Gum metal (Ti-36Nb-2Ta-3Zr-0.3O) is a recently developed multifunctional bcc titanium alloy that exhibits high strength, high ductility and high yield strain. The ideal strength (Tmax) of Gum metal is suggested to be comparable with the actual strength, implying that deformation can occur via ideal shear without any dislocation activity. Calculations indicate that C approaches zero when the average ela value is around 4.24 in Ti-X binary alloys, li is suggested that this is attained in Gum metal, whose e/a is 4.24. The effect of processing route and chemical composition on the deformation mechanisms and mechanical properties of Gum metal were also investigated. ed. A more cost effective processing route involving ingot metallurgy was trialled and the mechanical properties were comparable to the alloys produced via powder metallurgy.

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The subject of this dissertation is the study of the effects of fabrication history (prestraining, welding and heat treatment) on the mechanical properties of austenitic stainless steel thin wall boiler tubes. These tubes are usually cold bent to shape, and sometimes swaged, prior to interconnection by welding. The bends require solution heat treatment before welding. In addition, subsequent to welding, the residual stresses should be relieved. It is sometimes not practically feasible to follow these constructional practices strictly especially when a whole boiler is constructed as a single unit and becomes too large and complex and contains different tubing materials. As a result of this fabrication history, the mechanical properties of boiler tube materials can be significantly altered. Sample tubes simulating the fabrication steps were supplied by British Energy for this project. The primary aim of the study was to determine spatially resolved room-temperature tensile properties using digital image correlation (DIC) by testing cross-weld specimens machined from the thin wall welded tubes (with plain or prestrained base metal) before and after the heat treatment. The experimental procedure which is used to retrieve the tensile properties from these integrated tests was validated through finite element simulation. Digital image correlation, which is a full-field strain measurement technique, was implemented in order to obtain the local stress-strain curves from regions less than a square millimetre in area and to extract the corresponding local tensile properties such as offset proof strength. The variation of the 0.2% offset proof strength was successfully obtained along these specimens. Evidence of strain hardening due to constraint and weld thermomechanical cycles was found in the plain base metal near the weld pool and evidence of softening was seen in prestrained base metal. On the other hand, after the heat treatment, the effect of prestraining and welding is cleaned out and the strength along the specimen was almost homogenized. However, aswelded cross-weld specimens with prestrained base metal have demonstrated unusual local stress-strain behavior in the weld-affected region. For a better understanding of this behavior, tension test of a cross-weld specimen with a high strength mismatch between the weld metal and base metal was simulated using the finite element method. It was found that the strength mismatch in the specimen, in combination with the experimental procedure, may cause some anomalies in the local stress-strain curves. It was also confirmed that these anomalies are not very detrimental for the determination of the proof strength on the specimens with strength mismatch. Material characterization of the welds and detailed hardness surveys on crossweld specimens were performed. Plastic strain is known to be detrimental for high temperature performance of austenitic stainless steel tubes, therefore, the degree of the plastic deformation should be known before these tubes enter service. DIe, hardness, electron back-scattered diffraction and neutron diffraction (peak width and anisotropy strain) were used to determine the amount of plastic strain in the as-welded tubes. It was observed that there is a good agreement between the predictions of plastic strain in 20% prestrained and welded tube.

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The present study was motivated by a need to model and understand creep in an ageing power plant particularly when residual stresses are present. A number of methods of generating residual stress in laboratory specimens are presented. Both numerical and experimental studies were conducted. The finite element (FE) analysis used in the numerical study was validated with an extensive experimental study of residual stress field characterisation in the test specimens. Two novel means of generating highly triaxial residual stress field in the laboratory specimens were identified and were followed up with design and manufacture. The specimens with internal residual tensile stresses of high triaxiality were used in the subsequent creep study. Using specimens containing residual tensile stress field with variable triaxiality and plastic strain the influence of prior straining in the creep cavitation initiation was also highlighted. A mechanical strain relief deep-hole (DH) drilling residual stress measurement technique used in residual stress measurement of specimens with high triaxiality highlighted a potential difficulty in using a simple elastic approach to data reduction in the DH measurement method. A comprehensive FE study modelling the DH using an elasto-plastic analysis and an incremental trepanning process in contrast to a previous elastic analysis with a single-step trepanning process [George 2000] was carried out to study further the influence of DH process on the residual stress field. The reconstructed residual stress distribution obtained in the DH FE analysis considering the influence of the electro-discharge machining (EDM) trepanning showed a good correlation with the DH measured residual stress distribution in the components with residual stress field with high triaxiality. A further DH FE study of the repair welded pipe three material models were considered and the result was found to be highly sensitive to the material model selected. The DH FE study of repair weld revealed that the original residual stress distribution was generated even considering the cutting process of the DH method. Consequently the deep hole drilling technique was used to characterise the residual stress field in a number of selected locations in practical components including thick-section steel welds consisting of a welded nozzle component which was thermally aged at 550�C for 19,644 hours and a pipe with repair weld.

The mechanism of low stress creep of uranium dioxide is identified as that modelled by Ashby and Verrall. This is supported by the agreements between predicter and experimental creep behaviours and creep rates. Evidence from the microstructures of superplastically deformed specimens confirms the topological aspect of the Ashby-Verrall model in these specimens. Superplastic strains of about 100% were obtained without failure of the specimens in a batch of 2um grain size specimens. The microstructures of these specimens revealed an even distribution of fine pores at triple point junctions. A mechanism is proposed to account for the superplastic behaviour of these specimens.

Assessing the damage level of in-service components and obtaining material properties for welded structures exposed to creep is essential for the safe operating of power generation industry. Standard creep testing techniques require relatively large volumes of material for the machining of testing samples. For that reason they are not usually suitable for obtaining creep properties of in-service structures. It has been found that significant amount of the failures in welds exposed to elevated temperatures occur in an area formed due to the complex thermal and cooling cycles during the welding process. Because of this a different approach is needed for the derivation of creep properties from small amounts of metal. The small punch creep testing method is considered to be a, potentially, powerful technique for obtaining creep and creep rupture properties of in-service welded components. However, relating small punch creep test data to the corresponding uniaxial creep data has not proved to be simple and a straightforward approach is required. The small punch creep testing method is highly complex and involves interactions between a number of non-linear processes. The deformed shapes that are produced from such tests are related to the punch and specimen dimensions and to the elastic, plastic, and creep behaviour of the test material, under contact and large deformation conditions, at elevated temperature. Owing to its complex nature, it is difficult to interpret small punch creep test data in relation to the corresponding uniaxial creep behaviour of the material. One of the aims of this research is to identify the important characteristics of the creep deformation results from 'localized' deformations and from the 'overall' deformation of the specimen. For this purpose, the results of approximate analytical methods, experimental tests and detailed finite element analyses, of small punch tests, have been obtained. It is shown that the regions of the uniaxial creep test curves dominated by primary, secondary and tertiary creep are not those that are immediately apparent from the displacement versus time records produced during a small punch test. On the basis of the interpretation of the finite element results presented, a method based on the reference stress approach is proposed for interpreting the result of small punch experimental test data and relating it to the corresponding uniaxial creep data. Another aim of this study is to investigate the effect of friction between the sample and the punch as well as the effects of the basic dimensions, on the small punch creep testing data.

P92 steel is a high-alloy steel that has been specifically designed for operating at high temperatures (600°C - 650°C) and has found wide use in the power generation industry, particularly since 2005. For the successful installation and use of this advanced steel, all aspects of its behaviour, in terms of both metallurgy and in-service behaviour, must be investigated. Investigating all the relevant material aspects is beyond the scope of a single PhD, and so the Supergen consortium funds a number of projects working on different material aspects. The purpose of this thesis is to investigate, and seek a greater understanding of, the behaviour of welds in P92 steel so that their in-service behaviour may be better understood particularly the response of the material to post-weld heat treatments (PWHT), the optimum weld consumable composition and the microstructural development during creep-rupture. This understanding has been achieved through a combination of microstructural characterization, thermodynamic modelling and mechanical testing. Specifications for weld metals define a range of compositions; thermodynamic modelling has enabled a better understanding of how the composition affects the final microstructure of P92 weld metal (given that this work is based upon thermodynamic predictions, the understanding developed here is applicable to both parent and weld metal). Precipitation strengthening is important to the creep resistance of P92 and the modelling has revealed how precipitate levels vary based on composition. Using this knowledge, quality checks on P92 used by industry can better ensure the fitness for service of a material if an accurate composition is known; furthermore, this understanding will enable manufacturers to further tailor compositions to produce the strongest possible material. Following welding with P92 fillers, post-weld heat treatment is carried out, and there is a desire to perform this heat treatment close to the A1 temperature of the materials involved. As such, it is important to accurately know the A1 temperature of the materials being heat treated. A combination of thermodynamic modelling, experimental thermal analysis and microstructural characterization was used to investigate the key transformation of ferrite to austenite. This investigation focused on the effect of composition on the transformation temperature, A1, and the rate at which austenite could form during PWHT. An equation to predict the Ae1 temperature of P92 is produced and validated. The knowledge of how composition affects the A1 temperature is useful for both welds and parent material, enabling the design and selection of P92 material that will not undesirably transform during heat treatments. It is proposed that the equation for Ae1 allows the determination of maximum safe heat treatment temperatures and will reduce the likelihood of poor quality material entering service. Experimental work has demonstrated that during PWHT (or parent material tempering), equilibrium conditions are approached, confirming that Ae1 should be used to determine maximum heat treatment temperatures instead of the AC1 temperatures which are currently employed. Creep testing of three different weld consumables was carried out to determine which had the best properties for use in service, and to understand the microstructural features which controlled creep behaviour of these weld metals. Creep testing of weld metal has indentified that δ-ferrite causes early failure as the resulting precipitate-free zones (PFZs) are creep weak. The presence of localized δ-ferrite is caused by an inhomogeneous distribution of ferrite stabilizers, particularly tungsten within the weld metal, resulting in greater stability of δ-ferrite and its retention in the weld. Using this knowledge, alloy specifications of weld consumables and corresponding welding procedures can be improved to ensure a homogeneous distribution of elements so that localized weaknesses in a weld can be avoided. There is tentative evidence that tungsten plays an important role in the creep ductility of P92 and that variations in tungsten and silicon could lead to an optimization of creep strength. The outcomes of this thesis facilitate a better understanding of P92 parent metal and welds and provide results that are immediately applicable and useful to the power generation industry.

The core aim of this research project was to improve understanding of the effects of microstructure and crystallographic texture on the high strain rate plastic deformation behaviour of the industrially important Titanium alloy, Ti-6Al-4V. To facilitate this study, four rolled plates of Ti-6Al-4V, with varying thermo-mechanical processing histories, were provided by TIMET Corp., the world’s largest supplier of Titanium product. To determine the nature of each plate’s microstructure and the crystallographic texture of the dominant α phase, the four Ti-6Al-4V plates were microstructurally characterised using techniques such as optical microscopy and electron backscatter diffraction (EBSD). To determine the effects of the measured microstructures and textures on the strain rate dependent plastic deformation behaviour of the four Ti-6Al-4V plates, uniaxial compression and tension tests were carried out in the three orthogonal material orientations at quasi-static (10^-3 s^-1) and high strain rates (10^3 s^-1) using a standard electro-mechanical test device and split-Hopkinson pressure bars (SHPB), respectively. To provide further understanding of the effects of microstructure and texture on the plastic deformation behaviour of Ti-6Al-4V, this time under complex impact loading conditions, the classic Taylor impact experiment was adapted to include an optical measurement and geometry reconstruction technique. A novel experimental setup was designed that consists of an ultra-high speed camera and mirror arrangement, allowing the Taylor impact specimen to be viewed from multiple angles during the experiment. Using the previously mentioned optical measurement and geometry reconstruction technique, it was then possible to gain valuable, previously unobtainable, data on the deformation history of Taylor impact specimens in-situ, such as the major/minor axes of the anisotropically deforming elliptical specimen cross-sections as a function of time and axial position, true strain as a function of time and axial position, and the true strain rate as a function of axial position. The technique was verified by testing a specimen cut from the in-plane material orientation of a clock-rolled high purity Zirconium plate. The output measurements from a post-deformation image frame were compared with measurements of the recovered specimen made using a coordinate measurement machine (CMM), with analysis showing excellent agreement between the two techniques. The experiment was then carried out on specimens cut from the two orthogonal in-plane material orientations of one of the four Ti-6Al-4V plates. Analysis of the data from these experiments gave significant insight into the plastic deformation behaviour of macroscopically textured Ti-6Al-4V under complex impact loading. Recovered Ti-6Al-4V specimens from the outlined Taylor impact experiments were then sectioned along specific planes and microstructurally characterised using EBSD, with comparisons made between the pre and post-deformation microstructures. From this analysis, and the previously discussed geometry reconstruction technique, insight was gained into the effects of micro-texture on the general anisotropic plastic deformation behaviour of Ti-6Al- 4V plate materials and in particular the role of micro-texture on the formation of deformation twins. Finally, the understanding gained from these experiments, and a detailed review of the literature, was used to inform a novel, physically based material modelling framework, capable of capturing the effects of microstructure and texture on the strain rate and temperature dependent plastic deformation behaviour of Ti-6Al-4V. The model was implemented in the computational software package, MATLAB, and verified by comparison with the mechanical characterisation results from one of the Ti-6Al-4V plates. A number of frameworks are discussed for implementing the new Ti-6Al-4V model within finite element (FE) analysis software packages, such as ABAQUS, LS-DYNA and DEFORM. It is hoped that the new Ti-6Al-4V model can be used to optimise the design of Ti-6Al-4V components and structures for impact loading scenarios.