Dynamic Strain-Aging and Neutron Irradiation in Mild Steel

Abstract

“Blue brittle” or dynamic strain-aging studies were carried out on mild steel in vacuum-annealed, irradiated, and irradiated decarburized conditions. The effect of neutron damage is to decrease or eliminate the concentration of free interstitial impurity elements responsible for locking the dislocations. Progressively increasing the irradiation dose from ~ 1016 to ∼ 1019 nvt (> 1 MeV) resulted in a transition from “serrated” to “jerky” flow at and beyond ~ 1017 nvt. The activation energies for serrated and jerky flow were determined to be the same, ∼ 18 kcal/mol (75 kJ/mol), identifiable with the migration energy of carbon or nitrogen in iron. Thus neutron irradiation does not seem to affect the mechanism of dislocation locking. Neutron irradiation was found to result in the following: (1) yield stress increases (hardening); (2) ductility decreases (embrittlement); (3) temperature of strainaging increases; (4) temperature region of unstable flow decreases; (5) degree of locking decreases; (6) Luders bands become relatively more diffuse; (7) the transition from serrated to jerky flow results in a complex temperature dependence of yield stress and Luders strain; (8) at temperatures below unstable flow, Luders strain increases and, at the highest doses (∼ 1019 nvt), fracture occurs during Luders extension; and (9) radiation hardening in mild steel is thermally activated. There was no evidence of serrated flow in decarburized and irradiated decarburized material up to about 280°C (553 K). The effect of neutron damage was found to be similar to that of dry hydrogen treatment. In both cases a transition from serrated to jerky flow was observed. Both the appearance and disappearance of serrations were found to be dependent on the concentration of nitrogen in solution. These results imply that free nitrogen is primarily responsible for dynamic strain-aging in mild steel, both archive and irradiated.