Helium is conveniently introduced into nickel­bearing alloys through thermal neutron irradiation. Although helium is usually detrimental, especially if the material is to be subsequently welded,63 it offers a method to simulate the production of helium expected in the very hard spectrum of a fusion reac­tor. In the case of vanadium and other refractory metal alloys, the effect of helium has been studied using two primary methods of introduction of helium. One method is implantation of a-particles with an accelerator; the other is the use of the decay of tritium. Tritium rapidly diffuses into group V refractory metals at elevated temperatures. The elevated temperature serves more to dissolve the protective oxide layer than to accelerate the kinetics of dissolution. The tritium thus introduced is permit­ted to decay, by p-decay with a residual nucleus of 3He. Helium-doped specimens have subsequently been neutron irradiated to study the synergistic effects of helium and atomic displacement damage.

A limited number of experiments have used tech­niques to simultaneously implant He and produce atomic displacements through an irradiation environ­ment of Li. The concept of introducing tritium into an irradiation capsule with the specimens in contact with lithium has been investigated to study vanadium

alloys with the He/dpa ratio characteristic of a fusion environment. The tritium charge, the production of tritium from lithium, and the production of tritium from 3He are some of the important considerations in the design of the experiment.64 Although conceptually valid, the desired results have not yet been obtained with experiments of this type.

Cyclotron-implanted helium has been used, also to study the effects of the fusion irradiation environ­ment. Tanaka showed severe embrittlement with the introduction of 90 and 200appm He at 700 °C in V-20Ti.65 Grossbeck and Horak showed that a level of 80 at. ppm He implanted as part of the same experiment had no significant effect on elongation in V-15Cr-5Ti at 700 °C.52 Braski also observed no sig­nificant effect on ductility in V-15Cr-5Ti at 600 °C with similar levels of He introduced from decay of tritium.66 The alloys, Vanstar-7 and V—3Ti—1Si, were also investigated, in some cases with an improvement in ductility upon introduction of helium.66 Follow­ing irradiation, severe embrittlement was observed in V-15Cr-5Ti at 600 °C in tritium trick samples by Braski66 and at 625 °C in cyclotron-implanted samples by Grossbeck and Horak.52 Irradiation experiments with refractory metals, unless using a Li environment, frequently subject the specimens to contamination by interstitial impurities, also leading

to embrittlement.52,67

Unalloyed molybdenum, Mo-0.5Ti, and Mo-50Re were irradiated in EBR-II by Wiffen at exposures of

Test temperature fC)

3.5—6.1 n cm~2 (E > 0.1 MeV) (18-32 dpa).68 Although Mo alloys are known to exhibit increased ductility with increasing temperature in the unirradiated con­dition, at temperatures above 400-550 °C, all three materials suffered plastic instability with uniform elongations below about 0.5%. This effect is shown in Mo in Figure 2 968 where irradiation temperature is shown to be the critical parameter and where speci­mens irradiated at 455 and 1136 °C were embrittled even in room temperature tests.

This class of alloys is discussed further in Chapter

4.6, Radiation Effects in Refractory Metals and Alloys.