Stability of (Ti, M)C (M = Nb, V, Mo and W) carbide in steels using first-principles calculations

The lattice parameters, formation energies and bulk moduli of (Ti, M)C and M(C, Va) with the B1 crystal structure have been investigated using first-principles calculations, where M = Nb, V, Mo and W. The replacement at 0 K of Ti by Mo or W in the TiC lattice is found to be energetically unfavorable with respect to the formation energy. However, it decreases the misfit strain between the carbide and ferrite matrix, a factor which is of critical importance during the early stages of precipitation, thus favoring the substitution of Ti by Mo, as is observed in practice. The effect of Mo in enhancing the coarsening resistance of (Ti, Mo)C precipitates is discussed in terms of its role in the nucleation process, but followed by a more passive contribution during coarsening itself. The role of tungsten has been predicted to have a similar effect to molybdenum on the nucleation and coarsening process. Analysis of precipitates in Ti-, Ti–Mo- and Ti–W-bearing steels shows results consistent with the calculations.