Microstructure Analysis of Peritectoid Decomposition of Lamellar Equiectoid and Crystallization Process of Nanophases of FeC Monocarbide and Fe₂C ε-Carbide

In the course of studying the previously unknown process of homogeneous and heterogeneous crystallization of nanophases of iron monocarbide FeC and its cocrystallization with ε-carbide Fe₂C from a supersaturated solid solution based on ε-carbide Fe₂C or a polycarbide quasi-eutectic formed in the process of peritectoid decomposition during long-term heating (isothermal annealing) of lamellar ledeburite eutectoid in cast eutectic white cast iron, the crystallization of 2D-monolayers of FeC monocarbide allotropes in the form of translucent extended and elastic crystalline nanofilms was experimentally proven. Carbide phases in white cast iron can be characterized as a single isomorphic and isostructural polycarbide solid solution, which structurally crystallizes in the form of a mixture of carbide phases as a quasi-eutectic, the carbon content of which can freely vary within wide limits without identifying the actual carbide phases of the stoichiometric composition. The product of the decomposition of the lamellar eutectoid as a result of the peritectoid transformation during isothermal annealing is, in general, a polycarbide with a gradient crystal lattice of solid solutions corresponding in carbon concentration to one or another stoichiometric composition of iron carbide. The process of nucleation and growth of lamellar single crystals of ε-carbide Fe₂C and two-dimensional film crystals of allotropes of FeC monocarbide begins with the formation of globular or spherical nuclei with a diameter of 20–40 nm, which during the growth process form wickers or filiform nanocrystals with a diameter of up to 50 nm and an average length from 100 to 300 nm. Chemical analysis showed that globular and filiform inclusions correspond to the chemical composition of monocarbide FeC (18.0% C) and ε-carbide Fe₂C (9.6% C), and the ledeburite framework is a polycarbide quasi-eutectic including a continuous series of solid solutions based on carbides FeC (18.0% C); Fe₃C₂ (12.5% C); Fe₂C (9.6% C); Fe₅C₂ (8.0% C); Fe₃C (6.67% C). The lamellar inclusions correspond to the chemical composition of ε-carbide Fe₂C (9.6% C). Numerous studies of the carbon content in ledeburite white cast iron have shown that its composition in most cases does not correspond to the generally accepted carbon content, which corresponds to the carbon content in cementite θ-Fe₃C (6.67% C).