We have used first-principles calculations to investigate the mixing enthalpies, lattice parameters and electronic density of states of the ternary nitride systems Ti1−xAlxN, Cr1−xAlxN, Sc1−xAlxN and Hf1−xAlxN in the cubic B1 structure where the transition metals and aluminium form a solid solution on the metal sublattice. We discuss the electronic origins of the possible isostructural decomposition in these materials relevant for hard coatings applications. We find that in the systems Ti1−xAlxN and Hf1−xAlxN the electronic structure effects strongly influences the phase stability as d-states are localised at the Fermi level in AlN-rich samples. This leads to a strongly asymmetric contribution to the mixing enthalpy, an effect not present in Cr1−xAlxN and Sc1−xAlxN. The lattice mismatch is large in Sc1−xAlxN and Hf1−xAlxN, giving a symmetric contribution to the mixing enthalpies in those systems.

In order to investigate the stability of the cubic phase of Cr 1−x Al x N at high AlN content, first principles calculations of magnetic properties, lattice parameters, electronic structure, and mixing enthalpies of the system were performed. The mixing enthalpy was calculated on a fine concentration mesh to make possible the accurate determination of its second concentration derivative. The results are compared to calculations performed for the related compound Ti 1−x Al x N and with experiments. The mixing enthalpy is discussed in the context of isostructural spinodal decomposition. It is shown that the magnetism is the key to understand the difference between the Cr-and Ti-containing systems. Cr 1−x Al x N turns out to be more stable against spinodal decomposition than Ti 1−x Al x N, especially for AlN-rich samples which are of interest in cutting tools applications.

The effect of nitrogen substoichiometry on the isostructural phase stabilities of the cubic Ti1−xAlxN1−y system has been investigated using first-principles calculations. The preferred isostructural decomposition pattern in these metastable solid solutions was predicted from the total energy calculations on a dense concentration grid. Close to the stoichiometric Ti1−xAlxN1 limit, N vacancies increase the tendency for phase separation as N sticks to Al while the vacancies prefers Ti neighbors. For nitrogen depleated conditions, N sticks to Ti forming TiNδ (0<δ<1) while Al tends to form nitrogen-free fcc-Al or Al–Ti alloys.

Please refer to published version for the most recent bibliographic citation information. If a published version is known of, the repository item page linked to above, will contain details on accessing it.