Ni-15Cr-25W合金の高温ガス炉近似ヘリウム中における低サイクル疲労挙動におよぼす長時間時効の効果

抄録

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.