Growth characteristics of primary M7C3 carbide in hypereutectic Fe-Cr-C alloy

Abstract The microstructure of the hypereutectic Fe-Cr-C alloy is observed by optical microscopy (OM). The initial growth morphology, the crystallographic structure, the semi-molten morphology and the stacking faults of the primary M 7 C 3 carbide are observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The in-suit growth process of the primary M 7 C 3 carbide was observed by confocal laser microscope (CLM). It is found that the primary M 7 C 3 carbide in hypereutectic Fe-Cr-C alloy is irregular polygonal shape with several hollows in the center and gaps on the edge. Some primary M 7 C 3 carbides are formed by layers of shell or/and consist of multiple parts. In the initial growth period, the primary M 7 C 3 carbide forms protrusion parallel to {"Equation missing"} crystal planes. The extending and revolving protrusion forms the carbide shell. The electron backscattered diffraction (EBSD) maps show that the primary M 7 C 3 carbide consists of multiple parts. The semi-molten M 7 C 3 carbide contains unmelted shell and several small-scale carbides inside, which further proves that the primary M 7 C 3 carbide is not an overall block. It is believed that the coalescence of the primary M 7 C 3 carbides is ascribed to the growing condition of the protrusion and the gap filling process.