To understand the formation of bimodal ferrite grain structures (i.e. mixed coarse + fine grain sizes) in TMCR HSLA steel plates, as-continuously cast HSLA steel slabs with varying microalloying additions (Ti, Nb and V) were reheated to temperatures in the range 1000-1300 °C and deformed at 1110 °C and 980 °C temperatures in Gleeble 3500. The interdendritic segregation during continuous casting and the consequent inhomogeneous microalloying precipitate distributions (with interdendritic regions being the precipitate-rich regions) has resulted in severely bimodal austenite grain structures under certain reheat conditions (due to the significantly higher pinning force in solute-rich regions compared to solute-poor regions). The segregation of microalloying elements can also promote bimodality during deformation by affecting the local recrystallisation kinetics. Notch-bend fracture tests were performed at –160 °C to investigate the effect of a bimodal grain size distribution on fracture toughness by comparing local fracture stress values for uniformly fine, uniformly coarse and bimodal ferrite grain structures. Analysis of local fracture stress values suggests that bimodality can raise the scatter in the fracture test results and therefore, it is undesirable. Current methods of measuring bimodality are not useful at consistently quantifying small differences in bimodality between microstructures of steel, and hence, two easy-to-measure parameters (peak height ratio, PHR and peak grain size range, PGSR) have been suggested in this study to quantify bimodality in HSLA steels.

In the current study, copper nanowires are fabricated by filling the nanochannels of anodic aluminium oxide (AAO) templates using electrodeposition. The nanowires with different growth orientations as well as twin boundaries in some cases are obtained. Cu micropillars are machined from bulk materials using focused ion beam (FIB) milling. The micropillars oriented in the [235] and [156] directions, with and without a twin boundary, with a cylindrical or square shape and with different sizes are prepared. These pillars are then compressed by a nanoindenter equipped with a flat tip, and the stress-strain curves are obtained. The morphologies of the pillars are observed using scanning electron microscopy (SEM). Thin foils of the pillars are prepared by FIB and examined using transmission electron microscopy (TEM) and scanning TEM (STEM). Finally, the dislocation structures are analysed to shed light on the investigation of the deformation mechanisms.

Since mechanical properties of cast Al-Si-Mg alloys are directly influenced by microstructural defects, understanding the characteristics of these defects and any interactions between them is important for improving the properties of castings. This research studied the effect of the most common defects normally found in cast aluminium alloys namely, double oxide films, Fe-rich phase, and porosity. A different level of each defect was introduced into the castings to investigate their effects on tensile properties with the results analysed by Yate’s algorithm and Weibull statistical analysis to determine their relative effects. The most important defect in this research was the oxide film that had an effect on UTS and elongation of about 21 MPa and 0.9% respectively and on the UTS Weibull modulus and elongation Weibull modulus of about 37 and 3 respectively. Increasing Fe content from 0.1 to 0.5 wt% influenced the tensile properties of the castings by decreasing the UTS and elongation by about 28 MPa and 1.7 % respectively. Although Fe-rich phases produced the greatest effect on tensile properties, their relatively high Weibull modulus showed that the reliability of the castings was at least predictable compared to the effects associated with oxide films. The porosity defects caused by an increase in hydrogen content from 0.1 to 0.45 ml/100g metal were the least detrimental to tensile properties. The most important interaction found in this research was the interaction between oxide films and porosity suggesting a mechanism for porosity formation in which entrained oxide film acted as initiation sites for pore formation in the castings. The main factor in the formation of porosity was hydrogen and shrinkage, since both could encourage the expansion of the oxide film defects to become gas porosity or shrinkage porosity in the castings. The other interaction between the microstructural defects observed in this research was that oxide films were found to be substrates for the nucleation and growth of Fe-rich phases, particularly the β-Al5FeSi phase. An interaction between all three defects were also observed and it further influenced the tensile properties of the cast Al-Si-Mg alloys by decreasing UTS and elongation by about 5 MPa and 0.5% respectively and the UTS Weibull modulus and elongation Weibull modulus of about 8 and 0.6 respectively.

Five API grade steels designed for linepipe applications produced using different processing routes and with varying microstructures were studied against differences in work hardening and work softening behaviour obtained from mechanical data. The rolling history and wt % additions of alloying elements will determine how the microstructures perform under reverse deformation schedules commonly seen during large diameter linepipe fabrication as steels can undergo work softening in the reverse direction of deformation, otherwise known as the Bauschinger effect. The Bauschinger effect is known to be dependent on the initial forward pre-strain, volume fraction (VF) of carbo-nitride particles and initial dislocation density. The effects of grain size and solid solution strengthening are a matter of debate in the literature and the combined effects of all five strengthening mechanisms have rarely been quantified. TEM investigations determined the dislocation densities to be between 2.2 x1014 m-2 - 5.8 x1014 m-2 in the as received condition. Observed trends presented and discussed in this body of work have given a greater insight into the influence microstructure has on the mechanical properties across a wide range of HSLA steels of similar strength grades, which are of important consideration for future development of low carbon steels designed for the petrochemical industry.

Blends of poly(ethylene terephthalate) (PET) and polycarbonate (PC) have been prepared by twin-screw extruder with and without added a transesterification catalyst - lanthanum acetylacetonate hydrate. The blends without added catalyst are completely immiscible over the study range while the blends with added catalyst show partial miscibility. The mechanical properties are slightly improved in the latter blends. Studies were made on the crystallization and melting behaviour of PET and both of blends. It has been found that the crystallisation was inhibited in the presence of PC, particularly in the blends with added catalyst for which the equilibrium m.pt. showed considerable depression greater than the other samples. Multiple melting endotherms are due to dual lamellar thickness distributions and recrystallisation during heating. Miscibility, melting and crystallisation were also studied by MTDSC which proved to be a powerful tool separating reversing and non-reversing events. The crystallinities of PET and blends were measured using the First Law method and consistent with those measured by density and WAXD procedures. Both blends annealed at higher temperature the thermal properties, phase structure and transesterification have been investigated as a function of time. Kinetics parameters were also determined.

To improve the efficiency of turbine aero engines, higher operating temperatures and weight savings are being investigated. Alloys such as RR1000 are being trialled as they perform better at higher temperatures than current nickel-based superalloys. To achieve weight savings, inertia welding is being trialled for turbine discs but current post weld heat treatments reduce fatigue life. In this investigation, a number of novel post weld heat treatments were trialled aimed at improving post weld microstructure and fatigue properties. Extensive microstructural characterisation and mechanical testing were used to assess the effect of these treatments on both parent and weld materials. Post weld heat treatment (PWHT) was found to have a great effect on the size and distribution of γ' and carbides, particularly when a PWHT temperature of 980ºC or above was used. The effect of this microstructural change on the hardness of the weld and parent material has also been characterised. Extensive total life fatigue testing was carried out at 650ºC. It was found that failure can occur in both the parent and weld material, although it is deduced that the yield stress of the weld needs to be surpassed to see weld failure (plasticity in the weld). Increasing dwell time at peak load reduced the life of these components. Two mechanisms for crack growth were observed with initiation either at the surface or at a large Hf rich particle subsurface. Oxidation was found to have a large effect on both initiation and growth of fatigue cracks. By introducing a sharp pre-fatigue crack into samples, static load testing was used to determine a threshold value of K (stress intensity factor) for growth and growth rates were plotted at different K values. It was seen in these tests that PWHT had a large effect on growth rates and threshold values of K.

Co-Cr alloys are the most widely used metallic biomaterials for metal-on-metal joint prostheses. However, concerns about increased revision rate associated with metal-on-metal replacements have been raised recently. This research was aimed at studying the response of medical grade ASTM Fl537 Co-28Cr-6Mo alloy with different phase constituents to low-temperature plasma carburising (LTPC) to address the tribocorrosion problems of the current metal-on-metal joint prostheses. To this end, n series of heat treatments were designed to produce samples with an a-FCC dominated (>95%), an c-HCP dominated (>97%) and a dual phased (40% c-HCP) microstructures. An optimised low-temperature plasma carburising (LTPC) treatment wns applied to the heat-treated samples. The microstructure of the plasma carburised surface layers were characterised by XRD, GDS, SEMIEDX and TEM, and the properties of the surface carburised layers were evaluated using micro- a•nd nano-indentation, reciprocating wear and electrochemical corrosion tests, and tribocorrosion tests a t different electrochemical potentials. The experimental results have showed that the response of the Co-28Cr-6Mo alloy to the LTPC treatment is highly dependent on the phase constituent. The LTPC treatment can significantly increase the hardness, load bearing capacity and dry wear resistance and tribocorrosion properties of all the Co-28Cr-6Mo samples with different phase constituents.

A study of the processing and characterisation of sintered NdFeB magnets made from recycled feed stock was undertaken. Initially the hydrogen decrepitated (HD) powder was investigated using two different milling techniques. The powders were analysed with optical microscopy, with the aid of a magnetic field. It was found that with light milling the HD powder breaks up to a similar particle size to that of the grain size of the starting material. A data logging system was built to investigate the desorption behaviour of green compacts during sintering. Desorption traces showed desorption from the matrix phase and the intergranular Nd-rich phase. The start of desorption was seen to shift to lower temperatures as the mean particle size of the green compact was reduced. For the processing route used in this work intergranular additions of neodymium hydride were required to increase the density and magnetic properties. To investigate the oxidation behaviour of lightly milled HD powder, powder was exposed to air for varying times. The exposed powder was aligned pressed and sintered. The Nd-rich desorption peak reduced with exposure time, the density and magnetic properties also reduced. Post exposure additions of Intergranular neodymium hydride to the powder recovered density and magnetic properties.

High strength aluminium alloy AA2024 is susceptible to localized corrosion in the form of pitting and intergranular corrosion (IGC). The corrosion behaviour of this alloy is strongly influenced by the presence of different intermetallic particles. In this study, the effect of applied stress and the role of intermetallic particle removal by surface treatment on the initiation and propagation of localized corrosion are investigated. It was found that applying a stress to as polished AA2024 leads to a drop in breakdown potential. The development of micro/nano crevices adjacent to the intermetallic particles is thought to act as initiation sites for localized corrosion, lowering the breakdown potential of stressed AA2024. The electrochemical behaviour of specific intermetallic particles and the particle free matrix of AA2024 have been studied as a function of stress state using the micro-capillary electrochemical cell in combination with an in situ stressing stage. These experiments emphasized the role of intermetallic particles as the possible key contributing factor in determining the corrosion properties of the alloy under stressed conditions. In situ X-ray synchrotron microtomography experiments were used to observe the evolution of corrosion attack as a function of continuous exposure time in both unstressed and stressed AA2024. Quantitative growth rate measurements indicates that application of stresses of 70% yield strength or above has a significant effect on the localized corrosion propagation.

In this study, the inertia welding process was studied by both an FEM model and three analytical models. The thermal analysis shows that there is a steep temperature gradient near the mating surface, which is the cause for the existence of a band of high hydrostatic stress near the weld line. The holding effect of this high static stress is the reason for the presence of the very soft material at the welding interface. The models were used to predict the displacement of the weld line (upset) with a lambda model to describe the constitutive relation of IN718 at high temperature. The results from the different models are in broad agreement. The shear stress induced by friction at the interface is found to enlarge the upset value; its effect must be taken into account if the upset is to be predicted accurately. The extrusion of the burr during the last second of the welding is a direct result of the quick stop of the rotating part due to the balance of the momentum, which is clearly explained by the analytical mechanical model put forward in this work.