Microstructural stability and creep of elevated temperature alloys

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Creep refers to time dependent strain - or slow shape change during extended periods of loading. Creep deformation and failure afflicts many elevated temperature applications of engineering materials, such as gas turbine engine components and many heat exchanger structures. Undertaking laboratory creep tests is relatively straightforward, but understanding the physical metallurgical factors controlling creep as a function of the applied temperature and load is much more challenging. Several theories to explain the creep of metallic crystalline structures have been developed which rely on the response of crystalline defects and crystal grain boundaries to explain the accummulation of strain during creep. The research proposed will systematically investigate the influence of grain boundary structure on the primary creep response of three alloys designed for use at elevated temperatures. In particular, the grain boundary structure will be modified from 'planar' to 'serrated' morphologies. It is reasonably well established that serrated boundaries delay creep failure by hindering the growth of creep cracks along boundaries that are normal to the main loading direction. However, very little is understood regarding the influence of grain boundary morphology on the primary creep behaviour - the initial period during which the rate of strain accummulation continuously decreases. For many engineering applications the duration of primary creep determines component service life. The result of this research will be an improved understanding of the factors controlling creep of engineering alloys. This understanding can be applied to the microstructural design of alloys with improved creep resistance.