Is neutron radiation exposure always detrimental to metals (steels)?
It is well established that irradiated metals and alloys exhibit increased mechanical strength accompanied by reduced ductility, commonly referred to as irradiation strengthening and embrittlement. We talk about radiation damage and environmental degradation of metals following radiation exposure. Indeed, there have been numerous conferences and symposia held and planned on this subject, which include research work and discussions with the central theme being the damage created in materials by neutron radiation exposure. Radiation embrittlement in metals is believed to be due mainly to (1) changes in flow properties because of the interaction of dislocations with irradiation-produced defects1, and (2) precipitation of transmutation-produced gases and irradiation-induced segregation at grain boundaries which are potential fracture sites2. Contrary to the previously observed radiation embrittlement in metals, I report here that neutron irradiation of mild steel improved both the mechanical strength and ductility at 373 K even at relatively high (∼1018 neutrons cm−2), fast (>1 MeV) neutron doses. To my knowledge, such improvement has never been observed before.
Dynamic strain-aging and neutron irradiation in mild steel
"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 ∼ 10 1 4 to ∼ 10 1 9 nvt (> 1 MeV) resulted in a transition from "serrated" to "jerky" flow at and beyond ∼ 10 1 7 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 strain-aging 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 (∼ 10 1 9 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.
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