Microstructure, thermal stability and tensile properties of a Ni–Fe–Cr based superalloy with different Fe contents

Abstract

The effects of Fe content on microstructure, thermal stability and tensile properties of a Ni–Fe–Cr based superalloy, mainly chosen as turbine rotor material in 700 °C advanced ultra-supercritical power plants, have been systematically investigated. The results show that the major precipitates of the heat-treated alloys with different Fe contents are all γ′ phase, MC and M23C6 carbides, whereas the precipitation of γ′ phase is suppressed in Fe-rich alloys. Fe content significantly improves the ductility of alloys, but slightly deteriorates the strength resulting from the decreased volume fraction of γ′ phase. After long-term thermal exposure, the morphology transformation of γ′ phase from spherical to cubic shape is observed in the alloy with 35% Fe content. The coarsening rate coefficient of γ′ phase decreases from 485.7 nm3/h to 280.8 nm3/h as increasing Fe content from 15% to 35%. The main reason is that there is a large concentration gradient of Fe element in the vicinity of γ/γ′ interface, which obviously restricts Fe atoms from diffusing away. The change of yield strength with particle size can be explained by the strongly coupled dislocation shearing and Orowan bowing models. When Fe content is up to 35%, the yield strength of the alloy reduces more rapidly than the theoretical value due to the precipitation of harmful Laves phase in the intermediate stage of thermal exposure.