Radiation hardening in α-iron and mild and low-alloy ferritic steels is comprehensively reviewed and rationalized in te rms of the neutron damage production mechanisms and the migration, impurity trapping, and clustering behaviour of the resulting vacancies and interstitials. The effects of the principal metallurgical (grain size, initial strength, purity) and irradiation (neutron dose, dose rate, spectrum, irradiation temperature) variables on the magnitude of the hardening are described and are discussed in terms of these basic processes. The deformation characteristics of the irradiated materials and the post-irradiation recovery of the hardening are also considered. Finally, an account is presented of the analyses of the rate-controlling deformation mechanisms after irradiation.

Neutron-irradiation hardening in irons and ferritic steels

During low temperature reactor irradiation up to a neutron dose of 2.7 : times; 1018 cm−2 the electrical resistivity increase was measured at 19 K on purified niobium and niobium doped with nitrogen up to 850 at. ppm. From the equality of the extrapolated saturation value of the resistivity with that after electron irradiation it is concluded that the defect cascades are loosely packed. A strong initial decrease of the damage rate of the purified Nb can be quantitatively accounted for by assuming that a relatively large fraction of the self-interstitials is produced by means of long range focusing collision sequences. A pronounced increase of the damage rate with nitrogen content is discussed in terms of capturing of focusing collision sequences at interstitial impurities and of a reduction of the fraction of self-interstitials produced by such collision sequences.

Defect production in purified and nitrogen doped niobium during low temperature neutron irradiation

The role of depleted zones in neutron irradiated metals

A series of α-iron specimens containing carbon and nitrogen in interstitial solid solution were irradiated simultaneously at 4.5 K with fast neutrons to a fluence of 4.7 × 1017 n/cm2 with E > 0.1 MeV. In situ electrical resistivity measurements during irradiation showed that the apparent damage rates were dependent on the C or N concentration. In particular, the value of the initial resistivity increment per unit flux (S), passed through a minimum value at 0.1 at. per cent N and became larger than the value for pure iron above 0.14 at. per cent N. The results can be explained qualitatively as interstitial impurity-induced differences in production rates and dispersion of defects or semi-quantitatively as defect-induced deviations from Matthiessen's rule.

J.A. Horak

Papers

ELECTRICAL RESISTIVITY INCREASE RATES IN Fe--C AND Fe--N SOLID SOLUTIONS DURING NEUTRON IRRADIATION AT 4.5 K.