Studies on modification of some first wall materials using 3-6.8 MeV He ions

Because very few results are reported in the MeV region, we have started a systematic investigation concerning the fluence and energy dependence of bl istering induced in some useful materials for the first wall (stainless ste els, Ni, Cu, Mo). They have been irradiated with 3.0, 4.7 and 6.8 MeV He io ns. The irradiation effects have been investigated by means of a TEMSCAN-20 0-CX electron microscope and two metallographic microscopes. Irradiation ph enomena as sponge- and wave-like structures, submicronic cracks, multilayer flaking, micro-conglomerates, secondary blisters, microcraters were observ ed. An interesting effect is the amorphous phase in the Ti-modified austeni tic steel 12KH18N10T, phase formatted during irradiation with 6.8 MeV He+ i ons up to a dose of crater occurrence (7 x 10(18) ions per cm(2)), the temp erature of specimens (100 mum thick) being maintained during irradiation in the range 30-60 degreesC. The X-ray microanalysis shows the presence of th e major alloy elements in the amorphous area, whereas in that embedding a m icrocrystallite it is to be noticed the Ni depletion and the presence of Si and Mo. A potential explanation of the energy dependence of average bliste rs diameter d and blister critical dose could be stressed-induced model, be cause, in our cases (medium energy He ions), d-t(m), where t is the blister skin thickness. For our high energy data, an m=1.8 value is relatively con venient, except Mo, where m=1.5 is more suitable. (C) 2000 Elsevier Science B.V. All rights reserved.