In situ X-ray scattering study of the cubic to hexagonal transformation of A1N in Ti1-xA1xN

In the present work, we have studied the decomposition of arc-evaporated Ti(0.55)A1(0.45)N and Ti(0.36)A1(0.64)N during heat treatments in vacuum by in situ synchrotron wide-angle X-ray scattering primarily to characterize the kinetics of the phase transformation of AIN 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-MN-rich domains prior to the onset of the transformation to h-A1N. We report the fractional cubic to hexagonal transformation of MN in Ti1-x A1(x)N 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 similar to 50 K lower in Ti(0.36)A1(0.64)N compared to Ti(0.55)A1(0.45)N where the high Al content alloy also has a significantly higher transformation rate. A critical wavelength for the cubic domains of similar to 13 nm was observed for both alloys. Scanning transmission electron microscopy shows a c-TiN/h-A1N microstructure with a striking morphology resemblance to the c-TiN/c-A1N microstructure present prior to the hexagonal transformation. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.