Effects of thermal aging on the low cycle fatigue behavior of Ni-15%Cr-25%W alloy in the simulated HTGR helium.

Abstract

The effects of thermal aging on the low cycle fatigue behavior of Ni-Cr-W alloys were investigated on KSN alloy (Ni-15wt%Cr-25wt%W) at 1 273K in the simulated helium (He-2) for High Temperature Gas-cooled Reactor (HTGR). Fully reversed strain control tests were made at a strain rate of 1×10-3 s-1 with holdtimes of 0, 6 and 60min. The thermal aging at 1 273K for 1 000hr initiated 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 increase of the period of holdtime for both solution annealed and aged KSN (KSN-SA and-AGE), the value of fatigue life was significantly longer for KSN-AGE than 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.