The characterisation of displacement-cascade collapse in Ni-Cr-Fe alloys

Displacement cascades produced by energetic lattice recoils are the primary damage state in neutron-irradiated metals, among which austenitic stainless steels are an important class of technological alloys. The vacancy constituent of this damage that survives in the form of collapsed vacancy loops is a major component of this damage state, and has been investigated in the present work as a function of alloy composition and the temperature. This has been done by irradiating single crystal foils of a range of high-purity model ternary Fe-xNi-15%Cr alloys (x = 15-70%) with heavy ions, and then analysing the resulting damage by transmission electron microscopy. A full analysis has been achieved by measuring the areal density of loops and their size distribution for each irradiation condition, and hence obtaining the important parameters defect yield and collapse efficiency. By appropriate choice of ion energy, ion mass, ion dose and specimen temperature, we have been able to vary the factors such as cascade size, cascade energy density, cascade defect density and cascade overlap, and thus study their influence on cascade collapse. The results have been assessed in terms of current models of cascade processes in pure metals and alloys. (orig.)