Fault-induced polytypism in (Cr, Fe)2B

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Abstract
The microstructures of chromium and iron borides formed during laser surface alloying of AISI 1045 steel with CrB were examined by transmission electron microscopy combined with selected-area electron diffraction and X-ray-dispersive spectroscopy. It was found that the heavily faulted (Cr,Fe)2B phase contains a microsyntactic existence of several polytypes, including one-dimensionally disordered structures as well as fully developed lamellae of ordered polytypes, similar to those found in SiC. Classification of (Cr, Fe)2B as a polytype was made for the first time and is described in detail. It was found that in this case polytypism originates from the (Fe, Cr)2B → (Cr, Fe)2B transformation in the solid state by the introduction of stacking faults in the (Fe, Cr)2B phase, owing to a close similarity between the structures of both phases. This transformation is directly responsible for the mentioned microsyntaxy and was experimentally observed through transmission electron microscopy.