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The high clamping forces in lap-joints in the body and wings of an aircraft result in fretting fatigue damage at the contacting surfaces. Shot peening may provide an efficient technique to delay or eliminate fretting fatigue failures. In this study, the fretting fatigue behaviour of unpeened and peened aluminium alloy 2024 T351 in flat-on-flat contact with rectangular bridge pads of aluminium alloy 2024 T351 was investigated and compared with results from previous research where steel pads were used. Contact normal pressure was maintained at a constant value during the tests while the axial load was cyclic with a stress ratio of -1 and a sinusoidal waveform of 20 Hz. The friction force acting over the contact area was measured by strain gauges mounted on the bridges. There was a considerable reduction in fatigue strength due to fretting, although improved performance for the peened condition. In all cases, fretting fatigue life was found to decrease with an increase in the normal pressure, up to a critical value of normal pressure. Above this critical value, a further increase in normal pressure tended to increase fretting fatigue life. This behaviour was observed at three different axial stresses and indicates a changed sensitivity to pressure for the fatigue damage mechanism. In order to distinguish and quantify the parameters that control such a phenomenon, both an experimental and an analytical methodology have been utilised. Fretting fatigue life was investigated in terms of the contact region and the specific roles of friction, crack growth and surface modification. The test data was used to develop and assess a fracture mechanics model based on the mechanics of the fretting fatigue process, constructed to predict fretting fatigue life. Components of friction stress estimated from surface roughness profiles, axial stress and normal stress were combined to account for the biaxiality of the loading and the benefits of shot peening were modelled by incorporating the effect of a residual stress.

Super 13% Cr steels are used for offshore applications and have to meet strict tensile strength, yield strength, toughness, elongation, and hardness specifications. The mechanical properties of these steels are strongly dependent on the proportions of retained austenite and martensite, and consequently small changes in heat treatment parameters result in major changes in properties. In this work, a detailed investigation of the effect of heat treatment parameters on microstructural features, hardness and tensile properties of the commercial supermartensitic 13Cr-5Ni-2Mo steels in the asreceived and re-heat treated state were undertaken. The re-heat treatment was performed by re-austenitising the samples at 950 °C for 2 h, air-cooling, and then tempering (single and double temper) in the range of 550-700 °C. The effects of tempering temperature, time, and their combination P (P = (273+T (°C) x (40 + login t (h)) x 1000'1), the Holloman-Jaffe parameter, on retained austenite volume fraction, hardness and tensile properties were investigated. Retained austenite content increased with P to a peak value at P-36.9 and then decreased due to the formation of fresh martensite. However, the second temper increased retained austenite due to the re-transformation of fresh martensite to austenite. This resulted in refined grain size and a high dislocation density. An increase in P gave a decrease in the C and Ni content in austenite on tempering while the austenite grain size increased. The combination of these two effects led to a decrease in the stability of the retained austenite with P, as shown by the increased M. For the first temper at P > 36.9, the austenite present after the second temper was more stable than after the first temper as a result of re-distribution of C and Ni from the martensite to the austenite. 0.2% proof strength and hardness were inversely related to retained austenite content with P. Both hardness and 0.2% proof strength decreased linearly with increase in retained austenite content, but elongation showed the reverse trend. However, the slope of the relationships depended on whether fresh martensite was present or not. The results have shown that retained austenite with volume fraction between 16-30 vol% and size < 208 nm gives the optimum combination of strength, ductility and hardness.

This work forms part of the EPSRC Supergen 2 Lifetime Extension of Conventional Power Plant project. In particular this research investigates the remaining life of a number of ditferent YzCr YzMo 'i4 V power plant steels which have been in service for considerable lengths of time and which have subtle differences in chemical composition, microstructure and mechanical properties. The low alloy ferritic steels all operate in the creep regime and therefore resistance to creep is an important microstructural characteristic. There are a number of key questions which this research aims to address: Having endured this extensive time in service how do engineers within the power industry know that the component is still safe to run? Even as the components reach and often exceed their original design life, it is important to safely extend the lives of existing conventional plant in a bid to ensure that the country's electricity supply remains intact.

An investigation was carried out into the variability of tensile properties of A201 aluminium alloy investment castings. Fracture surfaces and microstructures of tensile specimens were examined by scanning electron and light microscopy. Chemical analysis of phases was carried out using energy dispersive x-ray analysis. Casting composition variations were measured using spark emission spectroscopy. The main factors that contributed to the variation of properties were incipient grain boundary melting during heat treatment, the macro segregation of chemical alloying elements and interdendritic porosity that had not sealed during the hot isostatic pressing (HIPping) process. Simple step plate castings were produced to investigate the effect of changes in section thickness on the local segregation of alloying elements. The compositional variations occur due to the flow of solute rich liquid through the dendrite interstices in the mushy zone. An investigation was carried out into the effect of titanium diboride particles on the formation of macro segregation and interdendritic porosity. It was found that by increasing the concentration of titanium diboride particles above 2.7 +/- 0.2 % the tendency for macro segregation was reduced to negligible levels. The formation of surface connected pores, as revealed by dye penetrant examination, was also reduced when this. concentration was exceeded. The change in the solidification behaviour is due to a lack of coherent dendrite formation in the mushy zone. This results in solidification contraction being compensated for by mass feeding rather than interdendritic fluid flow of liquid through the dendrite interstices. An additional effect of the presence of the particles was to prevent the formation of a low melting point'phase containing copper, magnesium and silver. This reduced the susceptibility of the alloy to grain boundary incipient melting during solution heat treatment. In the light of the findings of the experimental work the concept of casting reliability is discussed. The importance of preventing the variability of tensile properties by robust alloy design is highlighted. The future design and development of reliable components with regard to process specification and property prediction is discussed.

The process of solidification is fundamental to a wide range of engineering processes. In most practical cases, such as in grain refinement of aluminium alloys, heterogeneous nucleation is the predominant route by which the transformation of liquid to solid is initiated. The mechanism by which solid crystal grows from heterogeneous particles is widely investigated but not fully understood. This study explores a novel approach to investigating nucleation through the use of synchrotron X-ray scattering techniques. The novel approach provides unprecedented access to the interactions at the solid-liquid interface and permits in-situ data collection as phase transformations proceed. Liquid aluminium was used as a relevant test case, and solid Al2O3 and TiB2 substrates are used to represent, respectively, a model case and industrially relevant scenario for grain refinement. Experimental results reveal the undercooling required for nucleation in the Al/Al[subscript 2]O[subscript 3] system. They also provide detail on thermal expansion in the aluminium prior to melting. Crystal Truncation Rod analysis reveals the existence of a ‘transition layer’ between Al[subscript 2]O[subscript 3] and Al, which is suggested to arise to accommodate residual strain in the solidified aluminium. For the Al/TiB[subscript 2] system, an innovative sample preparation method facilitated investigation of the interface between the materials, providing an experimental model of a TiB[subscript 2] particle within an aluminium melt. Results indicate the proclivity for the formation of Al[subscript 3]Ti in the system, and lead to the proposal that this is likely to occur even in situations in which the assumed Ti content suggests otherwise.

Nickel-based superalloys are a class of materials that have been specifically developed for high temperature applications for use in aero-engines and power generation. The evolution of superalloy chemistry and solidification structure has made the control and prevention of defect formation in single-crystal components increasingly challenging. This study examines the formation and avoidance of Surface Scale and Surface Melting, both of which are detrimental to the production cost and the performance of single-crystal turbine components. Surface Scale is a defect found on the surface of as-cast single-crystal components identified as a region of discolouration. A systematic analysis reveals that Surface Scale forms in the solid-state during casting due to the combined effects of (1) differential thermal contractions between mould and metal and (2) transient oxidation of the exposed metal casting surface at temperature below ~1300°C. The resultant transient oxide film is 0.5-1μm thick whilst the remaining surface undergoes a mould-metal reaction during the initial stages of casting, at temperatures above the liquidus, producing a 1-2μm thick protective layer of Al[subscript 2]O[subscript 3]. Surface Melting is identified on the heat-treated blade surface as an area of localised melting within the Scaled surface area. Through an in-depth analysis of the microstructural evolution it was found that Surface Melting is a consequence of elemental evaporation of γ-phase stabilisers. The subsequent change in chemistry results in the transformation of the γ phase to γ’ and TCPs and a concomitant occurrence of incipient melting at the surface during solution heat treatment. This thesis concludes that the prevention of Surface Scale is made possible by the use of an inert atmospheric gas, such as Argon, within the casting furnace. However, the subsequent formation of Surface Melting during heat-treatment can only be prevented by pre-oxidising the as-cast components, thus forming Al[subscript 2]O[subscript 3] in order to form a protective barrier to evaporation.

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No description available.

No description available.