Improving the High-Temperature Creep Resistance of a Cast Co-Cr-W-Ni Superalloy with Additions of B and Zr

In this study, the creep deformation behavior of conditioning-treated Co-Cr-W-Ni-based cast superalloy with and without B and Zr additions was investigated using constant load creep tests conducted at temperatures of 700 & DEG;C (973 K) and 800 & DEG;C (1073 K) and stresses ranging from 200 to 400 MPa. For both alloys, secondary creep strains were analyzed in terms of minimum creep rate and activation energy. Also, modified Monkman-Grant relation was used to analyze the experimental data, and a good correlation between minimum creep rate and rupture time was observed. Creep results revealed that the samples with B and Zr additions exhibited remarkably higher creep strain and longer creep life for the same loading condition. The failure mechanism was investigated with scanning electron microscopy (SEM), and the results show that the alloy without additions exhibits a failure mechanism caused by nucleation and void coalescence within skeleton-type M23C6 carbides. In the case of the alloy with additions, it was caused by nucleation and growth of voids in the interface of the matrix and bulk M23C6 carbides. Finally, electron backscatter diffraction (EBSD) results unveiled that the matrix shows low creep-induced plastic strain in the case of the alloy without additions. While in the case of the alloy with additions, extended plastic strains are seen in the & gamma; matrix that can be related to its higher creep strain, lower creep rate, and longer creep life.