In situ X-ray scattering study of the cubic to hexagonal transformation of AlN in Ti1−xAlxN

In the present work, we have studied the decomposition of arc-evaporated Ti0.55Al0.45N and Ti0.36Al0.64N during heat treatments in vacuum by in situ synchrotron wide-angle X-ray scattering primarily to characterize the kinetics of the phase transformation of AlN from the cubic (c) NaCl structure to the hexagonal (h) wurtzite structure. In addition, in situ small-angle X-ray scattering measurements were conducted to explore details of the wavelength evolution of the spinodal decomposition, thus providing information about the critical size of the c-AlN-rich domains prior to the onset of the transformation to h-AlN. We report the fractional cubic to hexagonal transformation of AlN in Ti1−xAlxN as a function of time and extract activation energies between 320 and 350 kJ mol−1 depending on the alloy composition. The onset of the hexagonal transformation occurs ∼50 K lower in Ti0.36Al0.64N compared to Ti0.55Al0.45N where the high Al content alloy also has a significantly higher transformation rate. A critical wavelength for the cubic domains of ∼13 nm was observed for both alloys. Scanning transmission electron microscopy shows a c-TiN/h-AlN microstructure with a striking morphology resemblance to the c-TiN/c-AlN microstructure present prior to the hexagonal transformation.