Stability of Ti-B-N solid solutions and the formation of nc-TiN/a-BN nanocomposites studied by combined ab initio and thermodynamic calculations

The ab initio density functional theory (DFT) has been used to calculate the properties of binary face-centered cubic (fcc)(NaCl)- and fcc (ZnS)-TiN and BN, hexagonal-close-packed (hcp)-TiB₂, and ternary Ti_1-xB_xN and TiB_xN_1-x solution phases. In order to study the stability of the ternary fcc(NaCl)-Ti_1-xB_xN and of the nitrogen-deficient fcc(NaCl)-TiB_xN_1-x solution, their mixing energies and the phase stability diagrams were constructed over the entire range of compositions. The results show that the fcc(NaCl)-Ti_1-xB_xN should decompose by spinodal mechanism, whereas the substoichiometric fcc(NaCl)-TiB_xN_1-x should decompose via nucleation and growth. The relatively large lattice mismatch between the fcc(NaCl)-TiN and fcc(NaCl)-BN, and the high lattice instability of the fcc(NaCl)-BN with respect to fcc(ZnS)-BN, suggests that the spinodal decomposition will, in the later stages, be accompanied by transformation of the fcc(NaCl)-BN to a more stable phase.