often regarded as unwanted and detrimental. Previous work has not studied the combined effects of WPS and RS on the fracture behaviour of ferritic steel components. The purpose of this dissertation is to explore the combined effects of WPS and RS by means of robust statistical techniques. To describe the effects of WPS, two models are investigated. A local model using the near crack tip stresses and strains (often called the Beremin model) is usually used to predict the fracture probability of as-received material and can be adapted to include the effects of WPS. However, this requires the use of a detailed finite element analysis. The Chell model, on the other hand, is a deterministic model developed to explain the beneficial effects ofWPS on fracture toughness. Using a Monte Carlo (MC) simulation approach, the Chell model is extended to enable failure probability distributions to be determined. The failure probabilities are compared with both experimental data from the literature and results of an experimental test programme on compact tension specimens. It is found that the MCS approach used in conjunction with the Chell model provides robust and accurate predictions of the failure probability following WPS. To extend the analysis to include residual stress, a novel preloading rig model for introducing a known tensile RS into laboratory fracture specimens is investigated. It is found that the Chell model is to be preferred over the Beremin model. The Chell model provides robust and accurate predictions of the fracture behaviour after WPS, yet its application is straight forward and less time expensive than the Beremin model. The combined effects of WPS and a tensile RS field on the failure probabilities can be analysed by combining the Chell model with the preloading rig model and applying a Monte Carlo simulation approach. Contrary to expectations, it is shown that a tensile RS field can have a significant beneficial effect on the failure distribution after WPS.

Strain induced precipitation of microalloy elements during hot deformation of steels has been an area of Interest for several decades. Several researchers have produced models for strain induced precipitation phenomena that gave reasonably good predictions of precipitate evolution during hot deformation. These models were however, applicable only for single pass deformation. The present work aims to develop a new model that could be extended to prediction of precipitate evolution during multi-pass hot deformation during rolling. In this work, a new approach has been adopted in modelling strain induced precipitation of niobium carbonitrides during hot deformation. A dislocation geometry is proposed for a typical microband, which is considered as a key factor In the model. This microband geometry enables one to obtain the local solute concentration gradients in the regions between the microbands. Diffusion of solute atoms towards the microbands is considered to govern the growth rate of the precipitates on the dislocation nodes in the microbands; i.e., the solute atoms reaching the microbands are immediately consumed in either nucleating a new precipitate or growing an existing precipitate. Depending on the considerations of how precipitates nucleate on the microbands, different models have been proposed and their results analysed. Extending this model to a multipass model depends on the location of the next generation microbands after a second/subsequent deformation. The local solute concentration, already obtained after the first pass, will govern the precipitation potential of the nodes on the second generation microband. Plane strain compression tests were conducted on Fe-30wt.%Ni-Nb alloys at different temperatures and deformation conditions. The test samples were subjected to thin foil transmission electron microscopy to obtain the particle size distribution of the niobium carbonitride particles. The experimental results were then correlated with the model results and found to be in reasonably good agreement.

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In this thesis, the effects of strain path reversal on microstructure evolution, in terms of austenite grain subdivision and its recrystallisation as well as continuous cooling phase transformations, are investigated using an API X-70 grade microalloyed steel and two non-transforming austenitic model alloys, namely 316L and Fe-30%wtNi with low and medium stacking fault energies (SFE), respectively. The flow stress-strain and static recrystallisation behaviours of the X-70 steel were studied by monotonic and interrupted hot plane strain compression (PSC) tests. A fraction softening parameter calculated from "double-hit" PSC tests has been found to be capable of quick outlining the partial recrystallisation temperature region without complicated microstructure analysis involved. When hot deformed to medium strains (Evm=O.3~0.5) in austenite non-recrystallisation temperature region by forward-reverse torsion tests, simple strain path changes do not impose significant influence on the microstructural evolution of this X-70 steel through continuous cooling phase transformations. The post-deformation cooling rate is the more influential factor which determines the transformation mechanisms and microstructure, therefore mechanical properties of the final products. However, when subjected to large accumulative strains (Evm2.0), the effects of multiple strain path reversals combined with small amplitude of each forward strain are found to be very profound on microstructural evolutions in austenite studied by hot cyclical torsion using two non-transforming model alloys. The formation of high angle boundaries (HABs) by austenite grain subdivision was significantly delayed by multiple strain path reversals in the Fe-30wt%Ni model alloy with medium SFE. Dynamic recrystallisation (DRX) of austenite in low SFE 316L stainless steel was almost completely suppressed by multiple strain path reversals. Dynamic strain induced transformation (DSIT) in the X-70 steel was also found to be inhibited by multiple strain path reversals when deformed to large strains between the Ac3 and Ar3 temperatures by similar hot cyclical torsions which were applied to model austenitic alloys. Interpretations of the observations were made by correlating the behaviour of the austenite in X-70 steel, which could not be directly observed easily, to the observations on microstructural evolution made in the two model austenitic alloys. This provides better insights into the transformation mechanisms of DSIT process.

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300M ultra high strength steel has been widely used for over forty years as a structural material in aerospace applications where a high strength is required. These parts are generally protected from corrosion by electroplated cadmium sacrificial coatings. However, there are concerns over this coating material due to its high toxicity and alternative coatings including Zinc-14%Nickel and SermeTel®1140/962 have been considered. It is known that applying electrodeposited coatings causes atomic hydrogen to be absorbed by the steel substrate producing delayed failure by direct hydrogen embrittlement. Hydrogen is also absorbed when a sacrificial coating undergoes corrosion in service and this process is known as re-embrittlement. The effect of electroplated Zinc-14%Nickel and aluminium based SermeTel®1140/962 sacrificial coatings in causing hydrogen embrittlement and re-embrittlement of 300M steel have been compared to that of conventional electroplated cadmium. AerMet®100 ultra high strength steel has been also considered as alternative replacement for the conventional 300M. Hence, the hydrogen embrittlement and re-embrittlement susceptibilities of AerMet®100 were studied when coated with cadmium, Zinc-14%Nickel and SermeTel®1140/962. In addition, two alternative alloys GifloM2000 and CSS-42LTM were also taken into consideration and only the extent of hydrogen re-embrittlement was assessed when coated with cadmium and SermeTel®1140/962, respectively. Slow strain rate tests, SSRT, were carried out for plated, plated and baked as well as plated, baked and corroded tensile specimens. The time to failure values were compared using a Weibull distribution, statistical ttests and embrittlement indices. Differences in hydrogen susceptibility of the high strength steels considered might depend upon their intrinsic hydrogen transport characteristics. These properties were studied and compared in terms of hydrogen diffusivity and solubility.