Role of Re and Co on microstructures and γ′ coarsening in single crystal superalloys

Seven alloys containing different contents of rhenium and cobalt are investigated to explore systematically the effects of rhenium and cobalt on microstructures and γ′ coarsening. The alloys are exposed to different periods at 950 °C and 1050 °C. In the alloys containing rhenium, γ′ coarsening is controlled by diffusion. Diffusion coefficient rises with the decreasing of rhenium and the increasing of cobalt. In the alloys containing 4 wt.% Re and Co free, there is TCP phase precipitation when exposed at 1050 °C. Additions of 3 wt.% Co to the alloy suppress the formation of TCP phase completely. Co reduces or eliminates the adverse effects of Re. In the Re-free alloys, there is no TCP formation and the coarsening mechanisms are complex. The competition in rate-controlling between diffusion and interface reaction on coarsening kinetics is dependent on the elements and the diffusion behavior. A model describing the process of forming rafts is also established.