The effect of Co on the microstructure and stress rupture properties of a single crystal superalloy

The influence of cobalt (Co) on the as-cast microstructure of an Ni-based single crystal superalloy and its chemical segregation, incipient melting temperature and stress rupture properties was investigated. The results show that the dendrite arm spacing in the alloy did not vary with the addition of Co. The γ' size in the as-cast microstructure decreased with increasing Co content. The segregation ratio of the γ'-forming elements titanium (Ti) and tantalum (Ta) became higher in alloys with high Co content (5%, 10%), while the segregation ratio of the γ-forming elements chromium (Cr) and molybdenum (Mo) first increased then decreased with increasing Co content, attaining the minimum around 10% Co. The incipient melt temperature decreased with increasing Co content. The stress rupture life under 1010°C/248 MPa increased with the increasing of Co content up to 10%, but decreased at 15% Co. The related mechanisms are also discussed in this paper.