Solute Behavior in the Initial Nucleation of V- and Nb-Containing Carbide

The three-dimensional atom probe (3DAP) is used to characterize the solute behavior at the initial nucleation stage of carbide that precipitates in a quench-temper processed multicomponent V-Nb microalloyed steel. The equilibrium (V,Nb)C phase does not precipitate directly from the ferrous matrix with martensite structure, whereas it is preceded by the formation of isostructural diffuse atmosphere, Fe-depleted V-Nb cluster, and metastable phase (Fe,V,Nb)C successively. The transient phases are considered to largely reduce the required high energy, which is often accompanied by the appearance of the fresh incoherent carbide/matrix boundary. During the evolution of these phases, vanadium and niobium diffuse to the previously existing carbon-enriched cluster concurrently but with different mobilities from the aspect of the kinetic pathway, leading to heterogeneous distribution of solutes in the transient phases.