There are increasing interests to alternate the microstructure and hence the properties of steels that are applied in various environment conditions using a work-efficient and energy-saving manner. The desirable microstructure evolution is often not achievable by means of conventional thermo-mechanical processing and solid-state phase transition. This thesis has considered four fundamental engineering problems, namely (i) the possibility of anti-aging processing for the aged steels in service at high temperature, (ii) the recovery of the lost strength for the steels at high temperature, (iii) the suspension of crack initiation and propagation during cold-working of steels with eutectoid microstructures and (iv) the regaining of strength during tempering of a steel containing martensite. Phase stability in the processing environment is the primary concern in each of the list problems because it presents, in thermodynamically, the possibility to achieve the goals using the designed processing. Electropulsing processing has been considered and integrated with the conventional thermomechanical processing in the development of this PhD thesis. The so-called electropulsing treatment utilises electric current pulses with high peak current density and short pulse duration. Due to the nature of the short duration pulse, the energy consumption is very low. The high current density enables a very strong impact of electropulsing on the microstructure evolution and hence is work efficient. Following results have been obtained through the study: • Using the appropriate electropulsing parameters, the formed secondary phase (e.g. X-phase) by precipitation in 316L stainless steels at elevated temperature can be dissolved. Electropulsing processing can supress the precipitation and homogenize the alloying elements in the stainless steel. The stability of the secondary phases in the stainless steel has been changed by the imposed electropulse. • Electropulsing treatment is able to alternate the delta-ferrite phase transition. This has been proved in the treatment of 2205 duplex stainless steel. The new format of phase transition causes strengthening of the steel at high temperature. The stability of phases in the steel has been affected by the applied electropulsing treatment. • For the light steels containing high aluminium composition, electropulsing is able to affect the thermodynamic stability and grain morphology of κ-carbide. This leads to significant improvement of steel formability. • Application of electropulsing processing to dual-phase automotive steel changes the stability of martensite phase. The processing improve the mechanical properties and refined the microstructure of this steel. The fundamental understanding of the experimental observations has been developed based on the thermodynamic and kinetic analysis.

Electropulsing is a process characterised by the application of short duration (~100μs) electrical current pulses. The process is claimed to be capable of stimulating microstructure change in a variety of metals at low temperatures and energy levels. In this thesis, electropulsing is applied to examine its potential to produce nanostructured, strong yet ductile pearlitic steel. Electropulsing of cold-rolled 0.92wt%C pearlitic steel plate was conducted at room temperature across a range of variables (J ~ 10⁷ - 10⁹ A·m-2, f ~ 1 - 100Hz, d ~ 80 - 160μs, N ~ 1 - 90000), in order to identify the parameters required to induce microstructure change. A microstructure of equiaxed ferrite and spheroidised cementite was obtained under high frequency, current density and pulse duration electropulsing. Martensitic transformation and Widmanstätten structures were also observed. No microstructure change was produced by electropulsing treatments of low current density, frequency and pulse duration. Elevated temperature electropulsing tests on 0.73wt%C pearlitic rod, which combined pulsed current treatments (J ~ 10⁷ A·m-2) with furnace annealing (660-740°C), were conducted to attempt to overcome any initial energy barrier threshold, in order to highlight any current-based athermal effects. Grain size analysis did not reveal any variation between electropulsed and furnace treated samples. Secondary, transmission and backscatter electron microscopy were used for microstructure characterisation, and mechanical and electrical properties were also experimentally determined. Atom probe tomography was applied to select samples to analyse the steel composition after electropulsing. Resistive heating-based temperature effects were calculated and compared with similarly treated Gleeble-processed samples, to understand the impact of heating rate on the microstructures observed. Electropulsing was determined to have influenced the microstructure of pearlitic steel through resistive heating. An athermal mechanism could not be clearly discerned.

The high susceptibility to corrosion limits the broad application of magnesium alloys, and therefore, the corrosion protection of magnesium is of major concern in practical conditions. A great effort has been made in the last few decades to solve this problem. Various types of surface coatings have been developed to provide corrosion protection for magnesium alloys, among which plasma electrolytic oxidation (PEO) is one of the most promising techniques. The PEO treatment can produce a hard ceramic-like oxide coating on magnesium and its alloys, leading to significantly enhanced wear and corrosion resistance. However, the intrinsic porous morphology of the PEO coatings still limits their effect of corrosion protection. The objective of the present work is to overcome this microstructural drawback, and further improve the corrosion protection ability of PEO coatings on magnesium and its alloys. Different approaches have been adopted to reduce the degradation rate of PEO coatings, including optimisation of the PEO treatment itself, sequential processing combining PEO coating with various post-treatments and formation of smart self-healing PEO coatings inspired by biological systems. The PEO process was investigated by analysing the current/voltage transients, and the PEO coatings were systematically characterised by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). Fourier transform infrared spectroscopy (FTIR) was also used to study the chemical composition of the plasma enhanced chemical vapour deposition (PECVD) coatings on the PEO coated magnesium alloy. The corrosion resistance of the PEO coatings in 3.5 wt.% NaCl solution was investigated by the electrochemical methods, including open circuit potential (OCP) monitoring, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation scans (PDP). In addition, the mechanical behaviour of PEO coatings was also examined by scratch testing. It was found that both voltage and frequency have significant effect on the properties of PEO coatings, and a more compact coating was produced by pulsed bipolar voltage mode. The PECVD post-treatment was proven to be an effective method of improving the corrosion protection ability if appropriate precursors were used, as this method could cause both positive and negative effects. Coating degradation could also occur during immersion post-treatments, although the corrosion resistance of the PEO coating was also improved by the Ce deposition and benzotriazole (BTA) adsorption. In this case, a better corrosion protection was achieved by combining the PEO coating with Ce-based immersion post-treatment, as the insoluble Ce-containing compounds provided both sealing effect and the inhibition of cathodic reaction. Finally, the self-healing PEO coating incorporated with inhibitor loaded nanocontainers was developed and shown a good potential for providing a long-term corrosion protection for magnesium alloys, even though the corrosion resistance was not significantly increased compared with conventional PEO coating. However, none of the above approaches was perfect, indicating that there is still plenty of work to be done in the future.

The degradation of material surfaces is a 'valve' that has limited technological progress through the ages. Today's most challenging environments are those where component surfaces are exposed to the simultaneous or sequential actions of corrosion and mechanical wear. These phenomena can be mitigated by a better understanding of the interactions between the multiple degradation mechanisms and the systematic development of alternative material surfaces. In this work, PVD coatings based on the Fe-Cr-Ni, Cr-Ti, Cr-B-(N) and W-(B) material systems were developed using unbalanced magnetron sputtering and mainly applied to AISI 316L, Ti-6AI-4V and Ortron 90 substrates respectively. For Fe-Cr-Ni and Cr-Ti coatings, post deposition heat treatments were conducted to stimulate the formation of the cr-FeCrNi and Laves Cr2Ti intermetallic phases respectively. In all cases, the morphology, chemical composition, structure, mechanical properties, corrosion behaviour and damage tolerance of the various coated materials were investigated. Special emphasis was placed on the response of the coated materials and uncoated test-pieces to the simultaneous action of corrosion and mechanical wear that resulted from sliding an Ah03 ball against the coated and uncoated materials in a 0.9 wt%NaCI solution. For one case only, W-(B) coated AISI 52100, lubricated rolling/sliding contact durability was assessed. For the corrosion-wear tests, the removal and regeneration of the passive layer (type I corrosion-wear) dominated the material loss. A high coatings hardness for'Fe-Cr-Ni and CrTi coatings was often beneficial to reduce the mechanical wear but the corrosion due to wear frequently remained high. Partial post deposition oxidation of Fe-Cr-Ni coatings was very effective in reducing the latter. Also of importance was the roughening of the Ah03 counterface due to the formation of tribo/transfer films or grain pull out. The material loss for Cr-B-(N) coatings was dominated by a material transfer mechanism and Ah03 grain pull out resulted in high mechanical wear of amorphous Cr-Ti coating. W-(B) coatings generally had low material loss and their crystalline variants displayed a low p.. The latter became inappropriate for conditions which promote high W dissolution. In lubricated rolling/sliding tests W-(B) coated AISI 52100 performance was·dependent on coating crystal structure and boron supersaturation. The crystalline coatings with low boron supersaturation displayed the best rolling/sliding damage resistance.

This thesis describes the development and uses of equipment and methods based upon the use of quartz crystal microbalance (QCM) technology for measurements involving major Flow Assurance issues, namely wax, asphaltene and hydrate in addition to ice formation in processing facilities, deposition of diesel performance additives in injectors and evaluation of anti-depositional paint coatings. For wax, the use of QCM for accurate measurements of the solubility of wax in synthetic binary and quaternary mixtures of n-alkanes is demonstrated and validated against literature data and model predictions. The use of the QCM for measurements of wax appearance temperature (WAT) and wax disappearance temperature (WDT) for stabilised and live reservoir fluids is presented. The development of QCM based equipment for investigating the effect of temperature gradient on wax deposition tendency at ambient and high pressure is described. The development and validation of the application of QCM technology for comparing and optimising dose rates for wax inhibitors at atmospheric and high pressure is presented. Wax case studies employing the developed equipment and methods are included. In the case of asphaltenes the potential use of QCM based equipment for measuring asphaltene onset in standard solvent titration measurements is shown. In addition by comparing step-wise and continuous injection results, potential errors in asphaltene stability measurements are highlighted. QCM tests with live fluids show that asphaltene onset can be readily detected in reservoir fluids at high pressure/high temperature conditions. In addition reversibility of asphaltene deposition can be demonstrated. Measurement of the effectiveness of asphaltene inhibitor treatments in terms of reducing solids deposition is demonstrated at operating conditions. Asphaltene case studies using the developed equipment and methods are presented. With hydrates, the development of QCM based equipment for measurement of hydrate dissociation points is presented. The use of QCM to identify solids forming in a dew pointing and mercaptan removal unit is described. The development of high pressure/high temperature equipment to detect deposition of diesel performance additives in injectors is presented. Finally the evaluation of anti-deposition coatings for scale and wax is described.

No description available.

The research presented is part of a larger study, dedicated to investigating flow boiling in small to microchannels. The test facility, originally designed by Huo (2005) and since used by Chen (2006) and Mahmoud (2011), has been used to investigate flow boiling of R134a across a range of channel diameters and both seamless cold drawn and welded channels. These previous studies concluded that one of the reasons for discrepancies in reported data is the result of surface characteristics. The objective of this current study is to further investigate the effect of channel characteristics and changing the refrigerant to R245fa. Surface characteristics are investigated with stainless steel, copper and brass channels, all seamless cold drawn and 1.1 mm internal diameter. Experiments using R245fa were initially conducted in the same stainless steel channel used with R134a by Mahmoud (2011). This allowed for the surface characteristics to be negated and the comparison to be based purely on the changes in the thermophysical properties between R134a and R245fa. Experiments were conducted at inlet pressures of 1.85 and 2.45 bar, mass fluxes of 100 – 400 kg/m2s, heat fluxes from 1 – 60 kW/m2 and vapour qualities from 0 – 0.95. The test section surfaces were evaluated based on scanning electron microscopy (SEM) and confocal laser microscopy (CFLSM). SEM allowed for a visual inspection of the channel surface, with clear differences in the surface stricter evident. The surfaces were then compared based on two CFLSM profilers. The values of the surface parameters differed between the two profilers but the same trend was seen, brass being the roughest surface and copper the smoothest. Changes in the surface parameter values were found to be a function of the scan area, scan resolution and cut-off value. A borosilicate glass tube, at the test section exit, allowed for flow visualisation. Mahmoud (2011) reported bubbly, slug, churn and annular flow for R134a, with no effect of hysteresis. Churn and annular flow were present for R245fa with an increasing heat flux. This was a result of a higher surface tension for R245fa which facilitates annular flow. Hysteresis was evident for R245fa, with bubbly, slug, churn and annular flow seen with a decreasing heat flux. The hysteresis effect is a result of nucleation sites activating during the increase in heat flux and remaining activated as the heat flux is decreased. The activation of nucleation sites depends on the size, which was constant due to the same channel being used, and the wall superheat. The wall superheat is lower for R245fa which does not allow for the nucleation sites to be initially activated with an increasing heat flux. The same effect of hysteresis was evident for copper and brass. Differences in the exit vapour quality and heat flux at which flow patterns occurred were seen between the three materials. The heat transfer coefficient varied in both magnitude and trend between R134a and R245fa. Mahmoud (2011) reported an almost constant heat transfer coefficient with vapour quality at a higher magnitude than seen for R245fa. R245fa showed an increasing trend with vapour quality. Peaks in the heat transfer coefficient were seen to be a result of surface flaw, evident when plotting as a function of the axial location. The test section was reversed in orientation, moving the location of the peak from near the entry of the test section to near the exit. A similar heat transfer coefficient peak was seen at the same axial location, near the exit of the test section, confirming that the peak was a result of a surface flaw and a result of the flow developing. The heat transfer coefficient changed in magnitude and trend for copper and brass. The magnitude of the recorded heat transfer coefficient did not follow the same trend as the surface parameters. The heat transfer correlations in literature did not predict the increase in the heat transfer with vapour quality, performing poorly compared with R134a. The best correlation for the prediction of both refrigerants was that of Mahmoud and Karayiannis I (2012). The pressure drop for R245fa was over 300 % higher than that of R134a, with a steeper increase with heat flux. This is attributed to a higher liquid viscosity and lower vapour density for R245fa. The pressure drop was highest for the roughest channel, brass, but lowest for stainless steel which had the intermediate roughness. The smoothest channel, copper, showed the largest difference in the effect of inlet pressure on the measured pressure drop and the roughest surface, brass, the smallest difference. The effect of surface characteristics on pressure drop is greater than the effect of changes in the fluid properties with inlet pressure. Pressure drop correlations performed poorly for R245fa in comparison with R134a, with the majority under predicting the pressure drop. Only one pressure drop correlation included a function of the surface parameters, Del Col et al. (2013), but this correlation under predicted the effect of the surface parameters on pressure drop. There was no one correlation which gave satisfactory results for all three materials.

As surface engineering develops, the methods of applying coatings and the materials used have improved significantly. One method of coating deposition that is becoming increasingly popular is physical vapour deposition (PVD). This research investigates PVD coatings of single layered systems, dual layered systems and multilayered systems with varying layer thicknesses. The properties of the coatings are investigated along with the dependence of those properties on the coating materials, the coating architecture, the hardness and surface finish of the substrates used as well as the deposition conditions. Tests were carried out on the coatings to investigate their tribological and corrosion resistant properties compared against industry standard benchmark coatings of electrodeposited chromium and nickel respectively. The base materials chosen were as follows: CrN (hard wearing) and Graphit-iC (low friction) for the durable and low friction coatings; corrosion resistant IN625 and chromium for the scratch and corrosion resistant coatings. The results showed that multilayering can be an effective tool for increasing the hardness and scratch resistance of a coating system; however, no benefit was seen in terms of the wear and corrosion resistance of the selected coating systems through multilayering. In terms of corrosion behaviour of the coated systems, the coatings themselves were corrosion resistant, and therefore the system behaviour depends upon the barrier properties of the PVD films. PVD coatings contain growth defects which provide a route for exposure of the substrate to the corrosive media. Irrespective of the film architecture, the substrate surface finish was seen to play a significant role in determining defect density, resulting in a lower defect density for coatings deposited on substrates with a finer surface finish, which resulted in an improvement in the corrosion barrier properties of the resulting films.

The growth in the use and wear of Ti-based alloy components in mining and offshore explorations has led to a search for techniques to re-engineer such components for reuse. The most desirable method of restoring/protecting the component surfaces is by hard-facing to enhance longevity in service. Laser cladding is one of the viable techniques to achieve a thick coating on such components which involves the addition of reinforcing particulates to improve surface properties such as hardness, wear and erosion resistance amongst others. A fundamental study and understanding of the resultant microstructure-property of the laser clad, hard-facing composite becomes necessary. In this study, laser cladding of Ti-6Al-4V wire with Spherotene particulate reinforcement and laser cladding of modified pre-blend of Ti-6Al-4V and TiB_2 powder were undertaken. The resulting physical and microstructural characteristics, hardness, and performance characteristics of laser clad composites were investigated. Samples from the as-deposited laser clad composites were characterised using optical microscopy, scanning electron microscopy (with chemical microanalysis) and X-ray diffraction. Performance characteristics were examined via erosion testing of the laser clad Ti-6Al-4V/Spherotene using plain and abrasive water jetting, and tensile testing of the laser clad Ti-6Al-4V/TiB_2 composite. The results showed that a crack and pore free clad containing as high as 76 wt.% Spherotene in the Ti matrix was achieved at an energy density of 150 J.mm^-2, 275 mm/min traverse speed, 700 mm/min wire feed rate and 30 g/min powder feed rate. The microstructure of the laser clad Ti-6Al-4V/Spherotene is characterised by nano-sized precipitates of reaction products (W and TiC) uniformly distributed in a beta-Ti solid solution matrix. Matrix hardness is enhanced by the presence of the reaction products in the Ti ranging between 410-620 kgf.mm^-2. Moreover, the modication made to the 90 wt.% Ti-6Al-4V/10 wt.% TiB2 feedstock by attaching the TiB2 to Ti-6Al-4V allowed uniform distribution of reinforcing element in the deposited composite. The composite microstructure on solidication is characterised by TiB eutectic needle-like features uniformly distributed in a Ti-rich primary phase. The hardness of the composite ranged between 440-480 gf.mm^-2. Tensile tests showed that the mean elastic modulus of Ti-6Al-4V/TiB_2 composite is 145 GPa, which is a 27% improvement when compared to that of Ti-6Al-4V. Erosion test indicated that the Ti-6Al-4V/Spherotene composite offered as high as 13 and 8 times resistance of that of Ti-6Al-4V when subjected to PWJ and AWJ impacts respectively.

There is growing interest in Al-TM (transition metal)-RE (rare earth) amorphous alloys because of the combination of their good mechanical properties and corrosion resistance. However, the high critical cooling rate required to form the amorphous structure leads to difficulties in generating bulk amorphous material. Therefore, there has been greater interest in producing amorphous Al-TM-RE alloys as surface layers. In this study, wedge mould casting, laser surface melting (LSM) and large area electron beam (LAEB) surface melting were used to fabricate Al Co-Ce alloys in both crystalline and amorphous form. An eutectic Al 33Cu (wt.%) alloy was also used to quantify the solidification conditions based on the well-known relationship. The microstructures formed by different processes were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, image analysis and X ray and glancing angle X-ray diffraction. In particular, LAEB surface melting was found to be able to provide a sufficiently high solidification velocity for the generation of an amorphous layer on the remelted surface of bulk crystalline Al Co-Ce alloys. Experimental results show that the LAEB treatment can remelt, homogenise the multiphase crystalline starting material and generate a predominantly amorphous layer, although it also caused cracking of the treated layer. However, the cracking was largely reduced in the laser refined starting microstructure. Laser microstructural refinement also improved the homogenisation and amorphisation generated by the subsequent LAEB treatment. The temperature field of multi-pulse LAEB irradiated Al-Co-Ce and Al-Cu alloys was numerically simulated through a finite difference method. The simulation results were generally consistent with the experimental results. The corrosion behaviour of Al-Co-Ce alloys with different microstructures was studied through potentiodynamic polarisation tests. Al-Co-Ce amorphous layer exhibited an enhanced corrosion resistance compared to the crystalline counterpart, although cracking in the amorphous layer greatly influenced the effectiveness of the amorphous layer protecting the substrate.