Modeling Characterization of Time-Dependent Fatigue Crack Propagation in Ni-based Solid-Solution-Strengthened Superalloys

摘要

Ni-based solid-solution-strengthened superalloys, INCONEL 617 and HAYNES 230, obtain their high temperature strength and creep resistance mainly through solid solution strengthening of Cr, Co, W and Mo, and precipitation hardening of Ni3(AlTi)-type y' phase, Ti(C,N), M6C and M23C6 carbides [1]. Alloy 617 has been well characterized and selected as the prime candidate material for the intermediate heat exchanger (IHX) applications in the Very High Temperature Nuclear Reactor (VHTR) in next generation nuclear plant (NGNP) program, while alloy 230 is less characterized and considered as potential backup material for IHX. Preliminary studies have shown that both alloy 617 and 230 would display the time-dependent fatigue crack propagation (FCP) behavior at elevated temperature, when alloys were subjected to hold time fatigue (or called creep fatigue) [2]. The environment effect, SAGBOE (stress assisted grain boundary oxygen embrittlement), was thought to be responsible for the accelerated fatigue crack propagation rate at time-dependent stage. In this study, a semi-empirical model, based on thermodynamic theory, is employed to characterize the time-dependent fatigue crack propagation behaviors of alloy 617 and 230. A time-dependent factor is introduced to correlate the crack propagation rates with hold times and temperatures to predict the crack propagation process.