Microstructural Effects on Creep Properties in a Co-Base Single Crystal Superalloy

Abstract

In our previous study, Co-Al-W-Ta-Ti quinary alloys were found to show excellent creep resistance at elevated temperature. In order to clarify the influence of the initial microstructure on the creep behavior of a Co-base single crystal superalloy, the tensile creep behavior of the Co-base alloy with different initial microstructures was investigated at 900 °C and 420 MPa and the relationships between the initial microstructures and the creep properties, such as the rupture life and the minimum creep rate, were analyzed. It is suggested that the γ′ size increased with increasing aging time or temperature, while the volume fraction of this Co-base alloy remained similar through heat treatments at 900 and 1000 °C for 50–1000 h. Moreover, the initial dislocation density also increased with increasing aging time but varied negligibly with higher aging temperature. On the other hand, four distinct creep stages were observed during the creep process, and the rupture life exhibited an optimum peak as a function of γ′ size, which was also influenced by the initial γ/γ′ lattice misfit. Meanwhile, the minimum creep rate increased with an increase of the initial dislocation density. This study can serve as a guide for future microstructural design and optimization of Co-Al-W-base superalloys.