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 M7C3 carbide are observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The in-suit growth process of the primary M7C3 carbide was observed by confocal laser microscope (CLM). It is found that the primary M7C3 carbide in hypereutectic Fe-Cr-C alloy is irregular polygonal shape with several hollows in the center and gaps on the edge. Some primary M7C3 carbides are formed by layers of shell or/and consist of multiple parts. In the initial growth period, the primary M7C3 carbide forms protrusion parallel to {} crystal planes. The extending and revolving protrusion forms the carbide shell. The electron backscattered diffraction (EBSD) maps show that the primary M7C3 carbide consists of multiple parts. The semi-molten M7C3 carbide contains unmelted shell and several small-scale carbides inside, which further proves that the primary M7C3 carbide is not an overall block. It is believed that the coalescence of the primary M7C3 carbides is ascribed to the growing condition of the protrusion and the gap filling process.
Highlights
He Fe-Cr-C system has been investigated many times over the years
The mobile atoms in the molten alloy can be adsorbed on the steps so that the M7C3 carbide preferentially grows along the c-axis
The primary M7C3 carbide in Fig. 1(d) consists of two visible parts and the right part exhibits a gap on the edge
Summary
He Fe-Cr-C system has been investigated many times over the years. The Fe-Cr-C system contains kinds of carbides such as M23C6, M7C3 and M3C21. Leško et al.[18] have put forward a theory on the growth of M7C3 carbide in the basis of the dissolved screw dislocation core It held that the inherent screw dislocations on the basal-plane of the hexagonal M7C3 carbide can form the revolving steps along the c-axis. It is found in many studies that the primary M7C3 carbide is irregular polygon shape with several hollows and some gaps on the edge, rather than the simple shape mentioned above[20,21]. These phenomena can be hardly explained by the two theories mentioned above. Carbide is proposed, which provides reasonable interpretation for the irregular polygonal shape of the primary M7C3 carbide with several hollows in the center and gaps on the edge
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