Understanding surface and sub-surface phenomena in nickel-based superalloy castings

N/ANickel-based superalloys are high-temperature materials, created to operate in the aggressive conditions found within gas turbine engines. Single crystal turbine blades are investment cast, followed by a complex series of operations including heat treatment, machining, blasting, etching and polishing to produce a product ready for coating. Coating systems provide environmental protection, tribological resistance and enable efficient gas-path sealing. This thesis seeks to examine the root causes of various defects and features that render components unsuitable for service. The impact of increasing our understanding of these expensive materials is to increase process yield, and thus lower unit costs Investment casting of turbine blades is highly challenging; despite tight process control some variability in manufacturing operations results in scrapped components. Internal surface melting was found on intermediate pressure turbine blades following high-temperature heat-treatment; it consists of a layer enriched in aluminium and tantalum on the surface of the internal cooling passage. The discovery led to a systematic investigation of the silica-based ceramic cores used to form the cooling passage to better understand core / casting interactions. The silica-based core displayed a degree of pressure assisted liquid phase sintering at those locations where the nickel-based superalloy exerted a crushing force on the surface due to differential contraction after solidification. The areas affected matched that which later displayed surface melting. This was clearly related to casting height since the severity of pressure assisted liquid phase sintering could be mapped and decreased with the height of the casting. Further findings include the formation of a thin alumina layer at the interface between the superalloy and the silica-based core, this layer prevents volatilisation or sublimation from the surface of the component during the heat treatment. In the bespoke castings undertaken to further explore this phenomena clear evidence emerged of zircon particles, an integral constituent of the core, pinning the superalloy during the casting process. The result was a rough undulating surface with depressions formed by the hard, immovable zircon particles and peaks where the small silica grains where able to sinter and retreat against the pressure of the superalloy. A theory has been proposed that these zircon particles effectively indent the casting in specific locations at high temperatures. At the top of the casting where more effective stress and strain are experienced, this can translate into local stresses reaching the threshold for inducing recrystallisation during subsequent heat treatment.Rolls-Royce Plc, EPSR