Enhanced oxidation resistance of nanocrystalline ODS ferritic alloy in high-temperature steam through addition of a small amount of Si

Abstract

Nanocrystalline (NC) oxide-dispersion-strengthened (ODS) alloys with a high fraction of grain boundaries (GBs) have recently been demonstrated with significantly improved performance, such as ultra-high strength, high creep resistance and irradiation tolerance, in comparison to conventional coarse-grained ODS alloys. Here, we report an enhanced high-temperature steam oxidation resistance of a Zr-Si co-alloyed NC ferritic ODS alloy (14YWTZS, with a nominal composition of Fe-14Cr-3W-0.4Ti-0.8Zr-0.3Y₂O₃-1Si (wt%)) developed by our research group, and reveal the oxidation mechanism in terms of the oxide scale evolution through detailed microstructural analysis. The oxidation behaviors of ultrafine-grained (UFG) 14YWT, Zr-alloyed NC 14YWTZ and Zr-Si co-alloyed NC 14YWTZS ferritic ODS alloys are comparatively investigated in high-temperature steam at 650 °C. The Zr-alloyed NC 14YWTZ alloy shows significantly enhanced oxidation resistance when compared with the UFG 14YWT alloy. This improvement is primarily attributed to the high-fraction GBs, which serves as short-circuits for the outward diffusion of selective solutes (mainly Cr). The oxidation resistance is further improved by alloying 1 wt% Si, forming a dense oxide scale accompanying with a continuous SiO₂ inner layer. The SiO₂ inner layer results mainly from the short-circuit transport of the segregated Si along the GB-network, facilitating the formation of a continuous SiO₂ layer at the early oxidation stage. This work demonstrates the synergy of composition design (low content Si alloying) and microstructure refinement (Zr alloying-induced nanocrystallization) for the development of high-temperature oxidation-resistant NC ODS alloys, which are suitable for extremely hostile conditions in future advanced nuclear reactors.