He trapping and bubble formation in Ni, stainless steel 316 and amorphous alloys

Abstract

He trapping and the formation of bubbles in surface layers of crystalline and amorphous metals following multiple energy He + implantation (250 eV⩽ E⩽8000 eV) at various irradiation temperatures T i and fluences φ ∗ were investigated by means of transmission electron microscopy, ion beam depth profiling and thermal desorption techniques. No surface blistering is observed at these implantation conditions. Materials under investigation were Ni, stainless steel (SS 316) and amorphous alloys of different composition (VITROVAC Ni78Si8B14, VITROVAC Ni40Fe40B20, METGLAS B-Ni2 Ni82.4(CrFeSiB) 17.6). After room temperature implantation at fluences φ ∗⩽φ c ∗ ≈ 2 × 10 21 He + m −2 He is effectively trapped in small, presumably overpressurized bubbles (radius r ≈ 1 nm in Ni and SS 316). In crystalline material the He bubbles arrange in ordered domains where they are preferentially aligned parallel to {111} and {220} matrix planes (“bubble lattice” with lattice parameters a 1 ≈ 8 – 9 nm). The He containing surface layer of ≈ 150 nm thickness saturates at c He = 20–30 at.% in Ni and SS 316. The experimental observations in SS 316 indicate that He re-emission through channels takes place at fluences φ ∗ > φ c ∗ in good agreement with a model for He re-emission [2] that accounts for the microstructural evolution at high implantation fluences in Ni. In addition, irradiation-induced precipitation of new phases is observed. In amorphous materials only random arrangements of small bubbles ( r ≈ 1–4 nm) are formed. Considerably less He is trapped in the surface layer and depth profiles as well as thermal release spectra show distinct differences. He implantation into Ni and SS 316 at high T i (300–500°C) leads to random arrangements of facetted bubbles ( r ⩽ 25 nm). Evidence is found that coalescence of bubbles during continuous implantation results in the growth of bubbles that eventually intersect the surface and thus contribute to He release. At these temperatures the surface layer saturates at considerably lower He concentrations.