Microstructure evolution, failure mechanism and life prediction of additively manufactured Inconel 625 superalloy with comparable low cycle fatigue performance

Abstract

During hot end components service, i.e., startup and shutdown processes, the cyclic stress caused by the asymmetry loading conditions is introduced. To study the effect of mean stress on the cyclic response, the low cycle fatigue, failure mechanism and life prediction model of additively manufactured (AMed) Inconel 625 superalloy at room temperature (RT) and service temperature (600°C) with strain ratio R = −1 and 0.1 are conducted. Experimental results indicate that the different cyclic softening and hardening behavior at RT and 600 °C is the result of competition between dislocation slip, back stress hardening and second phase precipitation. According to the evaluation of the internal stress variables, the long-range back stress plays a crucial role to control the cyclic behavior of the alloy. At RT, the heterogeneous grain structure introduced by additive manufacturing causes the back stress hardening, making the material exhibit high fatigue performance. But this hetero-deformation induced (HDI) strengthening is inhibited at high temperatures due to the uniform distribution of dislocations. In addition, a low cycle fatigue life prediction model based on critical plane method is established which presents an easily assess to evaluate the low cyclic fatigue life of an AMed material under various test condition.