Interactions between irradiation defects and solute atoms in ferritic materials by resistivity recovery experiments

The basic properties of primary radiation defects in ferritic materials and their interactions with important solute elements in steels such as carbon (C) and nitrogen (N) were studied. For this purpose, the technique of electrical resistivity recovery was employed, which consists of (i) generation of defects by irradiation at low temperature (typically in the cryogenic regime) and (ii) using electrical resistivity to monitor defect evolution during post-irradiation annealing. Experiments were carried out in Fe doped with C and N, in Fe-10% Cr doped with C and in Eurofer97 steel, which is essentially an Fe-Cr alloy containing C and N as well as other solutes.

In the case of Fe containing C and N, the experiments show clearly that C and N interact with both SIAs and vacancies and form stable complexes. This impacts the migration properties of both radiation defects and solutes and may alter the nature of important precipitation reactions in irradiated iron and steel. Indeed, it was possible to observe carbide and nitride nucleation and the influence of radiation defects on this process. Particularly in Fe-N, it was possible to model the phenomenon using classical kinetic theory in conjunction with defect parameters obtained for this purpose with ab initio theoretical methods by Dr. Chu-Chun Fu (CEA, SRMP, France). Thus, the strong binding energy of vacancy N complexes was validated, which clarified previous discrepancies on this issue, and show how irradiation competes with nitride precipitation.

Experiments were also carried out in the more complex Fe-Cr alloy and in Eurofer97. It is actually the first time that such experiments are performed on a RAFM steel. However, the interpretation of resistivity recovery presents difficulties, even in the simpler Fe-Cr alloy, due to the many different radiation defect configurations that are possible due to Cr and the presence of many solute atoms in Eurofer97. The measurements on Fe-Cr show that in this material C does not play a significant role for the migration of radiation defects. However, in Eurofer97, it was established that the migration of vacancies is hindered due to their trapping by solute atoms, most probably Mn.