χ phase and its effect on the mechanical properties of a Mo-bearing high–Si austenitic stainless steel after aging at 650 °C

In this study, a Mo-bearing high–Si austenitic stainless steel was designed for lead-cooled fast reactor, and the precipitation behavior and mechanical properties of the experimental steel after 650 °C isothermal aging for 3000 h were investigated. The results showed that Mo-enriched χ phase was revealed to preferentially precipitate not only at grain boundaries, but also on the nano-sized secondary MC particles during long-term aging. The secondary MC carbides acted as nucleation sites of χ phase particles, leading to lenticular shape composite precipitates. Si can improve the precipitation driving force and was discovered to segregate in the χ phase, which indicates that Si can accelerate the formation of χ phase during aging. The precipitation of the χ phase dramatically reduced the plasticity and toughness of the steel, but did less harm to the strength.