Positron annihilation studies of vacancy defect accumulation in a cold-worked aging Fe–Ni–Ti alloy

The behavior of defects during irradiation in the real structure of iron-nickel alloys, which are model for austenitic stainless steels used in fast-neutron nuclear reactors, is studied by positron annihilation spectroscopy. The study discusses the efficiency of the absorption of point defects (interstitial atoms and vacancies) by dislocations (dislocation bias), as the main reason for vacancy supersaturation in steels, through dislocation pinning by oversized impurities or second-phase precipitates. Such studies are relevant in connection with the problem of the limited use of austenitic steels as structural materials for nuclear reactors due to the susceptibility of steels to vacancy swelling caused by vacancy supersaturation. The cold-worked Fe–Ni alloy is shown to accumulate vacancies under irradiation despite the high dislocation density, this being due to dislocation bias. In the cold-worked aging Fe–Ni–Ti alloy, the accumulation of defects during irradiation is significantly reduced from that of the cold-worked Fe–Ni alloy. Dislocations pinned by Ni3Ti precipitates have a lower efficiency of the absorption of interstitial atoms than that of free dislocations in the Fe–Ni alloy. Therefore, dislocation bias decreases and mutual recombination of point defects is enhanced. The here-obtained data are applicable to the prediction of radiation-induced damaging of austenitic stainless steels and alloys, as well as to the development of methods for improving their radiation resistance.