We investigated the high temperature decomposition behavior of wurtzite phase Ti$_{ 1-x}$Al$_{ x}$N films using experimental methods and first-principles calculations. Single phase metastable wurtzite Ti$_{ 1-x}$Al$_{ x}$N (x = 0.65, 0.75, 085 and 0.95) solid solution films were grown by cathodic arc deposition using low duty cycle pulsed substrate-bias voltage. First-principles calculated elastic constants of the wurtzite Ti$_{ 1-x}$Al$_{ x}$N phase show a strong dependence on alloy composition. The predicted phase diagram shows a miscibility gap with an unstable region. High resolution scanning transmission electron microscopy and chemical mapping demonstrate decomposition of the films after high temperature annealing (950$^{ \circ}$C), which resulted in nanoscale chemical compositional modulations containing Ti-rich and Al-rich regions with coherent or semi coherent interfaces. This spinodal decomposition of the wurtzite film causes age hardening of 1-2 GPa.
Categories: cond-mat.mtrl-sci cond-mat.other