Effect of minor additions on the microstructures and stress rupture properties of a directionally solidified Ni-based superalloy

The effects of C, Zr and Y/Ce on the microstructures and stress rupture properties of a directionally solidified Ni-based superalloys were investigated. The morphology of MC carbides does not change in as-cast alloys, as the C content is increased or the Zr is added. However, the Y/Ce additions promote the transformation of MC carbide morphology from blocky to script-like type. Compared with that in as-cast alloys, there is no obvious change in the morphologies of MC carbide or chemical compositions after heat treatment. The increasing of C content and the addition of Zr or Y/Ce significantly decreases and increases the amount of the γ/γ′ eutectic, respectively. After a solid solution treatment and subsequently a two-stage aging treatment, the γ′ phase is characteristic with two kinds of shapes and sizes, and the volume fraction of cuboidal γ′ phase is much larger than that of spherical γ′ phases. More MC carbide and residual eutectic result from the decreasing of cuboidal γ′ phase volume fraction. The decrease in volume fraction of cuboidal γ′ phase is principally responsible for the reduction of the stress rupture property. In addition, script-like carbide and residual eutectic remaining after heat treatment play the secondary role in the degradation of the stress rupture properties. The less regularly arranged cuboidal γ′ phase can damage the stress rupture life to some extent. The spherical γ′ phase re-dissolves into γ′ matrix widening the spacing between large cuboidal γ′ phase, and the addition of Zr inhibits grain boundary sliding and cracking; these two facts are beneficial to the rupture elongation.