Solidification and heat-treatment related defects in single crystal nickel-base superalloys

This thesis examines the causes of surface scale, surface inter-dendritic phase formation and surface melting. These effects hinder component inspection and lead to extensive rework operations and component scrap. Surface scale is a region of colouration observed on the external surface of cast components. It has been shown to occur in the solid-state and to result from the differential thermal contraction between the mould and metal, driving separation during the casting process. The regions of the casting that separate from the mould are exposed to the casting atmosphere, resulting in the formation of a thin oxide interference film, or surface scale. The separation of the mould and metal in the solid state can cause the semi-solid ‘mush’ above to contract, resulting in segregated liquid flow towards the surface and a layer of inter-dendritic phases at the surface of the casting. Therefore a close relationship between surface inter-dendritic phase formation and the spatial occurrence of surface scale is observed. After heat-treatment a number of components exhibit ‘blisters’ of melted material at the surface, even in under-solutioned components. Surface melting frequently occurs within the scaled regions of turbine blade components, however the reasons for this were previously not understood. The increased risk of inducing melting of these lower melting point inter-dendritic phases on the surface associated with surface scale rationalizes this observation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo