Formation and avoidance of surface defects during casting and heat-treatment of single-crystal nickel-based superalloys
thesisposted on 15.05.2014, 10:22 by Scott Simmonds
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.