The growth of a magnesioferrite precipitate in iron doped (˜lOOOOppm wt.) magnesium oxide crystals, heat treated at 973K in oxygen, is studied with torque, magnetisation and magnetic resonance measurements. The torque and magnetisation results are in agreement with a model which assumes that the precipitate grows by diffusion limited Ostwald ripening.The effects of the particle size distribution and cubic magnetocrystalline anisotropy of the orientated octahedral precipitate particles on the magnetisation and torque curves are calculated. A magnetometrie demagnetisation tensor is defined for assemblies of orientated dipoles, its variation with the assembly size is investigated and used to calculate the longitudinal demagnetisation factors of octahedra. The ferromagnetic resonance spectra obtained were not in agreement with the generally used theory of de Biasi and Devezas (J. Appl. Phys. (1978)49, 2466). A new theory, based on a spin Hamiltonian, of the FMR response of an anisotropic superparamagnet is proposed and compared with some of the experimental spectra.

Fine/nanoscale carbonitrides of microalloying elements such as Nb, Ti, and V play a significant role in the strengthening of HSLA steels. Site-specific analysis of the precipitates in different heterogeneous microstructural areas within realistic alloys is limited and the competition of different precipitates has not been discussed in detail. In this work, the relationship of precipitates/clusters with microstructure has been analyzed by site-specific methods and a simple model has been created to describe the competition between strain-induced precipitation and (Ti, Nb) (C, N). Firstly, the spatial distribution of precipitates and microstructure heterogeneity in an X70 steel were investigated by site-specific analysis method. The quantitative analyze the precipitates reveals that strain-induced precipitation of fine NbC particles (5-20 nm) on dislocations was suppressed by the large (Ti, Nb) (C, N) precipitates. The similarity of precipitates in each location suggests that the local features (such as strain and grain size) in the final microstructure arise from phase transformations during cooling. Secondly, the microstructural evolution during coiling and its effects on the mechanical properties of a vanadium microalloyed steel were investigated. Experimental findings showed that during holding at 500 ºC, nano precipitates (<10 nm) containing V and N nucleated heterogeneously, primarily in areas with high Kernel Average Misorientation (KAM) values. These areas contained a larger number of dislocations, which acted as nucleation sites for the precipitates. The effect of precipitation strengthening was not significant and was offset by softening caused by the aging of bainite and associated recovery of dislocations. Thirdly, in the HSLA steel with both V and Nb additions, nano precipitates were found to preferentially form around dislocations and grain/sub-grain boundaries in high KAM areas associated with bainite. Precipitates were frequently observed around cementite in low KAM areas, which were identified as granular bainite. Interphase clusters were also discovered in low KAM areas behind the ferrite/austenite interface. Analysis of the results indicated that the precipitation of micro-alloyed particles on cementite may reduce the contribution of precipitation hardening achievable through microalloying. Finally, a competition model between strain-induced precipitates (SIP) and epitaxial growth in micro-alloyed austenite has been developed. Using this model, it is possible to estimate the effects of process parameters (T, applied strain), the number density of pre-existing TiN particles, and steel composition on the precipitation process. Through the various studies achieved here, the aim to understand the relationship between the precipitates and different microstructures and develop the competition models has been accomplished. These works provide a relatively new workflow to investigate the precipitates within the steel, especially in site-specific areas, and also allow us to predict the precipitation of NbC by selecting desired temperature range, applied strain, and number density of pre-existing TiN precipitates. / Thesis / Doctor of Philosophy (PhD)

This thesis concerns the investigation of radioisotopes as indicators for precipitation reactions. As a precipitate forms in the presence of a radioisotope, adsorption may take place on its surface. If this adsorption changes markedly at the stoichiometric point it will be possible to use this variation as an indicator for the reaction.

The purpose of this investigation is to reveal the effects of certain factors affecting adsorption on some specific precipitates. It is hoped that the choice of precipitate types will enable extension of the information gained here to other precipitates similar to those investigated.

Precipitates of a series of alkaline earth metal (barium and strontium) carbonates, chromates, phosphates, and sulfates were formed at high supersaturation by diffusion through silica hydrogel, agarose hydrogel, and the freshly developed agarosesilica mixed gels. The reaction vessels could be a small test tube, a recently designed standard micro slide cassette and a enlarged supercassette. Homogeneous nucleation is thought to have taken place, and particle development led to the formation of an unusual category of materials, known as Induced Morphology Crystal Aggregates [IMCA], at high pH under far-from-equilibrium conditions. Standard procedures were developed in order to produce homogeneous gels. Particle development led to characteristic style of pattern formation, which I have called monster, spiral, and flake. Among these IMCA, barium carbonate, chromate, and sulfate were moderately easy to grow. Barium phosphate was very difficult to grow as IMCA due to formation of poorly crystalline spherulites. IMCA of strontium carbonate, chromate and sulfate could be developed at high basic pH in the presence of silicate. Strontium carbonate sheet morphology displays a unique property, double internal layer structure, which was identified by backscattering electron imaging (BEI). Selected electron diffraction (SAD) revealed a new crystal phase which was called "Dentonite". Precipitate particles were isolated using a non-destructive isolation technique. Optical microscopy was widely used to examine particles in situ and scanning electron microscopy and X-ray dispersive energy (EDX) spectroscopy were applied to particles ex situ, together with ESCA for surface analysis. Growth patterns were found to be strongly dependent on pH. Other related pattern formation processes were also investigated including normal and dendritic structures, spherulitic structures and periodic pattern formation. Some interpretations were proposed in terms of mechanism. Chemical additive effects were examined experimentally in the calcium phosphate system. The effect of external ionic strength was investigated, and it was found that a certain concentration of sodium chloride (0.2 M) approximately equals a fraction of pH unit (-0.2).

Over the last two decades there has been an increased interest in understanding the influence of trace boron additions in Ti alloys. These additions refine the prior β grain size in as-cast Ti alloys along with increasing their modulus and yield strength due to the precipitation of TiB. TiB also acts as a heterogeneous nucleation site for α precipitation and has been shown to influence the α phase morphology. B is completely soluble in liquid Ti but has a negligible solubility in both body centered cubic β and hexagonal close packed α phases of Ti. Thus, during solidification of hypoeutectic B containing alloys, B is rejected from β into the liquid where it reacts with Ti to form pristine single crystal whiskers of TiB. Despite a substantial amount of reported experimental work on the characterization of TiB precipitates, its formation mechanism and influence on α phase precipitation are still not clear. The current work is divided into two parts – (i) understanding the mechanism of TiB formation using first principles based density functional theory (DFT) calculations and (ii) elucidating how TiB influences the α phase morphology and crystallography in titanium alloys using electron microscopy techniques. TiB exhibits anisotropic growth morphology with [010] direction as its predominant growth direction and displays a hexagonal cross section with (100), (101), and (10) as the bounding planes. A high density of stacking faults has been experimentally observed on the (100) plane. The present study, by using DFT based nudged elastic band (NEB) calculations, elucidates for the first time that the diffusion of B through TiB is via an interstitial-assisted mechanism as opposed to vacancy-assisted mechanism hypothesized in literature. This one dimensional interstitial-assisted diffusion results in the anisotropic growth of TiB. In addition, the energetics of TiB- α interfaces was calculated to understand the hexagonal cross-section of TiB. The intimate mixing of B27 and Bf structures and their co-existence with stacking faults has been explained by calculating the interfacial energy of B27/Bf interfaces along with stacking fault formation energy. The boride precipitates have also been shown to modify the morphology of α phase from lath like to more equiaxed like. However, the influence of TiB on the crystallography of α precipitation has not been explored in great detail. The present study will clearly demonstrate, for the first time, the influence of various alloying elements present in the titanium alloy, on the resulting effect of B addition on α morphology and its crystallography in Ti alloys. Thus, the influence of B addition on α precipitation in two classes of commercial Ti alloys, i.e. β alloys and α + β alloys have been explored. It has been found that TiB nucleated α can either become equiaxed by a loss of Burgers orientation relationship (OR) with β or can retain the lath morphology in case of alloys containing a combination of Ti, Al and Mo.

The overarching goal of this research was to generate a menu of shape memory alloys (SMAs) actuator materials capable of meeting the demands of aerospace applications. Material requirements were recognized to meet the demand for high temperature SMAs with actuating temperatures above 85 °C and provide material options capable of performing over 100K actuation cycles. The first study is a preliminary characterization for the down selection of Ni-rich NiTiHf15 compositions chosen for a more in-depth examination of the nano-precipitation and evolution of the H-phase. To make this selection, the effect of Ni content in Ni-rich NiTiHf high temperature shape memory alloys (HTSMAs) on processability, microstructure, and hardness was analyzed for three compositions (Ni50.1TiHf15, Ni50.3TiHf15, Ni50.5TiHf15). Each composition was characterized under three conditions: homogenized, 25%, and 50% thickness reduction through hot-rolling. The second study emphasized the processing and aging response of an industrially produced, hot-extruded Ni50.3Ti29.7Hf20 (at%) HTSMA. The samples were sectioned into two halves with half remaining as-extruded and the other half hot-rolled to a 25% reduction in thickness. A portion of both conditions underwent conventional aging for 3 hours at various temperatures ranging from 450-750 °C, and the other portion was pre-aged for 12 hours at 300 °C followed by conventional aging treatments. After processing, the samples were characterized by differential scanning calorimetry (DSC), Vickers hardness (HV) testing, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and synchrotron radiation X-ray diffraction (SR-XRD). The relationship between the introduction of texturing, pre-aging, and aging on Ni-rich and high Hf-content compositions was investigated.

White Island is a highly active volcano with an acidic, S-rich hydrothermal system in the Bay of Plenty, North Island, New Zealand. In this acidic environment a series of subsurface Fe-rich layers are ubiquitous in the crater sediments at shallow depth and are capable of modifying the flow and gas flux dynamics in the system. The mineralogy of the subsurface Fe-rich layer(s) and the processes leading to their formation are unknown. Here the mineralogy and formation of the subsurface Fe-rich layers in relation to the surface and subsurface environment(s) within the Main Crater at White Island are assessed. Based on geochemical analyses, subsurface Fe-rich crusts are composed of a mix of jarosite and goethite, cementing crater fill sediments into cohesive layers. Saturation index (SI) and Eh/pH assessments identify that fluids evolved at White Island are undersaturated with respect to the mineral phases present in the Fe-rich subsurface layers. Formation of the Fe-rich subsurface layers is most likely related to the transition between atmospheric gases and/or meteoric water mixing with hydrothermal fluids. This transition zone creates an environment conducive to forming jarosite and goethite forming in the same layer. Additionally, subsurface sediments including the Fe-rich layers show a consistent organic carbon isotopic signature of -23 ‰. Microscopic investigations confirm diatoms and microbes are present in the subsurface Fe-rich layers. The full extent of microbial activity in relation to the Fe-rich layers at White Island still requires further investigation. Based on chemical extractions for isotopic analyses, Fe-rich layers are shown to preserve δ¹³C signatures indicative of microbial life. Interface zones such as those identified in the hydrothermal environment at White Island can create metal-rich deposits and habitable/preservative microbial environments as well as affecting the macroscopic dynamics of volcanic and epithermal systems.

The system of upper high nickel alloying steel seal ring and lower high nickel alloying steel seal ring, installed in the grooves of turbine, can extend out and fit with the wall of valve cage, resulting in forming a good seal under the pressure. In the project, the failure steel seal ring is considered. This situation had threatened the safety of the whole steam turbine system. The purpose of this study is to identify the failure cause of the steel seal ring used in nuclear steam turbines. New high nickel steel alloy seal ring was compared with the failed seal ring. The dimensions of macroscopic ring with clearly plastic deformation were measured using calipers. Surface morphology of ring was observed by optical microscopy through metallographic analysis. There is a lot of precipitation in the grain boundaries of used seal ring, along with smaller grain size than the new seal ring. To explore the composition of precipitation, scanning electron microscopy (SEM) with energy-dispersive spectrometer (EDS) were used. The results indicated that the concentration of titanium (Ti) and molybdenum (Mo) was higher in the precipitation of used seal ring. At the same time, the hardness and elastic modulus of used seal ring were reduced, measured by nanoindentation test. In-situ SEM tensile testing were used to record and analyze the generation of crack source and crack development under applied load. The reasons of the seal ring failure can be answered because of these experimental results at both macroscopic and microscopic scales. The main reason of the seal ring failure is a combination of long-term stress and elevated temperature during turbine operation. Complex work environment caused recrystallization and recovery, resulting in grain refinement and secondary phase precipitation. Further embodiment, recrystallization and recovery caused the elastic modulus and hardness of used seal ring decrease. Moreover, a lot of secondary phase precipitates appeared at grain boundaries during use. The appearance of secondary phase precipitates become the weakest part of used seal ring. The applied load lead to seal ring failure from the formation of microvoids to microvoids aggregated becoming microcracks until to the appearance of cracks at macroscopic scale. These changes of microscopic structure ultimately reflected in critical plastic deformation of used seal ring.

No description available.