Microstructure and crystallography of massive cementite layers on ferrite substrates

Abstract Upon gaseous nitrocarburizing, massive cementite layers were grown on ferrite substrates. By means of electron backscatter diffraction, it was shown that the Bagaryatsky orientation relationship holds between the cementite grains of the compound layer and the ferrite grains of the substrate. The experimental data exhibit a preference for specific variants of this orientation relationship: those variants corresponding to relatively low cementite–ferrite misfit-strain energy are favoured. Furthermore, the orientation relationship of neighboring cementite grains grown on the same ferrite grain allows the establishment of low-energy grain boundaries between these cementite grains. In contrast, at ferrite-grain boundaries intersecting the substrate surface, high-energy cementite-grain boundaries occur between cementite grains on adjacent ferrite grains. The latter cementite-grain boundaries promote carbon grain-boundary diffusion through the cementite, which is experimentally supported by the observation of relatively large cementite-layer thicknesses at these locations.