HVEM observations of [formula omitted] he-ion bombardments

Abstract

in-situ helium ion irradiation experiments on type 316 stainless steel, a candidate first wall material for the fusion reactor program, have been conducted by means of a 200 kV light ion linear accelerator mechanical1y linked to the objective lens of a 500 kV high voltage electron microscope. An ion energy of 80 keV, Ion fluxes up to about 2 × 10 15 cm −2sec −1, doses up to about 5 × 10 18cm −2 and sample temperatures ranging from about 500°C to 650°C were employed. During irradiation of stainless steel, after a minimum dose was achieved, tiny dislocation loops were formed whose density increased with fluence. As dose was increased, these defects were apparently masked by the appearance and growth of gas containing voids hereafter referred to as “bubbles.” With additional irradiation, bubble formation was followed by the development of surface blistering and/or exfoliation. Bubble formation was observed to depend strongly on the microstructure of the sample. In particular, bubbles preferentially grew on grain boundaries, on incoherent twin boundaries and in regions of high plastic deformation. Irradiation of samples first produced bubbles preferentially on slip bands followed by bubble formation in other areas. Irradiations at high temperatures produced a larger ratio of bubble size on grain boundaries to that in the grain interior than the ratio found for irradiations conducted at low temperatures. By means of a video system, the rates of growth of microstructural features, such as bubbles, blisters, and exfoliated areas, have been recorded. For comparison purposes, in-situ helium irradiation experiments on molybdenum were carried out and essentially reproduced the transmission electron microscope in-situ results reported by Mazey and co-workers for 18–60 keV helium ion bombardment of molybdenum.