Co Effect on As-cast and Heat-Treated Microstructures in Ru-Containing Single-Crystal Superalloys

The effect of Co on the as-cast and heat-treated microstructures was investigated in two experimental Ni-based single-crystal superalloys containing low levels of Re and Ru. The experimental results indicated that increasing the Co content from 7.9 to 15.8 wt pct decreased the volume fraction of (gamma + gamma') eutectic and the solidification segregation ratio of W. High levels of Co additions were also found to decrease the solvus temperatures of the gamma' phase and (gamma + gamma') eutectic as well as the solidus temperature. During the long-term thermal exposure at 1373 K (1100 A degrees C), no TCP phases precipitated in either alloy. However, the coarsening and coalescence of gamma' precipitates in the alloy containing 15.8 wt pct Co was slower than that in the other alloy with 7.9 wt pct Co. In the current study, high levels of Co additions decreased the equilibrium volume fraction of gamma' phase, leading to a change in the partitioning ratios of TCP-forming elements Cr, Mo, Re, and W between the gamma and gamma' phases. This change resulted in a lower degree of elemental supersaturation in the gamma matrix and improved the phase stability of the gamma/gamma' microstructure. These experimental results were then compared with those obtained from multi-component thermodynamic calculations, and good agreement was observed.