Kinetics and phase stability of nano-scale precipitates in Fe-based binary alloys during ion irradiations

Due to the high void-swelling resistance and good mechanical, thermal and chemical properties, FeCr based ferritic-martensitic steels are promising structural materials in future nuclear reactors. However, they suffer from the well-known “475 °C embrittlement” phenomenon due to formation of Cr-rich alpha prime (α’) precipitates. Despite the extensive observation of α’ after neutron irradiations, α’ formation was seldom reported after high dose rate ion irradiations.

In this study, high purity FeCr alloys with Cr concentrations of 5-18% were irradiated by 8 MeV Fe ions at -123 °C to 450 °C, 10^-5 – 10^-3 dpa/s to mid-range doses of 0.37-3.7 dpa, and by 800 keV protons at 250 – 450 °C, 2×10^-5 dpa/s to 2 dpa. Solute distribution and formation of solute clusters were characterized mainly by the atom probe tomography (APT) technique.

After heavy ion irradiations at the high temperature range (300 °C – 450 °C), Cr precipitates were observed in Fe-(12-18)Cr alloys in a large portion of irradiation conditions. These precipitates feature much lower Cr concentrations compared to the thermal equilibrium values, which can be attributed to the modifications introduced by displacement damage cascades. Furthermore, both the near-surface and implanted ion regions were observed to suppress α’ precipitation.

Proton irradiations at 350 to 450 °C resulted in the formation of homogeneously distributed α’, which have near-equilibrium compositions, and can be attributed to a radiation enhanced precipitation mechanism. At 250 °C, highly concentrated Cr clusters and their positional association with dislocation loops was revealed in Fe-(5-8)Cr alloys. These agglomerated clusters are attributed to radiation-induced precipitation due to the preferential coupling of Cr atoms with interstitials.

Irradiations at cryogenic- to room- temperature using heavy ions were conducted to study the radiation-induced dissolution rate. Both FeCr and FeCu alloys with pre-existing solute clusters were simultaneously irradiated up to 0.6 dpa. Gradual dilution of Cu precipitates was observed after irradiations. However, Cr precipitates show relatively high resistance to the dissolution induced by displacement cascades, in contrast to the behavior observed at 300– 450 °C. This low temperature behavior is inconsistent with the radiation enhanced diffusion/ballistic dissolution theory. Further investigations are required to elucidate these puzzling observations.