Abstract
Specimens of Nb and Nb-40V having grain sizes 21 and 17 μm, respectively, were irradiated in JMTR (Japan Materials Testing Reactor) to a total neutron fluence 1.0×1023 m−2 (E>1 MeV). Tensile test was conducted for the unirradiated and irradiated specimens at a temperature range 293 to 77 K. The deformation structure was observed by optical and transmission electron microscopy. From the surface observation of the fractured specimens, deformation twins were found to be more extensively distributed in the irradiated Nb-40V than in the unirradiated Nb-40V specimen. No deformation twins were observed in unirradiated and irradiated Nb.The irradiated Nb showed a representative behavior in the mechanical properties, as general bcc metals, that is, the radiation hardness and radiation embrittlement occurred. On the other hand, the irradiated Nb-40V showed an anomalous behavior in the mechanical properties. The radiation softening occurred in Nb-40V, that is, the yield stress decreased by the irradiation, and moreover, it was constant over a whole range of tested temperature. The elongation of irradiated Nb-40V was almost the same as that of the unirradiated at 293 K and also maintained a constant value in the whole temperature range.For the unirradiated Nb-40V, the relation between the grain size and the mechanical properties was investigated. The specimen having a large grain size (45 μm) showed the mechanical properties similar to the irradiated specimen with a smaller grain size (17 μm), that is to say, neutron irradiation had the same effect as the increase in grain size on the mechanical properties as well as the occurrence of deformation twins for Nb-40V.
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