Alloying effects and the development of Co-Ni superalloys

Abstract

This thesis presents the development of cobalt/nickel-based polycrystalline superalloys for gas turbine applications. The optimisation of the hot isostatic pressing (HIP) process and subsequent heat treatments has been investigated. Additionally, two powder alloys V208E and V208F have been characterised and investigated to determine the effects of 1.5at% titanium and 1at% niobium substitutions. Finally six alloys based on V208C, developed by M. Knop [113], were developed and characterised. Additions of 1at% chromium, 1at% aluminium, 1 and 2at% molybdenum, 0.5at% niobium and a swap in the carbon and boron level (0.20 and 0.15at% respectively) were investigated. Introducing a lower temperature stage of 950°C during the HIP process caused the carbides that form to reduce in size from approximately 1μm to 80-150nm, as well as confining them to grain boundaries. STEM-EDS confirmed that the small carbides formed are rich in tantalum and zirconium. Upon cooling from above the γ’ solvus, cooling rates below 0.5°C/s were found to cause grain boundary serration, which resulted in an increase in room temperature strength of approximately 100MPa. Titanium was found to increase the γ’ solvus by around 30°C, whereas niobium had little effect. Both alloys had an acceptable low cycle fatigue (LCF) strength with a life of over 0.25 million cycles at 650°C and 774MPa, which was above the yield stress. Titanium was detrimental to oxidation performance, with a damage depth of 4.7μm, compared to niobium with 1.1μm. Both titanium and niobium improved creep strength compared to Knop’s alloy V208C. In the V208C-based alloy series, niobium and aluminium were found to increase the γ’ fraction and solvus, with molybdenum and chromium reducing them. All additions improved oxidation performance except niobium. Molybdenum and niobium were found to be particularly beneficial to high temperature strength. Substitution of carbon for boron significantly improved both high temperature strength and oxidation performance.