Temperature dependence of the amorphization of NiTi irradiated with Ni ions

Abstract

NiTi was irradiated with Ni ions at various temperatures in order to study the temperature dependence of the irradiation-induced crystalline-to-amorphous transition. The irradiations were conducted above theAftemperature, and thus the specimens contained only the ordered B2 (CsC1) phase. The irradiations to similar doses at 150, 200, and 250°C showed that the amorphization kinetics slow down appreciably as the temperature is increased in this range. No amorphization was detected at irradiation temperatures of 350°C or higher, even after doses of 4 dpa. The small volume fraction of amorphous material observed after irradiation to 0.67 dpa at 250°C indicates that the cutoff temperature for amorphization is in the vicinity of this temperature. The amorphous regions of partly amorphous samples are distributed in a nonuniform manner and exhibit a morphology similar to the martensitic microstructure that existed in the specimens before heating to the irradiation temperature. Large amorphous regions in these samples exhibit some very fine crystalline debris which tends to disappear with increasing irradiation dose. Post-irradiation annealing experiments indicated that no thermally activated crystallization occurred during irradiation at temperatures up to 250°C.