Microstructure and stress rupture properties of polycrystal and directionally solidified castings of nickel-based superalloys

A new directionally solidified Ni-based superalloy DZ24, which is a modification of K24 alloy without rare and expensive elemental additions, such as Ta and Hf, was studied in this paper. The microstructure and stress rupture properties of conventionally cast and directionally solidified superalloys were comparatively analyzed. It is indicated that the microstructure of K24 alloy is composed of gamma, gamma', gamma/gamma' eutectics and MC carbides. Compared with the microstructure of K24 polycrystalline alloy, gamma/gamma' eutectic completely dissolves into the gamma matrix, the fine and regular gamma' phase reprecipitates, and MC carbides decompose to M6C/M23C6 carbides after heat treatment in DZ24 alloy. The rupture life of DZ24 alloy is two times longer than that of K24 alloy. The more homogeneous the size of gamma' precipitate, the longer the rupture life. The coarsening and rafting behaviors of gamma' precipitates are observed in DZ24 alloy after the stress-rupture test.