Modeling the metastable phase formation of compositionally complex face-centered cubic Ti-V-Nb-Ta-Al-N thin films

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Metastable phases are commonly obtained during synthesis of thin films due to kinetically limited processes, which render the prediction of their phase formation rather challenging. Recently, the metastable solubility limits of AlN in face-centered cubic (fcc) Ti1−xAlxN thin films have been investigated by us. On this basis we propose to predict the metastable phase formation of compositionally complex face-centered cubic Ti-V-Nb-Ta-Al-N thin films using a correlative experimental and theoretical approach. Utilisation of such higher-order material systems as multi-element thin films provides, in some cases, enormous untapped application potential with respect to both, enhanced thermal stability and superior mechanical properties. Our research strategy comprises the combinatorial synthesis of thin films in the quaternary Ti-V-Al-N, quinary Ti-V-X-Al-N (X = Nb or Ta), and senary Ti-V-Nb-Ta-Al-N systems and their experimental characterization in terms of composition and structure. Enthalpies of formation, lattice parameters, elastic constants, and activation energy barriers for surface and bulk diffusion will be obtained from ab initio calculations for various compositions using density functional theory-based methods. Incorporating the experimental and ab initio data as input, thermodynamic descriptions of compositionally complex systems will be obtained using the CALPHAD approach. This methodology – already established for ternary nitrides – will be critically appraised for these highly complex systems, yielding TiN-VN-AlN, NbN-TiVN-AlN, TaN-TiVN-AlN, and NbTaN-TiVN-AlN metastable phase formation diagrams. We expect that these metastable phase formation diagrams will provide a solid basis for future rationally-guided design efforts for compositionally complex, face-centered cubic thin film materials.