Microstructure evolution and improved mechanical properties of a cobalt modified nickel-based superalloy

Effect of cobalt on microstructures and mechanical properties of Nimonic 80A has been investigated. Room temperature tensile strength increases by about 8.8%, and stress-rupture life at 750 degrees C/310 MPa increases by about 53.0% of 5Co alloy. Area fraction of gamma' linearly increases from about 59.6% to 65.6% and average diameter of gamma' significantly decreases from about 21.0 to 6.6 nm with the increased Co content after full heat treatment. The gamma' exhibits a coherent orientation relationship with gamma on {100} and {110}. Area fraction of gamma' decreases and average size of gamma' increases after stress-rupture test for each alloy due to the coarsening of approximately spherical gamma' particles, but addition of Co can stabilize gamma' phase because the average size of gamma' in 5Co alloy is smaller than that in 2Co alloy. The lattice misfit of gamma'/gamma increases with the increased Co content both before and after stress-rupture test, but the lattice misfit for each alloy slightly decreases after stress-rupture test compared with that after full heat treatment. The addition of Co can easily lead to the appearance of numerous dislocations, and significantly varied interplanar spacing of {111} and {200} in gamma phase. The blocky Cr23C6 carbide prefers to precipitate at GBs with an orientation relationship with the nearby gamma matrix on {100}, the Cr23C6 carbide can suppress the movement of dislocations and grain boundaries, which benefits better mechanical properties.