Behavior of helium in steel 16Cr12W2VTaB under various implantation temperatures

Gaseous porosity formation and helium behavior have been investigated in the reduced activation ferritic-martensitic steel 16Cr12W2VTaB irradiated with 40 keV He+ ions up to a fluence of 5 x 1020 m-2 at temperatures from 290 to 900 K in two initial conditions: normalized + tempered (HT-1), and cold rolled + annealed (HT-2). After HT-1, cavitation was observed above 770 K, where bubbles with a maximum diameter 1 nm were formed. Large bubbles with a mean diameter 8 nm formed when irradiated at 900 K. On the other hand, a high density of small bubbles formed at the lower irradiation temperature of 570 K after HT-2. At 900 K, bubbles had an extremely nonuniform distribution in size, depending on their locations. The helium thermodesorption spectra had two intensive peaks owing to polymorphic transformation during heating. The peaks widened and gas release began at higher temperatures with formation of large bubbles