Abstract
The effects of neutron irradiation on both the fatigue and the creep-fatigue crack propagation performance of alloy 718 were investigated at 427°C at a level of ~7.0 dpa. The experimental results revealed that, for tests conducted with a continuous cyclic load at 0.17 Hz, the rates of crack propagation for both irradiated and unirradiated material conditions were nearly the same and were consistent with the previous results. However, for tests conducted with a one minute tensile hold period at the maximum cyclic load, the crack propagation rate for the irradiated material was found to be an order of magnitude higher than for material tested in the unirradiated condition. Fractographic observations have indicated that the basic mode of crack propagation for the unirradiated material was transgranular, regardless of hold time, as well as for the irradiated material tested with continuous cyclic loading. The crack propagation mode for the irradiated material tested with a one minute hold time was found to be intergranular in character in accordance with the higher crack propagation rates observed. The results suggest that the effects of irradiation exposure may severely reduce the crack propagation performance of alloy 718 during creep-fatigue loading conditions at even moderately elevated temperatures.
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