Microstructure of a rapidly-solidified Ni-base eutectic superalloy

Abstract

A γ- γ’ -MC eutectic nickel-base superalloy was rapidly solidified by melt-spinning. The resulting ribbon microstructure was studied using light microscopy and transmission electron microscopy. In the as-spun ribbon, a chill zone was found at the surface in contact with the wheel, consisting of grains 0.2 to 2.0 μm in diameter with Ta-rich MC carbides, 25 nm in diameter, decorating the grain boundaries. An intragranular cell structure was seen with 10 to 20 nm MC carbides at cell walls. A uniform dispersion of γ’ particles, 20 to 40 nm in diameter, was also present. In the ribbon center, a columnar dendritic structure was observed, with a slight increase in grain size. Carbides were found in this region at grain boundaries, along secondary dendrite arm boundaries, and in extended intercellular regions. A randomly-oriented dendritic structure was present at the ribbon top surface. The chill zone structure and the cellular-to-dendritic transition are described in terms of classical solidification models. Heat treatment of the as-spun ribbon for 2 hours at 1000 °C caused significant coarsening of the γ’ particles, but the grain size and carbide size and distribution were only slightly altered. An additional Re-rich sigma phase precipitated at grain boundaries during the heat treatment. After heat treatment for 2 hours at 1200 °C, some grain growth occurred, the MC and γ’ particles coarsened, and the Re-rich sigma phase was found.