Phase transformation of mixed Cr_1−xAl_xN nitride precipitates in ferrite

Nitriding of specimens with the composition Fe–1.5 wt% Cr–1.5 wt% Al (Fe–1.6 at.% Cr–3.1 at.% Al) at 853 K leads to the formation of mixed, ternary Cr1− x Al x N nitride platelets precipitated in the cubic, rock-salt structure type obeying a Bain-type orientation relationship with the ferrite matrix. Upon subsequent annealing (at 973 K) the mixed, ternary nitrides transform into the two equilibrium, binary nitrides, namely CrN of cubic, rock-salt structure type in the Bain orientation relationship with the ferrite matrix and AlN of hexagonal, wurtzite structure type, obeying a Pitsch–Schrader orientation relationship with the ferrite matrix. At the same time, the mobile excess nitrogen, dissolved in the ferrite matrix, diffuses towards the originally not nitrided core, where relatively coarse, cubic CrN and hexagonal AlN precipitates develop. The microstructure and (local) composition changes were analysed using X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray and electron probe microanalysis techniques. It was found that the transformation proceeds by Al depletion of the original mixed Cr1− x Al x N precipitates.