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 Ti1−xBxN and TiBxN1−x solution phases. In order to study the stability of the ternary fcc(NaCl)–Ti1−xBxN and of the nitrogen-deficient fcc(NaCl)–TiBxN1−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)–Ti1−xBxN should decompose by spinodal mechanism, whereas the substoichiometric fcc(NaCl)–TiBxN1−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.