Microstructural characterization of rapidly solidified nickel-base superalloys

Several Ni-Al-Mo-based eutectic superalloys were rapidly solidified using a chilled block melt spinning process. The effects of rapid solidification on the microstructure were studied using optical microscopy, transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). Results showed, except for the alloy containing chromium, that the microstructure varied as a function of ribbon thickness from segregationless solidification at the wheel side of the ribbon to dendritic solidification at the free side. In addition, alloys with the same solidification rate showed a large variation in microstructure depending upon the solid state cooling rate. The rapidly solidified eutectic Ni-Al-Mo alloy with a small amount of rhenium and vanadium did not show any improvement on delaying or prohibiting the formation of the embrittling-NiMo phase on ageing at 1000 C. This was determined from microstructural as well as chemical analysis using STEM. Differential thermal analysis was used to obtain melting temperature,-Ni₃Al solvus, and heat of formation for the alloys.