Abstract
Samples of pure nickel have been implanted with 500keV helium ions, at a dose of 10[17]ions/cm[2], followed by annealing in vacuo at 750°C (≈0.6T[m]) for various time periods to allow bubble nucleation and growth to occur. A transverse sectioning technique has been developed to allow TEM studies of the complete depth distribution of cavities, hence allowing the mechanisms for bubble growth at 750°C in nickel to be identified. It was found that after 2 hours annealing, a fine layer of cavities developed, corresponding well with the gas deposition profile calculated using the E-DEP-1 code. Subsequent annealing resulted in cavity growth on the periphery of the layer by vacancy collection, the principal vacancy sources being the irradiated surface and grain boundaries in the bulk of the material. Cavity growth in the peak implanted region was found to be suppressed due to the lack of vacancies and with bubble migration being hindered as a result of high bubble pressures, hence migration and coalescence did not occur until cavities approached their equilibrium pressure. The same bubble growth mechanisms were found to prevail after implantation of 5x10[16]ions/cm[2] and also after 250keV He implantation. The growth of helium bubbles has been compared to neon bubbles after implantation with 500keV Ne ions at two doses: 7.8x10[16]ions/cm[2] to obtain the same peak gas concentration and 2.9x10[15]ions/cm[2] to achieve the same peak displacement damage, followed by annealing. The cavity density was found to be established by the gas concentration, the displacement damage apparently having little or no effect, even after an approximately 27-fold increase. The growth mechanisms observed after Ne implantation appeared to be the same as those for He, although the bubbles after low dose Ne implantation achieved equilibrium conditions more rapidly, due to the lack of implanted gas. He and Ne have been compared after high energy implantation at 500°C, in the Variable Energy Cyclotron at Harwell to a peak gas concentration of 250ppm. For 4MeV He, an inhomogeneous cavity distribution was observed, compared to a relatively uniform cavity layer after 17MeV Ne implantation. However, the observed cavity sizes and number densities were found to be similar. Finally, nickel has been irradiated at 500°C with a mixed beam of 51MeVNi[6+]/17MeVNe[2+] ions, to 250ppm Ne together with 30dpa displacement damage, and compared to an irradiation with 51MeVNi[6+] ions without inert gas, as well as with 17MeVNe[2+] ions. The void number density profile resulting from the single heavy ion irradiation was similar to the computed damage profile, although the peak was ≈10% deeper than that predicted. A depression in the swelling profile was observed in this peak region resulting from a reduction in cavity size, a bimodal distribution being observed. The effect of simultaneous gas deposition was to increase the cavity nucleation and reduce cavity size. This phenomenon was found to be dominant in the region corresponding to the implanted gas layer, however the gas appeared to influence cavities produced at greater depths, with an overall reduction in swelling.