Effects of Thermal Aging on the Low Cycle Fatigue Behavior of Ni-15Cr-25W Alloy in the Simulated HTGR Helium

Abstract

The effects of thermal aging on the low cycle fatigue behavior of Ni-15Cr-25W alloy (KSN) were investigated at 1 273 K in the simulated helium gas (He-2) for High Temperature Gas-cooled Reactor (HTGR). Fully reversed strain control tests were made at a strain rate of 1 × 10^-3s^-1 with holdtimes of 0, 6 and 60 min. The thermal aging at 1 273 K for 1 000 h caused the precipitation of α₂ phase (bcc-W) in the matrix and on grain boundaries of KSN.
Although a degradation of fatigue lives was observed with increasing the period of holdtime for both solution annealed and aged KSN (KSN-SA and -AGE), fatigue life was significantly larger for KSN-AGE than that for KSN-SA. The morphology of intergranular fracture of two materials indicated that the existence of α₂ particles at the grain boundaries increased both cycles to initiation and those to propagation of fatigue crack. The role of α₂ precipitates was to reduce the weakening of grain boundaries and the oxidation rate by changing the corrosion products at a crack tip from Cr-oxides for KSN-SA to Ti/Al-oxides for KSN-AGE.