Yuzo Nakamura

Effects of He+ irradiation on the mechanical properties of molybdenum

Abstract The effects of 100 keV He ion irradiation on the mechanical properties of molybdenum single crystals were studied by examining the temperature- and dose-dependence of tensile behavior at 77 and 293 K. The yield stress of single crystals increased with increasing dose and the fracture stress decreased with increasing dose at 77 K. A ductility loss occurred in irradiated crystals at 293 K. These results show that the surface damage layer about 200 nm thick causes embrittlement at low temperatures and ductility loss at higher temperatures.

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

Abstract 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 1 × 10 21 ions / m 2 showed enhanced irradiation embrittlement and the DBTT increased by about 50 K at a dose of 1 × 10 21 ions / m 2 . 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 4 × 10 21 ions / m 2 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 grain boundaries does not occur.

Low temperature embritllement of low energy He implanted pure iron and iron-based alloys

Abstract Polycrystalline plates of pure iron, Fe-0.0018 wt% C and 0.0042 wt% C alloys, Fe-3.25 wt% Si alloy and JFMS steel were injected with 100 keV He ions up to a dose of 4 × 1021 ions/m2, and tensile-tested at 77 K. He-injected pure iron and Fe-Si alloy showed brittle fracture in intergranular and transgranular manners, respectively. Surface cracks, which started at grain boundaries, extended to a depth of several 10 μm. These results indicate that surface cracks trigger off brittle fracture of injected pure iron and Fe-Si alloy. He-injected Fe-C alloys and JFMS steel showed transgranular fracture and surface cracks which formed at grain boundaries did not extend into the interior. The present results were discussed in terms of the weakening of grain boundaries by implanted He bubbles and the ratio of the strength of the grain boundary to that of the matrix.