Microstructural evolution in high energy helium implanted nickel—I. Room temperature (t − 100°c) implantation

Abstract Nickel foils of about 12 μm thickness have been homogeneously implanted with 0 to 6 MeV He ions at room temperature. For helium concentrations ranging between 0.1 and 5 at.% the implantation induced changes of the microstructure have been investigated by measurements of the electrical resistivity, the length and the lattice parameter and by transmission electron microscopy (TEM). Beginning from the lowest implantation dose clustering of irradiation induced vacancies and self-interstitial atoms leads to the formation of stacking fault tetrahedra (SFT), dislocation loops and of a dislocation network. In addition a large number of small He-vacancy complexes is present that are invisible in the TEM at low He concentrations, c He , and can be identified as small cavities for c He ≳ 0.7 at.%. The He-vacancy complexes are characterized by a He-density that is rather low compared to the density necessary to activate the loop punching mechanism for bubble growth: i.e. c He is about one half of the vacancy concentration c V for c He ≲ 0.8 at.%. At higher implantation doses the ratio c He / c V increases slowly up to a value of 1.4. Hence, the limiting pressure for loop punching may be obtained only for c He ≈ 8 at.%. A “model” is proposed that describes this microstructural evolution by the continuous nucleation of stable He-vacancy complexes as a result of high energy collision cascades. This model directly explains the low average helium density observed during the early stages as well as the coexistence of small cavities and SFT's. A saturation of the bubble density at high doses and the slow increase of the He density in bubbles might be understood by cascade overlap.