Quantitative X-ray diffraction analysis of development of Z phase in 12%Cr-Nb-V-N steel

To study the evolution of nitrides (Nb,V)N, (V,Nb)N and Cr(2)N, and in particular the formation of the Z phase Cr(V,Nb)N, a model alloy with composition similar to that of 12%Cr steels for high temperature applications, microalloyed with Nb and V but with a very low carbon content, has been designed. A quantitative determination of the volume fractions of the extracted nitrides that had formed after aging treatments at 650 degrees C for up to 10 000 h was carried out by an X-ray diffraction procedure, based on the Rietveld approach. The investigation of the Z phase evolution by the Johnson-Mehl-Avrami-Kolmogorov kinetics at 650 and 700 degrees C revealed that, as the kinetic exponent is very close to unity, the formation mechanism of this phase is not associated with a conventional process of nucleation but hints at a gradual diffusion controlled transformation of the pre-existing V and Nb nitrides.