EFFECTS OF ION IRRADIATION ON FATIGUE OF FE-12CR-20MN STAINLESS-STEELFOR FUSION-REACTOR APPLICATIONS

To minimize waste disposal problems associated with the residual radio activity of the first wall material of a fusion reactor, fast induced radioactive decay (FIRD) alloys based on the Fe-Cr-Mn system are being investigated. The objective of this research was to evaluate the effe cts of irradiation on cyclic strain localization and fatigue crack ini tiation in a FIRD Fe-12Cr-20Mn alloy and to compare the response to co mmercially available 316 stainless steel. The alloys were irradiated w ith 200 keV Fe ions to a dose of 1 x 10(16) ions/cm(2) and 15.5 keV He ions to a dose of 7 x 10(15) ions/cm(2) to simulate the irradiation-i nduced defect structure and helium concentration that would be produce d in a fusion reactor. Irradiated specimens were fatigued in a cantile ver beam fatigue testing machine with the deflection set to produce a fully reversed total strain amplitude of 0.25 % on the surface of the specimen. Acetate replicas were obtained during the fatigue tests to p rovide a record of surface fatigue damage. Transmission electron micro scopy (TEM) analyses were performed to characterize the microstructura l changes resulting from the irradiations and interactions between fat igue-induced glide dislocations and the irradiation-induced defects. R esults indicate that the irradiated Fe-Cr-Mn alloy exhibits fatigue pr operties similar to 316 stainless steel. Glide dislocations produced b y fatigue cycling annihilate irradiation-induced defects. The defect a nnihilation causes the formation of cleared channels in which the cycl ic plastic strain is localized. Subsurface slip bands penetrate the ir radiated regions through the cleared channels and serve as fatigue cra ck initiation sites.