Positron annihilation lifetime study of Fe and Ni irradiated at low temperatures

In order to investigate fundamental behaviors of various kinds of defects in Fe and Ni positron annihilation lifetime measurements have been performed for these metals irradiated at low temperatures with neutrons or electrons. It was found that radiation-induced vacancies start to migrate at about 200 K in Fe and 300 K in Ni and above these temperatures vacancies form clusters, namely, microvoids in Fe, and collapsed type clusters like stacking fault tetrahedra in Ni. According to this process positron lifetime changes during isochronal annealing, that is, increases in Fe and decreases in Ni. Positron lifetime calculation has also been carried out to support these experimental results. In large enough model lattices a vacancy or vacancy clusters were constructed by full relaxation of the whole lattice by using the EAM type atomic potential, and positron lifetime was calculated for each defect by the atomic superposition method. Good agreements were obtained with experimentally obtained positron lifetimes. Electrical resistivity measurements were also performed to investigate the behaviors of self-interstitial atoms and some alloying effects were observed. (author)