  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.
