Scientists from the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS), in collaboration with researchers from The Hong Kong Polytechnic University, have clarified the dual regulatory role of niobium (Nb) in the microstructural stability of high-silicon austenitic stainless steel, offering a new strategy to suppress austenite decomposition during long-term service at elevated temperatures.
Lead-cooled fast reactors, a key reactor type in Generation IV nuclear energy systems, require structural materials that can withstand prolonged exposure to high temperatures, irradiation, and liquid lead-bismuth eutectic (LBE) corrosion. High-silicon austenitic stainless steels exhibit excellent LBE corrosion resistance, making them promising candidates for reactor vessels and internals. However, silicon addition significantly alters precipitation mechanisms and can induce austenite decomposition, threatening long-term microstructural stability.
Building on their previous work that revealed silicon-induced austenite decomposition and associated mechanical degradation, the team investigated how Nb addition regulates this decomposition during thermal aging at 550 °C. They discovered that Nb exhibits a "dual role." On one hand, Nb preferentially forms NbC, which suppresses the precipitation of M₂₃C₆ carbides and thereby delays austenite decomposition. On the other hand, the strong interaction between Nb and Si promotes the formation of (Nb, Si)-rich clusters, which further induce a new pathway of austenite decomposition.
The study elucidated a two-stage evolution mechanism. During intermediate aging, (Nb, Si)-rich clusters serve as favorable precursors for M₆C carbide nucleation. The growth of M₆C consumes Ni and C from the surrounding austenite, creating local Ni- and C-depleted regions that trigger the γ → α-ferrite transformation. Upon longer aging, these clusters further promote the nucleation of the G phase, leading to a eutectoid transformation of γ → G phase + α-ferrite, with the cooperative lamellar growth controlled by Nb diffusion in the austenite ahead of the decomposition front.
Guided by these mechanistic insights, the team proposed a stabilization treatment at 900 °C. This treatment precipitates dispersed secondary NbC particles while reducing the solute Nb and C content in the austenite, synergistically suppressing both M₆C carbide formation and Nb-partitioning-induced austenite decomposition. After the stabilization treatment, no obvious austenite decomposition was observed following 3000 hours of thermal aging at 550 °C.
The findings, published in Acta Materialia, provide a new theoretical basis for the composition design and microstructural stability control of long-life, LBE-corrosion-resistant high-silicon austenitic steels.
Nb partitioning-mediated austenite decomposition process. (Image by IMR)
Formation of (Nb, Si)-rich clusters and composition changes in adjacent austenite. (Image by IMR)
Two-stage evolution mechanism of Nb-regulated austenite decomposition in high-silicon austenitic steel. (Image by IMR)
Stabilization treatment further suppresses austenite decomposition. (Image by IMR)
