Mechanisms of neutron irradiation hardening in impurity-doped ferritic alloys

Mechanisms of neutron irradiation hardening in phosphorus (P)-doped, sulfur (S)-doped, and copper (Cu)-doped ferritic alloys have been studied by applying a rate theory to the temperature dependence of the yield strength. Hardening behavior induced by neutron irradiation at various temperatures (473 to 711 K) is characterized in terms of the variations in athermal stress and activation energy for plasticity controlled by precipitation or solid solution, and kink-pair formation with the content and type of impurities. In P-doped alloys, neutron irradiation below 563 K brings about a remarkable increase in the athermal stress and activation energy, due to the dispersion of fine (similar to 1.7-nm) P-rich precipitates that is more extensive than that for the Cu-rich precipitates reported in irradiated steel. During neutron irradiation above 668 K, precipitation hardening occurs to some extent in Cu-doped and S-doped alloys, compared to small or negligible hardening in the P-doped alloys. In alloys with a low to moderate content of various dissolved impurities subjected to high-temperature irradiation, the formation of kink pairs becomes considerably difficult. Differing dynamic interactions of dissolved and precipitated impurities, i.e., P and Cu, with the nucleation and growth of dislocations are discussed, giving rise to irradiation hardening.