Two Steady-State Creep Stages in Co-Al-W-Base Single-Crystal Superalloys at 1273 K/137 MPa

With the aim of understanding how to further improve the creep resistance of Co-Al-W-base superalloys at higher temperature (> 1223 K), the creep behavior of a novel Co-Al-W-base single-crystal superalloy at 1273 K/137 MPa was systematically investigated. Several interrupted creep tests were performed and the main deformation mechanisms of characteristic creep stages were identified and correlated to both the gamma/gamma' microstructural and dislocation substructural evolutions. Two steady-state creep stages were observed during the creep process. The interfacial dislocations and the formation of rafts were responsible for the first steady-state creep stage, while the second steady-state creep stage could be ascribed to interactions between stacking faults of different slip systems in an inverted gamma/gamma' microstructure, e.g., the formation of Lomer-Cottrell locks by the leading partials of the SFs. The observed creep mechanisms were compared to those typical of Ni-base superalloys at similar creep conditions. It is suggested that a high stacking fault energy of the gamma' phase and a higher gamma' volume fraction may be favorable for creep resistance of Co-Al-W-base single-crystal superalloys by promoting the double steady-state creep phenomenon.