Alloy 617의 저사이클 피로 거동에 미치는 변형률 속도의 영향

Abstract

Today the development of the generation-IV Nuclear reactor’s safety and economics are much more improved the current one. The very high temperature reactor (VHTR) is one of the most promising Generation-IV reactor types for the economic production of electricity and hydrogen. The VHTR may merge diversities of this baseline design to allow eventual operation at gas outlet temperatures up to 950℃. Alloy 617 is a nickel-based superalloy, and is considered as one of the candidate materials on the intermediate heat exchanger (IHX) for the very high temperature reactor. Such a startup and shut down on the power transmission, or temperature change of flowing coolant with a low loading rate of reactor system may impose a low cycle fatigue (LCF) and creep-fatigue (CF) damage in nuclear system major components. Many attempts have been made in the past two decades to evaluate the LCF and/or creep-fatigue behavior in Alloy 617 at ambient and elevated temperatures. Based on their results, it is very difficult to separate the effects of temperature, strain rate and environment as they have complex interrelationships. However, less experience with the LCF behavior of the Alloy 617 as a comparative study over wide ranges of strain (speed) rate as investigated in this study. Therefore, It is important to provide a baseline data of the LCF properties of Alloy 617 to satisfy the reliable design. The aim of this work is to investigate the influences of strain rate on the low cycle fatigue behavior of Alloy 617 at room temperature, as a start up temperature condition. We conducted the fully reversed () total axial strain-controlled LCF tests in an open air. The total strain controlled was 1.2% total strain range, with selected strain rates, i.e. , , and . The underlined results showed that the fatigue life was increased with increasing the strain rate. These findings are in accordance with the increasing in plastic deformation and ductility. Cyclic stress response behavior shows similar trend, with a small variability in initial stress state. The initial cyclic hardening behavior was observed below 100 cycles, and remained softening until sudden failure phase. The macroscopic view of primary crack was occurring at a lower fraction of fatigue life (about 40-50%) as the strain rate was increased. To conclude, the spacing of striations presents the segment of propagation rate, depending on the strain rate condition.