Evolution from clusters to precipitates in niobium-micro-alloyed ferritic steel: A combined in situ scanning transmission electron microscopy and atomistic study

Nano-sized carbides are essential for strengthening steel. In this study, we examined the evolution of metal carbide (M-C) clusters (M = Ti and Nb) into MC precipitates within a ferrite matrix through a reverse Bain transformation at the atomic level using the first-principles method. Through in situ transmission electron microscopy, the evolution of the Nb-C cluster into an NbC precipitate was observed for the first time. The Nb-C cluster was confirmed to maintain coherence with a limited carbon content, assuming a disc-shaped form that grew in a specific direction owing to misfit strain upon reaching a critical size. Interface dislocations were also formed. The NbC precipitate formation was subsequently completed through additional carbon diffusion. The findings of this study indicate that MC precipitation in a ferrite matrix can be attained through coherent cluster evolution with a specific carbon content.