The formation mechanism of complex carbides in Nb-V microalloyed steel.

[Display omitted] • HRTEM and APT were used to investigate the formation mechanisms. • Replacement and heterogeneous mechanism can appear simultaneously. • Two mechanisms both facilitate precipitation for the reducing interfacial energy. The formation mechanism of complex precipitates in Nb-V microalloyed steel has been systematically studied by high resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). The results reveal that there are two precipitation mechanism: replacement and heterogeneous mode. The core–shell precipitate has been observed with double direction Moiré fringe, which is formed by the heterogeneous mechanism, while for the precipitates with uniform distributed V and Nb atoms, is formed by the replacement mechanism. These two modes both reduce the lattice misfit and promote precipitation. In addition, the APT result reveals that the ratio of V to Nb increases with the increase of particle size, which matches well with the previous TEM results. [ABSTRACT FROM AUTHOR]