Influence of implanted helium on swelling behavior and mechanical properties of vanadium and V-alloys

The influence of helium implantation on the swelling behavior and ductility in Vanadium and V-Ti alloys has been investigated by transmission electron microscopy and tensile testing. V and V-20 wt. percent Ti has been implanted with He-ions of low energy (240 keV) at irradiation temperatures between 450 and 7000C to fluences ranging from 2.5 x 1014 to 5 x 1017 ions/cm2, which corresponds to peak concentrations of 125 and 250,000 at. ppM He respectively. The correlated ion-induced displacement damage is 1.1 x 10-2 to 22 dpa. For comparison, in the first wall of a CTR about 10,000 ppM He will be generated via (n,α)-reactions in V after a 10-year exposure. In pure V the threshold for bubble formation (d greater than or equal to 40 A) is 1 x 1017 ions/cm2 at 5000C; it decreases markedly with increasing irradiation temperature (7 x 1014 He+/cm2 at 6000C e.g.). The mean diameters increase hyperlinear with temperature and approximately linear with dose. The strong effect of temperature on the mean diameter can be explained by surface-diffusion controlled coalescence mechanisms. Generally two different growth processes are observed. The first one can be connected to vacancy trapping and the second one to coalescence. The observed maximum swelling is about 8 percent at 6250C. Additions of Ti reduce the concentration of bubbles and their distributions, but have no essential effect on the amount of swelling. Thus in this experiment swelling is mainly due to helium rather than to the displacement damage produced during the implantation. Pure vanadium tensile specimens were homogeneously implanted with 100 MeV He-ions up to 10 ppM helium between 200 and 3000C and tested between 750 and 9500C. No influence upon ductility or yield strength was found in agreement with earlier results on a V-3Ti-1Si alloy