Evolution of precipitates in Ni–Co–Cr–W–Mo superalloys with different tungsten contents

Abstract

The Ni–Co–Cr–W–Mo system is critical for the design of nickel-based superalloys. This system stabilizes different topologically close-packed (TCP) phases in many commercially superalloys with high W and Mo contents. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermodynamic calculations were applied to investigate the thermodynamics of the precipitates in two different W-contained Ni–Co–Cr–W–Mo superalloys (Alloy 1 and Alloy 2). Computational thermodynamics verifies the experimental observation of the μ phase formation as a function of temperature and alloy chemistry, but the kinetics for the precipitation of the M₆C phase do not agree with the experimental findings. The major precipitates of Alloy 1 at temperatures of 700 °C and 750 °C during long-time exposure are M₂₃C₆, γ′ phase and MC; for Alloy 2, they are M₂₃C₆, γ′ phase, MC, M₆C and μ phase. W addition is found to promote the precipitation of M₆C and μ phase during exposure. M₆C has higher W and lower Ni content than μ phase, whereas M₆C is an unstable phase that would transform into M₁₂C after 5000-h exposure at 750 °C. A great quantity of needle-like μ phases precipitated after exposure at 750 °C for 5000 h, which have no effect on the impact properties of Alloy 2.