The effects of cobalt content (0, 3, 6, 9.5 wt.%) on microstructure and high temperature mechanical properties in a cast type nickel base superalloy Rene 80 (Ni-14Cr-9.5Co-4Mo-4W-3Al-5Ti-0.17C-0.015B-0.03Zr) were studied. High temperature tensile tests were performed at 680$^\circ{C}$ as a function of cobalt content in the conventionally cast equiaxed condition. It has been found that high temperature tensile properties and room temperature hardness were not changed greatly by varing cobalt content. The results that yield strengthes were unaffected with cobalt content in this work were explained by the theoretical model of Copley-Kear relationship.
Creep-rupture tests at 760$^\circ{C}$ (664 MPa), 815$^\circ{C}$ (450 MPa), 982$^\circ{C}$ (160 MPa) were performed as a function of cobalt content in conventionally cast condition. Creep-rupture tests at 871$^\circ{C}$ (343.2 MPa), 982$^\circ{C}$(160 MPa) were performed as a function of cobalt content in directionally solidified condition. It has been found that rupture times increased with the increased cobalt content from 0 to 9.5 wt.% in both conventionally and directionally solidified condition.
Microstructural examination showed that the increase of cobalt content resulted in increased volume fraction of gamma prime $Ni_3$(Al, Ti) precipitates. It was also found that the amount of carbide phases decreased with the increased cobalt content in this alloy. The increased rupture times with increasing cobalt content is attributed primarily to the increased amount of gamma prime precipitates and decreased carbide phases.
