Phase transformation of mixed Cr1-xAlxN nitride precipitates in ferrite
Résumé
Nitriding of specimens with the composition Fe-1.5wt.%Cr-1.5wt.%Al (Fe-1.6at.%Cr-3.1at.%Al) at 853 K leads to the formation of mixed, ternary Cr1-xAlxN 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 have been analysed by XRD, (HR)TEM, STEM, EDX and EPMA. It was found that the transformation proceeds by Al depletion of the original mixed Cr1-xAlxN precipitates.
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