Understanding Creep-Fatigue Interaction in Fe-25Ni-20Cr (wt%) Austenitic Stainless Steel

Abstract

Gen-IV nuclear reactors require materials to operate under much harsher conditions necessitating the development of advanced structural materials. Sodium-cooled Fast Reactor (SFR) is a Gen-IV nuclear reactor with a high level of technology readiness. Alloy 709, Fe-25Ni-20Cr (wt%) alloyed with Nb and stabilized with nitrogen, is an advanced austenitic stainless steel having promising set of properties for SFRs. However, the creep-fatigue deformation behavior is unknown for this alloy. This work focuses on evaluating creep-fatigue interaction in this alloy. We report here a creep-fatigue study conducted at 750 °C using triangular waveform following the ASTM standard E2714–13 at 0.1 Hz frequency. The creep-fatigue tests were conducted in strain-controlled mode where strain amplitudes of 0.2% and 0.5% were used. The microstructural examination in as-received and post-deformation conditions were carried out using scanning and transmission electron microscopes. The creep-fatigue lives at 0.2% and 0.5% strain amplitudes were 17,416 and 526 cycles, respectively. The comparison of the TEM micrographs between as-received and creep-fatigue deformed at 0.5% strain amplitude showed higher dislocation density and loosely defined subgrains in the deformed alloy. The fractographs indicated that for the samples deformed at 0.2% strain amplitude, fatigue might have been the dominant mode of deformation whereas for the sample deformed at 0.5% strain amplitude, fatigue and creep both might have contributed to the overall deformation of the alloy.