Helium effects on tensile properties of powder metallurgical-processed tungsten for fusion reactor applications

Abstract Powder metallurgical processing of Tungsten (W) followed by thermomechanical treatment, including rolling and stress relief (SR) heat treatment, is a candidate fabrication process for W materials for fusion applications. Helium (He) is insoluble in almost all metals, and grain boundary embrittlement occurs on the accumulation of He on their grain boundaries. In this study, the effects of He concentration, test temperature, and strain rate on the mechanical properties of SR-W were investigated using tensile tests. The He effects were studied by the uniform implantation of high-energy He-ions into specimens by cyclotron accelerator irradiation and by tensile tests from 300 °C to 1300 °C. The examined He concentration was below 20 appm. Vickers hardness tests on an isochronal annealed specimen up to 1500 °C showed that the threshold concentration of He for suppressing the recovery and recrystallization of the microstructure of SR-W by annealing was between 2 and 20 appm. The tensile test showed a slight increase in the ultimate tensile strength (UTS) at He concentrations up to 20 appm; however, no clear effect on the elongation or fracture mode was observed. The results of the tensile test conducted at 1300 °C on a 20-appm He-implanted specimen showed that its softening and ductility by recrystallization were suppressed; however, there was no observation of grain boundary embrittlement by He implantation. The results suggest that the mechanism of the mechanical response of He-implanted SR-W was He suppression of the recovery and recrystallization of the layered structure unique to SR-W up to 1500 °C. Moreover, at the level of He concentration introduced by nuclear transmutation in the divertor of a fusion reactor without displacement damage, no severe effect occurred to promote the embrittlement of SR-W.