Influence of type and morphology of carbides on stress-rupture behavior of a cast cobalt-base superalloy

Abstract

The relationships between the type and morphology of carbides and stress-rupture property have been studied in a cast cobalt-base superalloy. Different types and morphologies of carbides are achieved by varying the heat treatment conditions. After heat treatment at 1240 °C/4 h, the primary M7C3 decomposes into M23C6, and the formed M23C6 and primary MC partially dissolve into the matrix. After heat treatment at 1280 °C/4 h, the primary M7C3 melts, resulting in the formation of a highly developed lamellar structure of M23C6, and the primary MC mostly dissolves into the matrix. The heat treatment at 1100 °C/300 h gives rise to the carbide transformation of M7C3 → M23C6 and a profusion of secondary M23C6 carbide around M23C6 and MC. The samples in stress-rupture test at 980 °C for 83 MPa manifest the following rupture life: 1280 °C sample > 1240 °C sample > as-cast sample > 1100 °C sample. The longer rupture life of the samples of 1240 °C and 1280 °C is derived from the stable microstructure, dispersed MC carbide morphology and the supersaturated matrix. The shortest rupture life of the 1100 °C sample is mainly attributed to the overaging. Additionally, the carbide transformations of M7C3 → M23C6, MC → M23C6, M23C6 → M6C take place in the as-cast sample during creep exposure.