FATIGUE TESTS ON A FERRITIC-MARTENSITIC STEEL AT 420-DEGREES-C - COMPARISON BETWEEN IN-SITU AND POSTIRRADIATION PROPERTIES

Strain-controlled fatigue experiments under simultaneous irradiation have been performed to investigate the specific loadings of structural materials in next-step fusion devices. Al irradiations were done on specimens made of the tempered ferritic/martensitic Cr steel MANET-I at 420 degrees C using a 104 MeV a-particle beam. Continuous push-pun cycling (R = -1) has been applied with total strain ranges Delta epsilon(t) between 0.5 and 1.0%. Under in-beam conditions at e.g. Delta epsilon, = 0.5% a number of cycles to failure of N-f = 42000, a He concentration of 400 appm and a damage dose of 1.6 dpa has been reached. This N-f is by about a factor of two below the average N-f-value of unirradiated reference tests, but seven times higher than N-f of comparable postirradiated specimens. It was found, that at least at 420 degrees C conventional postirradiation tests are a conservative approach to in-situ conditions, and that in strain-controlled tests N-f is mainly governed by the time evolution of the irradiation-induced hardening. Relevant microstructural features like the He-bubble morphology are correlated with the fatigue properties.