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.
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