Ultrahigh stability and strong precipitation strengthening of nanosized NbC in alumina-forming austenitic stainless steels subjecting to long-term high-temperature exposure

For precipitation-strengthened heat-resistant steels, property degradation resulted from phase coarsening is always one of the most critical challenges, and therefore, knowledge regarding the long-term influence of temperature on mechanical properties and microstructure is prerequisite for their applications. In this work, long-term thermal aging up to 10,000 h of typical alumina-forming austenitic (AFA) stainless steels was conducted, and evolution of microstructure and mechanical properties of these promising heat-resistant alloys were studied systematically. It was found that mechanical performance of the steel strengthened by secondary nanosized NbC showed not only much enhanced creep resistance, superior to that of most traditional heat-resistant steels, but also stable high-temperature strength even after long-term aging. The spheroidal secondary NbC particles showed extreme coarsening resistance with a low coarsening kinetic constant (almost six orders of magnitude smaller than that of the Laves Fe2Nb phase). The low interfacial energy of nanosize NbC, which was resulted from the cube-on-cube orientation relationship and semi-coherent interfacial structure with the matrix, gave rise to its sluggish coarsening kinetics. Our current findings not only confirm that the AFA alloys strengthened by dense precipitated, nanosized semi-coherent NbC is a promising structural material for long-term high-temperature applications, but also sheds new insights into understanding precipitation hardening mechanism for high-temperature materials in general.