Microstructure evolution and phase transformation in a nickel-based superalloy with varying Ti/Al ratios: Part 1 - Microstructure evolution

Nickel-based superalloy Waspaloy is a promising candidate material applied in power plant components for higher operation temperatures. Trial alloys based on Waspaloy with varying Ti/Al ratios (2.14, 1.44 and 0.74) were produced. The microstructure evolution, mainly focusing on strengthening phase γ’ and grain boundary carbides, were investigated and compared in these alloys during thermal exposure at 800 °C for up to 10,000 h. It is observed that lowering Ti/Al ratio leads to slight smaller average γ’ size and higher volume fraction of γ’. Moreover, γ’ in samples with low ratios kept spherical morphologies after long-term ageing compared to high ratio samples, which transformed to cuboidal shape in later ageing stage due to higher lattice misfit. A clear M23C6 carbides coarsening at grain boundary and growth toward grain interior in lower ratio samples were also observed during ageing process. The results showed higher thermal stability of lower ratio samples in terms of γ’ morphology and volume fraction evolution. This was reflected by the slightly higher hardness values in lower ratio samples in later ageing stages.