Effects of decomposition route and microstructure on h-AlN formation rate in TiCrAlN alloys

Abstract The phase evolution of cubic (c), solid solution Ti x Cr ∼0.37 Al 1−0.37−x N alloys with x = 0.03 and 0.16, and the kinetics of the hexagonal (h)-AlN formation are studied via in situ wide angle x-ray scattering experiments during high temperature (1000–1150 °C) annealing. Spinodal decomposition was observed in Ti 0.16 Cr 0.36 Al 0.48 N while Ti 0.03 Cr 0.38 Al 0.59 N decomposes through nucleation and growth of h-AlN, c-TiN and c-CrAlN. h-AlN is formed from c-CrAlN domains in both cases and the formation rate of h-AlN depends on the stability of the c-CrAlN domains. In Ti 0.16 Cr 0.36 Al 0.48 N, the c-CrAlN domains are stabilized by crystallographic coherency with the surrounding c-TiCrN in a microstructure originating from spinodal decomposition. This results in lower formation rates of h-AlN for this composition. These differences are reflected in higher activation energy for h-AlN formation in Ti 0.16 Cr 0.36 Al 0.48 N compared to Ti 0.03 Cr 0.38 Al 0.59 N. It also points out different stabilities of the intermediate phase c-CrAlN during phase decomposition of TiCrAlN alloys. Additional contributions to the low activation energy for formation of h-AlN in Ti 0.03 Cr 0.38 Al 0.59 N stems from precipitation at grain boundaries.