No description available.

No description available.

Rail steels rely primarily on possessing adequate wear and rolling contact fatigue resistance. These properties, together with the toughness, can in principle be optimized by implementing thermomechanical processing assisted by controlled niobium additions. The purpose of the current work is to develop a Nb-microalloying strategy in the context of high-carbon pearlitic and cementite-free bainitic steels. The conventional methods do not leave the critical regions of a rail section in a suitably processed state. An attempt has been made for the first time, to create a pancaked austenite grain structure, with an examination of the consequences on the final product. One of the major difficulties is to ensure that niobium does not segregate during manufacturing, since niobium is a strong carbide former and rail steels traditionally contain large carbon concentrations. Niobium solubility in austenite has been assessed critically and thermodynamic calculations for microsegregation have been taken into account. The aim is to ensure that any primary niobium carbide precipitated from solute-enriched liquid during non-equilibrium solidification, can be taken into solution in austenite during reheating, to mitigate potential effects of coarse precipitates on the final mechanical properties. Rail steels containing 0.01-0.02 wt% Nb have been designed and characterised. In as-cast condition, primary niobium carbides as large as ~10 µm can be observed, which dissolve slowly during reheating. An attempt has been made to develop a model to estimate the dissolution kinetics of the carbides. Dissolved niobium in reheated austenite precipitates during hot deformation as fine niobium carbides (< 50 nm) which inhibit austenite recrystallisation by pinning the austenite grain boundaries. Nb-microalloying increases the ‘no-recrystallisation temperature’ of deformed austenite during multi-pass compression tests. The topology of grain deformation has been analysed in terms of stereological calculations and dilatometric experiments have shown that transformation kinetics tend to accelerate when the austenite is deformed below the no-recrystallisation temperature, however the effect is relatively small. The microstructure and mechanical properties of the as-rolled Nb-microalloyed steels have been characterised along with their rolling-sliding wear performance and compared with their non-microalloyed counterparts. Increased austenite grain boundary area and increased dislocation activity due to pancaking, hinder bainite growth which leads to an increased retained austenite volume fraction. This in turn, leads to slightly improved ductility, improved toughness and improved wear resistance in Nb-microalloyed bainitic alloys. Microstructural refinement in Nbmicroalloyed pearlitic alloys does not have any significant effect on tensile and toughness properties, but wear resistance improves significantly. A Bayesian neural network model has been developed to estimate the wear of rails. Predicted trends have been found consistent with metallurgical experience and the perceived noise levels are consistent with reasonable repeatability of the wear testing method used. The model can be applied widely to estimate wear because of its capacity to indicate uncertainty, including both the perceived level of noise in the output, and an uncertainty associated with fitting the function in the local region of input space.

Copper (Cu) is an important micronutrient, predominantly occurring as organic complexes in marine waters. The composition of the ligands forming these metal complexes has implications for the bioavailability of the trace metals to marine microorganisms, both as nutrients and toxicants. A variety of samples from estuarine, coastal and open ocean waters were studied in an effort to further our understanding of the ligands responsible for controlling the bioavailability of copper. Presented here are findings on the nature and likely identity of such ligands, including humic substances and various thiols. A novel method was developed to measure copper-binding humic substances, which were discovered to be synonymous with iron-binding humic substances. Using the new method on samples from the Mersey Estuary, humics were found to account for around 70% of the total ligand available for copper complexation in the estuarine and coastal samples. Samples from a very different estuarine environment, Sapelo Nature Reserve, Georgia, were then analysed in order to study potential copper limitation in blooms of Thaumarchaeota, prolific to the region and with a high copper requirement. Again, it was discovered that around 70% of the total available ligand for copper were humic substances, but that over 90% of the copper was complexed to thiourea-type thiols, also present in excess of the copper concentration. Comparing competitive ligand exchange (CLE) titrations to independent measurements of thiols and humic substances, the L1 and L2 ligand classes obtained via titrations were found to correlate very well with thiols and humics respectively, providing an indication of the nature of the ligands responsible for copper complexation. Furthermore, these findings suggested that copper was predominantly complexed (90%) as Cu(I), contrary to our current understanding of copper speciation. A study across the seasonal cycle at these stations provided further insight into the complexities of copper speciation. Cu2+ was found to be exceptionally low throughout the study, at sub-femtomolar concentrations, and lowest during the Thaumarchaeota bloom itself. Although previously considered to be limited by Cu2+ concentrations lower than 2 x10-13 M (when induced by the presence of artificial ligands), here it was demonstrated that Thaumarchaeota must be able to access the naturally complexed copper in order to bloom at these levels, posing questions for our understanding of copper bioavailability to these organisms. Finally, the copper speciation of two profiles from Line P of the Northeast Pacific, coastal station P4 and open ocean station P26, were assessed in an attempt to characterise the ligands in ocean samples. Thiols and humic substances were detected at both stations but at concentrations lower than the ligand concentrations measured from titrations, suggesting additional ligands play a part in copper-complexation in the open ocean.

Nickel based superalloys are an integral material for gas turbines, where their excellent high temperature mechanical properties and corrosion resistance are utilised. Due to the increasing costs of raw materials, manufacturers are interested in repairing in-service and manufacturing defects in components. Unfortunately, superalloys such as Rene 80 are highly susceptible to welding defects such as liquation cracking and post-weld heat treatment cracking, which make repair welding highly difficult. The aim of the research in this thesis was to develop an improved understanding of welding defect production in nickel-based superalloys. In particular, the effect of repair process and its parameters were examined, with the ultimate aim to produce crack-free repair welds. The main theme of the work involved a large parametric study of the process parameter effects on welding defects in Rene 80 using a high power fibre laser. This work determined an optimised range of parameters which reduced the incidence of cracking. Furthermore, this work also identified a key relationship between the weld bead geometry aspect ratio and the incidence of cracking. This relationship was studied using neutron diffraction to determine the differences in strain and residual stresses between two welds with identical heat input but different geometry. An in-depth investigation of the cracks within the material, identified that as-welded cracks formed via liquation of secondary phases such as carbides, γ/γ’ eutectics, and secondary gamma prime. The post weld heat treatment cracks formed by the strain-age mechanism in Rene 80. From this work, a novel repair procedure avoiding the complications associated with using lower strength filler metal was developed, based on the optimised welding parameters. Finally, a number of advanced low heat input welding processes were also investigated for repair of superalloys.

Austempered ductile iron (ADI) has a microstructure consisting mainly of high carbon austenite, bainitic ferrite and graphite nodules, produced by a two stage austenitisation and austempering heat treatment. The resulting microstructure gives these materials a combination of high strength and toughness, making them attractive for a wide range of applications. To increase surface hardness, ductile iron alloys can also be cast into chilled moulds to induce carbide formation in the required areas of components. These chilled ductile iron alloys can also be subjected to austenitisation and austempering heat treatments, therefore further improving the mechanical properties of the components core, whilst retaining the hard carbides present in the surface layers. This work encompasses three main areas: two are concerned with the production of generic microstructure models, which work in conjunction with thermodynamic modelling software MTDATA; and one relates to high temperature X-ray diffraction experiments.

No description available.

The oxidation behaviour of a series of iron-chromium binary alloys containing 5%, 9%, 12%, 15% and 20% chromium at 400-600°C has been studied in CO2 -1% gas mixtures. The experimental approach has involved kinetic studies using a microgravimetric method together with detailed characterisation of the oxidation products using optical, X-ray and electron optical techniques. Several oxides are formed during the initial stages of oxidation. The nature of the oxidation product is shown to depend on both the crystallographic orientation and the initial composition of the substrate. These effects are explained by considering the maximum solubility of chromium in different oxide phases together with interfacial and strain energy factors. The kinetics of oxidation together with micrographic observations clearly indicate that as the oxidation proceeds spinel oxide, M3O4 at various sites on the substrate surface. Such sites are associated with asperities on the surface. The spinel nuclei grow both laterally and vertically until they impinge and coalesce. The scale subsequently thickens according to a parabolic rate law. Examination of scales reveals a duplex struc-ture. This is interpreted in terms of an outward diffusion of cations together with simultaneous growth of an inner layer in the space created by the outward movement of metal. Both layers are porous and hence provide a route for gas phase transport of oxidant to support the growth of the inner layer. A series, regularly spaced, of lamellar voids form in the inner layer under certain conditions. This is believed to be associated with a cyclic vacancy condensation process and it is shown that the spacing between lamellar voids is consistent with such considerations. Enrichment of the inner layer in chromium also occurs and a model is proposed to explain this segregation effect based on an analysis of the possible diffusion path networks in close packed oxides.

Magnetoresistivity tensor components have been measured as a function of orientation and magnetic field dependence in antimony and its p-type alloys with tin and germanium at 77 K, 196 K and 300 K. A special minimization program has been used to obtain the model parameters (the components of electron and hole mobility tensors, the carrier densities and the tilt angles of the Fermi surface ellipsoids) for a two band, multivalley ellipsoidal Fermi surface. The validity of the field dependent tensor method has first been checked on antimony itself; the model parameters of antimony found from components of field dependent tensors have been compared with those obtained using the low field method of Oktu; and Saunders (1967). The results show that the field dependent tensor theory can be extended to the treatment of galvanomagnetic effects of antimony and its alloys. The existence of Umkehr effect in the magnetoresistivity of antimony and its alloys has been established; this phenomenon can be understood on the basis of field dependent tensor description transport properties. Then using the field dependent tensor method, extensive details of the temperature and concentration dependence of the carrier mobilities in antimony and its alloys have been obtained. It has been found that each tin or germanium atom removes one electron. The carrier mobilities in antimony alloys are dominated by ionized impurity scattering; the Born approximation is a better fit for these alloys than for bismuth-lead alloys (Bhargava 1967). The tilt angles of Fermi surface pockets are invariant with temperature and concentration.

Specimens of various mild steels were deformed in split bar compression jigs at elastic strain rates of the order of 10³ sec-¹. Yield was obtained in times of 3μsecs., the upper yield stress, Yd, increased linearly with the elastic strain rate, E to five times the value, Yd, obtained statically, and is given by:— Yd Ys + KεE The strain rate sensitivity factor, K, varied with material between 50 and 100 (when the stresses were measured in p.s.i.) The variation of the yield stress between + 70 and —196°C was small except at low temperatures. The results are discussed in terms of existing theories of the damping of fast moving dislocations. Twins were observed to form in constant stress rate tests and this is believed to be evidence of the existence of dissociated dislocations in the B.C.C. lattice. A new model of the modes of yield near a fast moving crack is developed using these results. It is shown that dynamic yield can occur at some distance from the crack while delayed yield occurs closer in. The increase of Klc, seen by other workers, as the crack velocity becomes large is explained using this model.