Development of cobalt-based superalloys

This work presents an investigation into alloying effects on cobalt/nickel-based polycrystalline superalloys for gas turbine applications, focusing primarily on the Al/Cr ratio and C/B levels. Screening of the microstructure, physical properties and mechanical behaviour was carried out on alloys to elucidate the optimum alloying balance. Compositions between 10Cr-13.5Al and 13Cr-11Al were reviewed to determine the maximum Al and Cr that could be accommodated before five alloys with varying C and B contents were fabricated. Finally, two powder metallurgy (PM) alloys, V208G and V208H were characterised along with their ingot metallurgy (IM) counterparts. Alloying with more than 11.5 at.% Al led to extensive B2 NiAl precipitation, increased density, a reduced γ' solvus and diminished secondary γ' area fraction. NiAl did provide a beneficial effect to the alloy's oxidation performance, while increasing the Co:Ni dramatically decreased oxidation performance. This elevated Co:Ni was also found to be detrimental to a number of other alloy properties. Cr levels above 12.5 at.% were found to encourage decomposition of carbides after long term ageing at elevated temperatures. NiAl prevailed in alloys containing 13Cr-11.5Al, complicating interpretation of the slow strain rate tensile testing (SSRTT) behaviour. C was found to form Zr-rich MC and Mo-rich M23C6 while B promoted Mo-rich MB2 borides. Carbides were beneficial to SSRTT behaviour at 750°C, an increased C content encourages creation of carbo-nitrides (instead of C being lost as CO or CO2). Carbides at the surface experienced oxidation and volume expansion while their depths remained unoxidised. Atom probe tomography (APT) revealed unoxidised regions of carbides had nitrided. Formation of carbo-nitrides appeared to strengthen alloys during SSRTT. Scanning transmission electron microscopy energy dispersive X-ray spectroscopy (STEM-EDX) revealed that the oxidised carbides were enveloped in a layer of alumina. Borides oxidised, a discrete layer of B was found in the oxide scale in focussed ion beam mass spectroscopy (FIB-SIMS). V208G and V208H were fabricated via both IM and PM with compositions selected to build on the previous findings. V208H confirmed 11.5 at.% Al content is sufficient for B2 NiAl formation while V208G displayed no presence of NiAl, suggesting the optimum Al content lies between 11.5 at.% and 10 at.%. V208G had satisfactory oxidation behaviour without V208H's NiAl providing Al for alumina. Differential scanning calorimetry (DSC) indicated that the alloys have a small freezing range and an increased processing window compared to a current nickel-based superalloy.