FAULT-INDUCED POLYTYPISM IN (CR, FE)(2)B

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)(2)B 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)(2)B 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)(2)B --> (Cr, Fe)(2)B transformation in the solid state by the introduction of stacking faults in the (Fe, Cr)(2)B 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.