The mechanical properties of irradiated niobium

Abstract

The effect of heavy neutron irradiation (~10 20 n cm −2) at 16°C on the mechanical properties of niobium has been investigated. Only a moderate increase in yield strength occurred immediately on irradiation (61,000 lb/in 2 to ~77,000 lb/in 2) but on annealing in the temperature range 125°–175°C there was a further large increase with the formation of a yield drop at the start of the deformation. The activation energy has been measured, 1.30 ± 0.1 eV, and it is concluded that the process is due to vacancy migration, the yield drop being due to the condensation on dislocations of vacancies produced during irradiation. Recovery of irradiation hardening commences at 350°C and is complete after 1 hr at 600°C. Technologically the most serious irradiation effect is the very large reduction in uniform elongation before fracture (17 per cent to <2.0 per cent) a consequence of a complete lack of capacity to work harden in irradiated material. Hence deformation, once started, continues in the same place until fracture occurs. The reduction in area at the fracture is quite large in the irradiated material ( ~50 per cent c.f., 80 per cent in unirradiated niobium).