Development of Gas Porosity along the Ion Range in Vanadium Alloys during Sequential Helium and Hydrogen Ion Irradiation

Abstract

The development of helium porosity in vanadium and its alloys with tungsten, zirconium, and tantalum during sequential ion irradiation by 40-keV He⁺ ions at 650°C to a fluence of 5 × 10²⁰ m^–2 and 20-keV H⁺ ions at 20°C to a fluence of 5 × 10²⁰ m^–2 is studied by transmission electron microscopy. The microstructure and the development of porosity in the alloys are investigated along the ion range. Unlike He⁺ ion irradiation, the alloying elements during sequential He⁺ and H⁺ ion irradiation increase the gas swelling of vanadium: tantalum causes the maximum swelling and zirconium minimum one. Gas bubbles in the tantalum-containing alloys are located at the depths that are significantly more than the calculated helium and hydrogen ion ranges. Deep penetration of introduced gas atoms is shown occur mainly along the grain boundaries that are perpendicular to the irradiated surface. The largest bubbles (gas-filled pores) during He⁺ ion irradiation are found to grow at the depth with a high radiation vacancy concentration rather than the maximum helium concentration. In sequential He⁺ and H⁺ ion irradiation, a zone with large pores forms more deeply, in the ion range zone, and large pores in the 100-nm-thick layer transforms into high-density small bubbles.