Sublattice alloy design of high-strength steels: Application of clustering and nanoscale precipitation of interstitial and substitutional solutes

Recent advances in high-strength steels for automobile applications or surface-hardened steels have attracted interest on controlling the nanoscale dispersion of alloy carbides/nitrides in ferrite. Particularly, B1-type carbide/nitride precipitation and metastable alloy clustering can be understood as the coupled ordering and compositional phase separation of interstitial alloying elements i and substitutional alloying elements s, i.e., "spinodal ordering". In this paper, the behavior of i–s clustering and alloy carbide/nitride precipitation is revealed by nanoscale characterization, theoretical calculations, and evaluations of mechanical properties. Physical measurements and density function theory-based analyses revealed that strong i–s attractive interaction is responsible for metastable i–s clustering, which clearly indicated "hidden" nanoscale inhomogeneity in solid solutions. Further, we discuss a concept termed interstitial sublattice engineering in bulk and at interphase boundary for strengthening of alloyed steels.