We propose a design route for the next generation of nitride alloys via a concept of multicomponent alloying based on self-organization on the nanoscale via a formation of metastable intermediate products during the spinodal decomposition. We predict theoretically and demonstrate experimentally that quasi-ternary (TiCrAl)N alloys decompose spinodally into (TiCr)N and (CrAl)N-rich nanometer sized regions. The spinodal decomposition results in age hardening, while the presence of Cr within the AlN phase delays the formation of a detrimental wurtzite phase leading to a substantial improvement of thermal stability compared to the quasi-binary (TiAl)N or (CrAl)N alloys.

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Metastable solid solutions of cubic (c)-(TixCryAlz)N coatings were grown by a reactive arc evaporation technique to investigate the phase transformations and mechanisms that yield enhanced high-temperature mechanical properties. Metal composition ranges of y < 17 at. % and 45 < z < 62 at. % were studied and compared with the parent TiAlN material system. The coatings exhibited age hardening up to 1000 °C, higher than the temperature observed for TiAlN. In addition, the coatings showed a less pronounced decrease in hardness when hexagonal (h)-AlN was formed compared to TiAlN. The improved thermal stability is attributed to lowered coherency stress and lowered enthalpy of mixing due to the addition of Cr, which results in improved functionality in the temperature range of 850–1000 °C. Upon annealing up to 1400 °C, the coatings decompose into c-TiN, bcc-Cr, and h-AlN. The decomposition takes place via several intermediate phases: c-CrAlN, c-TiCrN, and hexagonal (β)-Cr2N. The evolution in microstructure observed across different stages of spinodal decomposition and phase transformation can be correlated to the thermal response and mechanical hardness of the coatings.

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Through a combination of theoretical and experimental observations we study the high temperature decomposition behavior of c-(TixZryAlzN) alloys. We show that for most concentrations the high formation energy of (ZrAl)N causes a strong tendency for spinodal decomposition between ZrN and AlN while other decompositions tendencies are suppressed. In addition we observe that entropic  effects due to configurational disorder favor a formation of a stable Zr-rich (TiZr)N phase with increasing temperature. Our calculations also predict that at high temperatures a Zr rich (TiZrAl)N disordered phase should become more resistant against the spinodal decomposition despite its high and positive formation energy due to the specific topology of the free energy surface at the relevant concentrations. Our experimental observations confirm this prediction by showing strong tendency towards decomposition in a Zr-poor sample while a Zr-rich alloy shows a greatly reduced decomposition rate, which is mostly attributable to binodal decomposition processes. This result highlights the importance of considering the second derivative of the free energy, in addition to its absolute value in predicting decomposition trends of thermodynamically unstable alloys.

On the day of the defence date the status of this article was Manuscript.