APT characterization of irradiation effects on MX phase in reduced-activation ferritic/martensitic steels

Reduced activation ferritic/martensitic steels (RAFMs), such as Eurofer 97 and F82H, are designed to enhance the high-temperature strength by forming the fine dispersive particles of the MX phase. However, the stability of MX phase is still not fully understood. The objective of this study is to determine irradiation effects on the MX phase of Eurofer 97 and F82H. The irradiation experiments were performed at the Swiss Spallation Neutron Source (SINQ) at a temperature of ∼300 °C to a dose of ∼20 dpa. The current work reveals the irradiation effect on the MX phase on an atomic scale by atom probe tomography (APT). The irradiation on the one hand can affect the composition of the MX phase, on the other hand, induces interface segregation. Compared with the nitride in the unirradiated Eurofer 97, the nitride in the irradiated specimen contains less V and Ta, while more Cr, Fe, Mn, and W. The irradiation-induced segregation shows consistency among different interfaces between the MX nanoprecipitate, the matrix and the M₂₃C₆ precipitate that the undersized elements, Mn, Si, and Cu are enriched, oversized elements W, V and Ta are depleted, while the spallation transmutants Ca, Sc, and K, being largely oversized elements, enriched at the phase boundary as well. The irradiation-induced changes of the MX phase indicates that in the current irradiation condition the MX phase endures an initial transformation process to Z-phase and in the meanwhile a dissolution process at the edge. Ta in the RAFM steels plays a similar role as Nb in the conventional ferritic/martensitic (FM) steels during the MX phase to the Z-phase transformation process where Nb diffuses out of the MX phase.