Dissolution of M23C6 and new phase re-precipitation in Fe ion-irradiated RAFM steel

Abstract

The M23C6 precipitate plays a major role in preventing the sliding of the grain boundary and strengthens the matrix in the reduced-activation ferritic/martensic (RAFM) steel. However, its stability might be reduced under irradiation. The microstructural instability of the M23C6 precipitates in the RAFM steels irradiated at 300°C with Fe ions up to a peak dose of 40 dpa was investigated by transmission electron microscopy. A “Core/Shell” morphology was found for the pre-existing M23C6 and a large number of new small phases appeared in parallel near the periphery of the precipitates after irradiation. The loss of crystallinity of the M23C6 periphery due to the dissolution of carbon atoms into the interface (C-rich “Shell”) actually decreased the size of the Cr-rich “Core”. The new phase that formed around the pre-existing precipitates was M6C (Fe3W3C), which was formed through the carbide transformation of M23C6 to M6C.