Behavior of helium cavities in ion-irradiated W-Ni-Fe ductile-phase toughened tungsten

Abstract

This study reports on the distribution of helium (He) cavities in a hot-rolled W-Ni-Fe ductile-phase toughened tungsten (DPT W) composite irradiated to a dose and a helium concentration that are comparable to those in the material after 5-year irradiation in a conceptual fusion power plant. The DPT W sample consists of W particles embedded in a ductile-phase NiFeW matrix with a nominal composition of 90W-7Ni-3Fe by weight. It was hot-rolled to a thickness reduction by 87% (87R DPT W). Sequential and individual irradiations of the material with 1.2 MeV Ni+ ions to a fluence of 2.15 × 1016 Ni+/cm2 and 90 keV He+ ions to 6.5 × 1015 He+/cm2 was performed at 973 K. Larger He cavities with a lower number density are observed in NiFeW than W. Helium cavities are aggregated preferentially along the NiFeW/W interphase boundary. This behavior is not observed along the W/W grain boundary under the same irradiation conditions. The average corrected cavity diameter or volume appears to be smaller in the sequentially irradiated sample than He+ ion irradiated sample. There is no evidence for formation of visible voids or Ni precipitates in W phase irradiated with Ni+ ions only. Diffusion, clustering and trapping of vacancies and He atoms during He+ ion irradiation at 973 K may be responsible for the formation of the He cavities.