Phase stability and sigma (σ)-phase precipitation kinetics in a new wrought CrCoNi-based medium-entropy superalloy

To date, most studies on precipitation of topologically close-packed (TCP)-phases in superalloys have focused on commercial alloys with low volume fractions (< 5 vol.%). This makes it difficult to accurately determine the temporal evolution of TCP-phase volume fraction and thus to study its precipitation kinetics. In this work, we investigate a new wrought medium-entropy superalloy, which contains ∼16 vol.% of γ’-phase in the precipitation-hardened state. Isothermal heat treatments between 800 and 1025 °C for times up to 3000 h were performed. After long-term aging at 800 °C, the alloy contains a significant amount of a TCP σ-phase (∼16 vol.%), making it a well-suited superalloy for studying its precipitation kinetics. A novel segmentation approach is hereby introduced for the analysis of precipitation at grain boundaries and within grains, thereby facilitating the identification of elementary processes. Within grains, kinetics are initially governed by the diffusion-controlled lengthening of a fixed number of σ-discs and their subsequent hard impingement at later stages. Along grain boundaries, almost continuous σ-films precipitate, whose diffusion-controlled thickening is decelerated due to the presence of γ’-layers forming at σ/γ interfaces. The combination of stereology with these in-grain and grain-boundary-dominated elementary processes enables to establish time-temperature-transformation diagrams for single crystals, grain-boundary-dominated precipitation, and any grain size. The present methodology has the potential to set up a new standard for future studies of TCP-phase precipitation in various alloys. Furthermore, the models presented here may serve as a guide for predicting the precipitation kinetics of other TCP-phases in commercial superalloys.