On α′ precipitate composition in thermally annealed and neutron-irradiated Fe- 9-18Cr alloys

Ferritic-martensitic steels are leading candidates for many nuclear energy applications. However, formation of nanoscale α′ precipitates during thermal aging at temperatures above 450 °C, or during neutron irradiation at lower temperatures, makes these Fe-Cr steels susceptible to embrittlement. To complement the existing literature, a series of Fe-9 to 18 Cr alloys were neutron-irradiated at temperatures between 320 and 455 °C up to doses of 20 dpa. In addition, post-irradiation annealing treatments at 500 and 600 °C were performed on a neutron-irradiated Fe-18 Cr alloy to validate the α-α′ phase boundary. The microstructures were characterized using atom probe tomography and the results were analyzed in light of the existing literature. Under neutron irradiation and thermal annealing, the measured α′ concentrations ranged from ∼81 to 96 at.% Cr, as influenced by temperature, precipitate size, technique artifacts, and, possibly, cascade ballistic mixing.