Effects of Ion Irradiation on Laser Welded V-4Cr-4Ti Alloy (NIFS-HEAT2)

Abstract

High purity V-4Cr-4Ti alloy (NIFS-HEAT2), fabricated by National Institute for Fusion Science (NIFS), was used for this study. The samples were annealed at 1,273K for 2 hours and bead-on-plate welds were performed using high purity argon gas. The measured oxygen concentration of the sample before welding, and weld metal are 139 and 158 wt ppm, respectively. A 2.4 MeV copper ion irradiation was carried out with the tandem accelerator at Kyushu University. The TEM (Transmission Electron Microscope) samples were sliced from the welded materials and irradiated at 573 K and 873 K up to the dose of 12 dpa. The microstructure before irradiation showed that relatively large Ti(C,O,N) precipitates which were commonly observed in NIFS-HEAT2 disappeared in the center of weld metal. After the ion irradiation at 873K, fine titanium oxides with {100} habit plane were detected at the dose of 0.75 dpa. Lower number density of the oxides was observed in the base metal after the same irradiation conditions. This means that the contribution of oxygen atoms, which dissolved from the large precipitates during the laser welding, is essential to the microstructural evolution of welded V-4Cr-4Ti alloys.