A proper assessment of TEM diffraction patterns originating from CrN nitrides in a ferritic matrix

Abstract

Upon nitriding of a Fe-3%Cr alloy at 550 °C during 48 h at a nitriding potential KN = 5 atm−1/2, finely dispersed CrN grains precipitate. Considering that the nitrides developing within the nitrided layer adopt a cubic, rock-salt structure, Transmission Electron Microscopy (TEM) investigations revealed the Bain orientation relationships (also called Baker-Nutting) existing between the matrix and the precipitates. Given the latter and the structures of both ferrite (bcc) and nitrides (fcc), we have shown that group theory predicts the existence of three nitrides variants, orthogonal to each other, developing in ferrite along the 〈001〉α-Fe directions, which is in agreement with our experimental observations. The nitrides adopt a very thin platelet-like morphology, energetically corresponding to an absolute extremum. Diffraction patterns obtained along the [001]α-Fe result from the simultaneous diffraction of the ferritic matrix and the three nitrides variants. The platelet-like nitride precipitates give birth to diffuse intensity lines, originating from the two families developing along the (100) and (010) ferrite planes, as well as possible appearance of additional reflections, located at the forbidden positions {100}α-Fe and {110}CrN of the remaining variant, driving the interpretation of such diffraction patterns complicated. Foil thickness has proven to play a key role in the occurrence of this phenomenon. When the geometric diffraction conditions are favorable (i.e. very thin foil along the zone axis and very fine precipitates platelets), TEM analysis permitted to attribute these extra reflections to the nodes located in the first layer of the third variant's reciprocal space.