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

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 mu 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 degrees C and 750 degrees C during long-time exposure are M23C6, gamma ' phase and MC; for Alloy 2, they are M23C6, gamma ' phase, MC, M6C and mu phase. W addition is found to promote the precipitation of M6C and mu phase during exposure. M6C has higher W and lower Ni content than mu phase, whereas M6C is an unstable phase that would transform into M12C after 5000-h exposure at 750 degrees C. A great quantity of needle-like mu phases precipitated after exposure at 750 degrees C for 5000 h, which have no effect on the impact properties of Alloy 2.