Mitigating time-dependent crack growth in Ni-base superalloy components

• Develop a methodology to treat cycle- and time-dependent crack growth.
• Model the effects of tertiary γ′ and grain size on time-dependent crack growth.
• Treat small-scale creep, oxidation, and stress relaxation at the crack tip.
• Reduce the risk of fracture of a gas turbine disk via microstructural control.
• Large grain size and tertiary γ′ size at the rim extend disk life and reduce risk.

Advanced Ni-based gas turbine disks are expected to operate at higher service temperatures in aggressive environments for longer time durations. Exposures of Ni-base alloys to these aggressive environments can lead to cycle-dependent and time-dependent crack growth in superalloy components for advanced turbopropulsion systems. In this article, the effects of tertiary γ′ on the crack-tip stress relaxation process, oxide fracture and time-dependent crack growth kinetics are treated in a micromechanical model which is then incorporated into the DARWIN® probabilistic life-prediction code. Using the enhanced risk analysis tool and material constants calibrated to powder-metallurgy (PM) disk alloy ME3, the effects of grain size and tertiary γ′ size on combined time-dependent and cycle-dependent crack growth in a PM Ni-alloy disk is demonstrated for a generic rotor design and a realistic mission profile using DARWIN. The results of this investigation are utilized to assess the effects of controlling grain size and γ′ size on the risk of disk fracture and to identify possible means for mitigating time-dependent crack growth (TDCG) in hot-section components.