Effect of Lanthanum on the Precipitation of NbC in Ferritic Steels

The effect of lanthanum on the precipitation of NbC in ferritic steel has been studied within the experimental and theory methods. The diffusion coefficients of Niobium resulted from the diffusion couple experiment shows that Nb diffuses slightly faster in the presence of La than that in pure a-Fe. Using the first-principles calculations based on the density functional theory, we studied the interactions between any two defects of La, Nb and vacancy, as well as the variation of diffusion activation energies caused by the presence of La. The interactions between La and Nb are calculated to be repulsive in both the first and the second nearest neighbor configurations, and the vacancy is expected to form in the La-Nb clustering area, which leads to the decrease of the diffusion activation energy of Nb. To elucidate the effect of La on the precipitation of NbC in ferritic steel, the above diffusion coefficients were substituted into the precipitation kinetics model. Agree with the experimental data, the simulated evolutions of the fractions and radius of NbC precipitate reveal that the addition of La leads to the faster precipitation kinetics of NbC in ferritic steels.