Defect cluster formation in vanadium irradiated with heavy ions

Irradiation data of vanadium alloys have been accumulated by intensive irradiation experiments in fission reactors. In evaluating irradiation performance of the alloys in fusion environments, we should consider the effects of high energy cascade damage and transmutation reactions under 14 MeV neutron irradiation. Effects of high generation rate of helium on microstructural evolution and mechanical properties in vanadium alloys have been studied by several techniques including dynamic helium charging experiments (DHCE) and boron doping. However, fundamental understanding on defect cluster formation under cascade damage in vanadium has not yet been clarified in detail. In this study, the effect of cascade damage on vacancy cluster formation was investigated as a function of energy transfer by cascades using several kinds of heavy ion irradiations to thin foils specimens. No defect clusters were observed by transmission electron microscope (TEM) in thin foils of 99.8% pure vanadium irradiated with 200 and 400 keV self-ions (V +) up to 1 × 10 16 ion/m 2 at room temperature. Thin foil specimens were also irradiated with Au + and Xe + ions to 1 × 10 16 ion/m 2. Energies of irradiating ions were 50, 100, 200, 300 and 400 keV. In the specimens irradiated with Au + ions, defect clusters of about 2–2.5 nm were detected by TEM. The areal density of the observed defect clusters increased with ion energy and was also found to be dependent on the thickness of the specimens. In the thin region of the specimens, density of the defect clusters per damage energy deposition increased with ion energy. These indicate that vacancy clusters are produced by high density of displacements in cascade damage. In the thicker region of the specimens, interstitials can easily annihilate vacancy clusters and form interstitial clusters. At the foil thickness of 20 nm, the minimum energy of gold ions to produce vacancy clusters was estimated to be 120 keV. This corresponds to the damage energy transfer density of 4.5 keV/nm/ion.