TEM and EBSD investigation of continuous and discontinuous precipitation of CrN in nitrided pure Fe-Cr alloys

Pure Fe-Cr alloys (1 and 3 wt% Cr) were gas nitrided (NH3, N2, N2O mixture at 823 K). Two modes of CrN precipitation: continuous (fine disc-shaped precipitates) and discontinuous (lamellae-like precipitates) were identified and investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Both types of precipitates presented a cubic NaCl-type structure and Baker-Nutting orientation relationship with respect to the ferritic matrix. A quantification procedure based on TEM images exploitation revealed that the size and the number of the fine precipitates vary inversely with nitriding depth. This result was compared to the profile of micro-hardness. SEM observations showed that only superficial regions in the Fe-3 wt% Cr were transformed by the discontinuous precipitation of CrN. Electron back-scattered diffraction (EBSD) studies of lamella revealed singular initiation and growth features. A qualitative mechanism of lamella initiation and growth is discussed.