Microstructure of Ni fatigued under neutron irradiation

The microstructure of fatigued and unfatigued Ni in the presence or absence of neutron irradiation was investigated using positron annihilation lifetime measurements and transmission electron microscopy (TEM). In irradiated unfatigued Ni, vacancy clusters were directly formed from cascades at doses of less than 10−4dpa, and started to grow at higher doses. In irradiated fatigued samples, vacancy clusters were directly formed from cascades at 1.5×10−4dpa. In unirradiated fatigued samples, single vacancies were produced. In all of the fatigued samples, dislocations introduced by the deformation acted as sinks and suppressed the growth of vacancy clusters. Following annealing at 373K, vacancy clusters were observed in irradiated fatigued Ni, but vacancies were trapped at dislocations in unirradiated fatigued Ni. No change in dislocation structure was brought about by annealing and no vacancy clusters could be observed by TEM.