The effects of W and Mo on the formation of early stage α’ solute clusters in ion-irradiated Fe-Cr-X (X = W/Mo) alloys

• Cr-enriched clusters in Fe-9Cr, Fe-9Cr-0.5 W/Mo alloys under self-ion irradiation were investigated using APT. • W effectively suppresses the formation and evolution of Cr-enriched clusters. • Mo promotes the formation of Cr-enriched clusters and facilitates the growth of larger clusters. Reduced activation ferritic-martensitic (RAFM) steels are known to suffer from 475 °C embrittlement, which is associated with the formation of Cr-enriched α’ phase, while dislocation loops also contribute to radiation hardening. The precipitation behavior in RAFM steels is highly complex, involving the formation of various types of precipitates as well as intricate interactions between different alloying elements. To date, only a limited number of studies have investigated the effects of minor alloying elements, such as Ni, Si, P, C and Al, under irradiation conditions. To enhance understanding of α’ phase formation in RAFM steels, this study conducted detailed experiments using Atom Probe Tomography (APT) on Fe-9Cr, Fe-9Cr-0.5 W and Fe-9Cr-0.5Mo alloys under self-ion irradiation to examine the effects of W and Mo additions. Ion irradiation was performed using sequential energies of 0.5 MeV, 1 MeV, and 2 MeV to achieve target maximum damage levels of 5 dpa and 10 dpa at 350 °C. Cr-enriched clusters were identified in these ion-irradiated samples. It was observed that W strongly suppresses the formation and evolution of Cr-enriched clusters due to its strong binding with vacancies, which reduces Cr diffusion. In contrast, Mo appears to promote the formation of Cr-enriched clusters and facilitates the growth of large clusters, possibly due to the positive interaction between Mo and Cr atoms