A transient irradiation creep in non-fissile metals

Abstract

Relaxation strains between 2 × 10⁻⁵ and 10⁻⁴ have been observed in iron, high carbon steel, stainless steel and zirconium, after fast neutron doses between 2 × 10¹⁹ and 3 × 10²⁰ cm⁻², at stresses between 1 and 3 kg mm⁻², and at temperatures about one-fifth of the melting temperature. In iron, the relaxation appears complete at a neutron dose of 2 × 10¹⁹ cm⁻², and its lack of temperature dependence indicates that it is not an acceleration of thermal creep. In the steels, correlation with other results shows that the relaxation is proportional to stress. Vacancies and interstitials created by irradiation give rise to a chemical stress on dislocations. Dislocation climb under this stress, the external stress, and the line tension of the dislocations gives a final creep strain which is independent of temperature and proportional to stress. For the stresses used here, a final strain of 5 × 10⁻⁵ is predicted in a material containing a random array of dislocations. The order of magnitude of the calculated initial strain rate is also correct.