The continuous advancement of Ni-based superalloys is crucial for achieving net- zero climate goals by enhancing jet engine operating temperatures and thus, their efficiency. The need for the development of novel compositions is of particular importance for turbine discs, where operating temperatures in excess of ∼800 ˚C are now sought, with adverse effects on mechanical performance, environmental stability, and microstructural control. Early studies on Co-based superalloys highlighted their excellent high-temperature strength due to the γ′-Co3Ti phase but found them unsuitable for turbine discs due to instability above 800 ˚C. Combining Ni-Al and Co-Ti systems enables the design of γ-γ′ alloys with enhanced strength, stability, and oxidation resistance. This work investigates the alloying effects of Ta and Ti co-additions, at a constant concentration of Ti+Ta = 6 at.%, on the microstructure, mechanical, and oxidation behaviour of three high Co-containing polycrystalline Ni-based superalloys for turbine disc applications.
In Chapter 3, the microstructure of the alloys was characterised to assess the impact of varying Ta and Ti contents on γ′ volume fraction, phase composition, and elemental partitioning. Results revealed differing phase partitioning between secondary and tertiary γ′ precipitates, and highlighted the positive effect of increased Ti concentrations on the γ′ volume fraction and precipitate size.
In Chapter 4, in situ synchrotron X-ray diffraction captured the load partitioning behaviour between γ and γ′ phases at high temperatures. The deconvolution of fundamental peaks provided insights into the micro-mechanical response under varying orientations, temperatures, and compositions, thus enabling an improved understanding of the effect of Ti:Ta ratio on bulk alloy tensile behaviour.
In Chapter 5, all alloys were subjected to isothermal exposures for 100 and 1000 hours at 800 ˚C to observe microstructural evolution due to oxidation, where the intermediate Ta and Ti alloy exhibit superior oxidation resistance at 100 hours, and the low Ti alloy after 1000 hours. After 1000 hours exposure, extensive internal oxidation and Nitrogen-rich precipitate formation beneath the alumina sub-scale was observed.
Importantly, the results from this work highlight the critical importance of experimental assessment and validation of thermodynamic calculations, providing fundamental insights that will guide the development of the next generation of powder metallurgy alloys for Rolls-Royce.
