Relatively few experiments have used helium implan­tation to investigate the embrittlement of nickel-based alloys, although this technique has been more widely used for austenitic steels. However, some data for high-Ni alloys have been published by Shiraishi et a/.95 and Boothby.96

Shiraishi et a/.95 compared the effects of helium injection and neutron irradiation on the tensile prop­erties of developmental y0-Ni3(Ti, Al) (alloy 7817) and y"-Ni3Nb (alloy 7818) precipitation-hardened 40Ni-15Cr alloys. A number of alloy conditions, including ST, aged, and cold worked, were tested. Cyclotron injections of helium were made at 650 °C to levels of 5 or 10 appm. Neutron irradiations were made at the same temperature to a fast fluence (E > 1 MeV) of 1.7 x 1024nm-2 and a thermal flu­ence of 5.9 x 1024nm-2. The helium content of the reactor-irradiated specimens was estimated to be ^45 appm, produced mainly from the thermal neutron reaction with 10B. Tensile tests were carried out at the implantation/irradiation temperature at a strain rate of ^5 x 10-4s-1. The results showed sim­ilar trends in helium-implanted and neutron — irradiated specimens, with the total elongation values tending to decrease with increasing tensile strength. Variations in tensile strength for each alloy were largely attributable to variations in the initial heat treatment and working schedules. However, there were some indications of softening and reduced ductility in the neutron-irradiated specimens com­pared to those injected with helium. Overall, the y0- hardened alloy 7817 exhibited relatively high tensile strength (typically >700 MPa) but low ductility fol­lowing helium implantation or neutron irradiation (with total elongation values generally <10% and as low as 1-2% in the highest strength conditions). The y"-hardened alloy 7818 showed lower tensile strength (typically 500-600 MPa) but maintained good ductility, with total elongation values always exceeding 10% and generally being above 20% in STA conditions.

Boothby96 examined the effects of helium and/or lithium injection on the tensile properties of STA (ST 1050 °C, aged 8h at 700 °C) Nimonic PE16. The effects of lithium were examined since this ele­ment is also produced from the 10B(n, a)7Li reaction in neutron-irradiated alloys. Helium and lithium were implanted at ambient temperature, either singly or in combination, to levels of10 appm each. Samples were tested in the as-implanted condition or follow­ing an additional aging treatment of 72 h at 750 °C. Tensile tests at a strain rate of 3 x 10-5 s-1 on as — implanted samples showed no effect of helium or lithium on ductility at 200-550 °C. However, at 650 °C, the as-implanted samples were all embrittled to a similar extent, with the total elongations gener­ally reduced to about half of the unimplanted levels regardless of whether He, Li, or (He + Li) ions were injected. Postimplant aging of samples containing He or (He + Li) resulted in further ductility loss in tests at 650 °C, with significant embrittlement also evident at 550 °C though not at 450 or 200 °C. Postimplant aging of samples containing only lithium, however, resulted in some recovery in ductility compared to the as-implanted condition at 650 °C but some duc­tility loss at 550 °C. Thus, although it was clear that lithium had a detrimental effect on ductility, it did not appear to exacerbate the effects of helium. A mechanism for lithium embrittlement was not identified, though ductility loss in both Li and He implanted samples was associated with an increased propensity for intergranular fracture.

Some additional, previously unpublished, data from a helium injection experiment conducted by Boothby and Cattle are given in Table 2. In this experiment, helium was implanted at ambient tem­perature to levels of 2, 10, and 50 appm into Nimonic PE16 that had been given a two-stage aging treat­ment (ST 1050 °C, aged 4h at 800 °C plus 16h 750 °C). As before, helium-doped samples were either retained in the as-implanted condition or given an additional aging treatment to coarsen the dispersion of gas bubbles. Tensile tests were carried out at 650 °C at strain rates of 3 x 10~5 and 3 x 10~6s~ The results show a significant loss of ductility even at 2 appm helium. In tests carried out at the higher strain rate, the total elongation values decreased progressively with increasing helium con­tent and were further reduced by postimplant aging. The ductility of as-implanted samples was generally lower but less sensitive to helium concentration in tests at a strain rate of 3 x 10~6 than at 3 x 10~5s~ However, there was little effect of the strain rate on the ductility of the postimplant aged samples.

Figure 15 illustrates grain boundary structures in a tensile-tested PE16 sample which had been aged subsequent to helium injection. Failure in this case appeared to occur by the growth and coalescence of cavities which were nucleated at grain boundary gas bubbles.97 The nucleation of unstable cavities at grain boundary helium bubbles requires the application of a critical tensile stress, which is an inverse function of the bubble radius, normal to the boundary. Cavity growth then occurs via the stress-induced absorption of vacancies. In the as-implanted condition, however,
the helium dispersion was too fine to enable grain boundary cavities to nucleate during tensile testing. Reduced ductility in the as-implanted samples appeared to be associated with grain boundary wedge cracking, where, as discussed by van der Schaaf and Marshall98 in relation to helium embrit­tlement of type 316 steel, the role of helium may be simply to decrease the effective surface energy for fracture.

Although it is evident from simulation experi­ments that helium alone can largely account for irra­diation embrittlement, it is more difficult to assess the significance of other radiation-induced effects such as matrix hardening and grain boundary segregation and/or precipitation. One experiment which examined the effect of the radiation-induced precipitation of the Ni3Si g’ phase on the ductility of a binary Ni-8 at.% Si alloy was described by Packan et al99 In this experi­ment, thin foil tensile specimens were bombarded with either protons or a-particles to damage levels of 0.1­0.3 dpa at 750 K; irradiation with a-particles resulted in the introduction of high helium concentrations of about 750 appm per 0.1 dpa. Proton and a-particle irradiations both resulted in the formation of g layers about 20-30 nm thick at the grain boundaries, but the material remained relatively ductile, exhibiting trans­granular failures, in tensile tests carried out at a strain rate of ^3 x 10~4 s-1 at room temperature and, for the proton-irradiated case only, 720 K. Unfortunately, no tests were carried out at higher temperatures and sam­ples which were irradiated with a-particles only at 750 K were not tested except at room temperature. However, low ductility intergranular failure was

induced in a test carried out at 720 K in a sample which was preimplanted with 1000 appm He at 970 K, then irradiated to 0.3 dpa, introducing an additional 2300 appm He at 750 K. Preimplantation of helium at 970 K produced grain boundary bubbles which were 10-20 nm in diameter, compared to 1.5-2 nm in mate­rial that was only irradiated with a-particles at 750 K. The results of this experiment therefore indicated that the radiation-induced precipitation of y’ at grain boundaries did not give rise to embrittlement unless helium was also implanted into the specimens.