Influence of 100 keV helium irradiation on tensile properties of pure iron and JFMS steel at low temperatures

The effects of 100 keV He+ ion irradiation on mechanical properties in polycrystals of pure iron and a ferritic-martensitic steel (JFMS steel) were examined at low temperatures. Pure iron polycrystals irradiated to doses higher than 1x1021 ions/m2 showed enhanced irradiation embrittlement and the DBTT increased by about 50 K at a dose of 1x1021 ions/m2. The brittle fracture occurred preferentially along grain boundaries, and cracks were formed along grain boundaries emerging on the irradiated surfaces. The embrittlement disappeared by removing the irradiated surfaces thicker than the projected range of He ions. Microcracks initiated in the He-implanted surface layers trigger off brittle fracture of irradiated pure ion polycrystals. JFMS steel specimens irradiated to a dose of 4x1021 ions/m2 and elongated at 77 K also exhibited surface cracks along grain boundaries. The cracks, however, did not grow into the interior and the specimens failed in a transgranular manner. Grain boundaries of JFMS steel may be so strong that crack propagation along boundaries does not occur. (orig.)