Effect of crystal orientation on hardness of He+ ion irradiated tungsten

The effect of crystal orientation on hardness in the as-received, irradiated and post-irradiation annealed tungsten samples was investigated using a nanoindenter. An effective irradiation method of He+ ions with a series of energy degraded from 200 keV to 20 keV was used to continuously irradiate polycrystalline tungsten at room temperature in order to obtain a relatively homogenous displacement damage and helium concentration from sample surface to a desired depth at a NEC 400 kV ion implanter. Some irradiated samples were then annealed at 900 °C. He+ ion irradiation induced hardness increase, oppositely for the post-irradiation annealing effect. Meanwhile, the hardness of the irradiated samples was decreased sharply in the initial stage of annealing from 0 to 1 h, and then slowed down in the latter stage from 1 h to 3 h. Crystal orientation had an obvious effect on the nanoindentation hardness. The (0 0 1)-oriented grains had highest hardness at the as-received and irradiated samples. During the annealing process, the hardness in the irradiated grains with (1 1 1) crystal orientation decreased more quickly than that in the (0 0 1)-oriented grains. The mechanism of the effect of crystal orientation on hardness was analyzed and discussed.