An investigation has been undertaken to examine the effect of microstructure on the mechanical properties of FV520B, a precipitation hardenable martensitic stainless steel. This high performance grade of stainless steel was heat treated to three commercially available material specifications, namely the peak hardened, standard and softened overaged conditions. These three precipitation hardened conditions were found to exhibit a range of tensile properties. In order to determine the role of the microstructure, a full materials characterisation programme was performed. The investigative techniques used to characterize the microstructures, were Transmission Electron Microscopy (TEM); Analytical Scanning Electron Microscopy (ASEM); optical microscopy; dilatometry and X-Ray Diffraction (XRD). The microstructural phases and features identified were measured and quantified wherever possible. The effect of the material microstructure and environment on the fatigue properties of FV520B have been investigated. Fatigue tests were performed under uniaxial loading conditions at a stress ratio R (omin/omax) of -1. The tests were undertaken using highly polished specimens to determine the fatigue strength of the three precipitation hardened conditions. The test conditions employed were air and a corrosive 3.5% sodium chloride environment, at pH2 and ambient temperature. The role of the microstructure and the effectiveness of the tensile strengthening mechanisms on the fatigue and corrosion fatigue strength have been discussed. Using SEM, the fatigue crack nucleation mechanisms prevalent within the three microstructures in air and the chloride environment have also been identified. For the peak hardened material, nonmetallic inclusions dominated the fatigue crack nucleation process in air and chloride environments. For the softened overaged condition, multiple site nucleation due to slip band cracking was the prevalent mechanism especially at higher nominal stress amplitudes. The tolerance of this high strength material to small defects at higher stress levels and the actual size of the critical microstructural defects initiating failure have also been highlighted. The microstructure has been shown to strongly influence the processes of fatigue crack nucleation, Stages I and II crack propagation and the concept of the microstructure acting as barriers and providing resistance to crack growth have been discussed. The effectiveness and the size of these microstructural barriers to crack growth have been considered. This discussion has led to the proposal of a model that facilitates flow stress and fatigue lifetime predictions as a function of the quantity of a key microstructural phase. The key microstructural phase, namely reverted austenite affected both the tensile and fatigue properties of FV520B as a function of the heat treatment. The standard overaged material was found to exhibit the greatest resistance to fatigue crack propagation.

The prime design considerations for modern nickel based superalloys for use in aero gas-turbine engines, are those of mechanical performance, namely good resistance to creep and fatigue with good toughness and microstructural phase stability. Design of the current generation of superalloys has attained these properties at the expense of environmental resistance. This design philosophy has lead to the widespread use of surface coatings technology to protect hot-section componentry from the harsh operating environment. The ongoing drive towards higher operating temperatures has lead to an interest, over the last few years, in thermal barrier coatings (TBCs). TBCs are duplex coating systems consisting of a thin, insulating, ceramic layer over a metallic bondcoat. The bondcoat provides both environmental protection and the necessary adhesive interface to maintain the adherence of the ceramic during the rigours of operation. Central to the performance of a TBC system is the integrity and adherence of the alumina scale promoted by the bondcoat. This study aimed to design and optimise a novel bondcoat system that was capable of out-performing the current generation of bondcoats and progress the resultant coating into a production ready status. This was achieved by comparing the performance of a range of bondcoats of both novel and standard compositions, using the modified scratch test in conjunction with hot isothermal and cyclic furnace tests. The down selected system was then analysed using a range of techniques including optical and electron microscopy, XRD, WDS and SIMS in order to understand the failure mechanisms. The results of the testing programme lead to bondcoat chemistry changes and processing improvements that enabled better performance to be achieved. The bondcoat was optimised and taken to a production standard by using the Taguchi Method of fractional factorial experimental design. The resultant coating system offered a higher TBC/bondcoat interface temperature capability and extended the life of the system at more moderate temperatures, beyond that offered by systems currently available. The coating system has subsequently been run as a bondcoat for EB-PVD TBCs and has successfully completed the duty cycles on a number of development and test engines.

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