Irradiation-creep in a materials testing reactor

Helical springs made from titanium, zirconium, Nimonic PE 16 alloy and two austenitic stainless steels namely Firth-Vickers FV 548 and a steel with a composition within the AISI 316 specification have been irradiated in the Dounreay Materials Testing Reactor, at temperatures between 85 °C and 100 °C and to neutron fluences up to about 4 × 10²⁴ m^?2 (> 1 MeV), whilst loaded in tension. Irradiation-creep was observed in all the materials studied and initial strain rates/unit stressneutron ranged from 1 to 1.9 × 10^?35 m⁴/N for Nimonic PE 16 and zirconium respectively. Data obtained from an earlier experiment are re-presented and compared with the present results.Springs which received no heat-treatment after coiling unwound during irradiation at rates which were independent of the supported loads. The phenomenon is attributed to the relaxation of internal stresses (produced during the manufacture of the springs) with an irradiation-creep constant which is an increasing function of prior cold-work.A mechanism of irradiation-creep is proposed which involves the re-arrangement of the dislocation network in a crystal as the dislocations climb by absorbing interstitials produced by irradiation.