Low cycle fatigue behavior of Hastelloy X under asymmetric load at room temperature and 650 °C

Abstract

Hastelloy X has been widely used in aero-engine combustors for its great mechanical performance at high temperature. However, low cycle fatigue (LCF) induced cracking is inevitable for the combustor under asymmetric load at elevated serving temperatures. The fatigue behavior and its impacting factors at actual working environment, which are yet not very clear, are important for the design and maintenance of the combustor. To this end, the LCF behavior of Hastelloy X under strain ratio R = 0 was assessed at room temperature (RT) and 650 °C. At RT and 650 °C, Hastelloy X showed initial cyclic hardening, followed by different types of softening until failure. The former temperature induced significant cyclic softening, while the latter temperature only resulted in slight cyclic softening. At RT, increased dislocation density was responsible for initial cyclic hardening, and the reduction in precipitate size was responsible for lateral cyclic softening. At 650 °C, initial cyclic hardening was caused by increased dislocation density, dislocation-obstacle interactions, and dynamic strain aging (DSA). In the later stage of LCF, cyclic hardening was related to the precipitation of M23C6 at the grain boundaries and its pinning effect on dislocations, while cyclic softening was due to the reduction in precipitate size. As a result, slight softening occurred at 650 °C. The Ostergren model was used to predict the LCF life, which is in good accordance with the experimental data.