Microstructure evolution and phase transformation in a nickel-based superalloy with varying Ti/Al ratios: Part 2 – Phase transformation

Nickel-based superalloy Waspaloy is one of the promising candidate materials for power plant components served in higher operation temperatures. Experimental alloys based on Waspaloy with varying Ti/Al ratios were produced and subjected to thermal exposure at 800 °C for up to 10,000 h. The phase transformations, mainly focusing on the η-Ni3Ti phase precipitation behavior and decomposition of two types of MC carbides during the prolonged ageing process, were investigated in this paper. η phases precipitated in the center of extra-large γ’ (EL-γ′), with or without facilitated by the small size MC carbides that were embedded within EL-γ’ as well. There were different decomposition routes for MC carbides with different sizes. Most of the MC carbides with larger size followed the usual transformation to M23C6 carbides, whilst the decomposition of MC carbides with smaller size was related to the nucleation of η phase and/or other phases such as σ phase in some cases. It is found that slightly lowering Ti/Al ratio (above 0.74) could not rule out the precipitation of η phase. The precipitation kinetics of η phase were discussed in terms of the density and stability of small size MC carbides. Furthermore, the effects of Ti/Al ratios on η formation were evaluated.