抄録
It is known that the impact toughness of polycrystalline Ni-base superalloy, commonly used in gas turbine blades decreases with increasing exposure time at an elevated temperature. For the present study, the change in impact toughness of the Ni-base superalloy Udimet 520, which had been used in service as a first-stage blade in a 1150℃ class gas turbine for 31626 hours, was investigated by small punch (SP) tests. Experimental results revealed that in the pitch line region where the metal temperature was highest there was a decrease of the SP fracture energy E_<SP>, although the E_<SP> measured on the root shank was higher than that on the unexposed blade. That is, in contrast with the root shank, the pitch line region of the present U-520 blade became embrittled due to prolonged in service exposure. It was also found that the increase in the amount of grain boundary M_<23>C_6 carbides, which provide sites for cracks to initiate and/or propagate, was a predominant factor in the thermal aging embrittlement of U-520. Additionally, the influence of rejuvenation treatment, which is usually conducted to restore creep strength, on the toughness has also been examined by SP tests
