The crystallization features of nodular cast iron alloyed with 19-25% Al have been investigated. Using the Thermo-Calc computer program, polythermal sections of the phase diagrams of the quaternary Fe-Al-C-Si system were obtained on an aluminum scale from 0 to 40% with varying carbon and silicon contents between 1.5–2.5 and 1.0–2.0%, respectively. The obtained results allowed us to identify the sequence of phase transformations during primary and secondary crystallization of cast iron. It has been established that the crystallization of cast iron begins with the separation of primary graphite crystals from the liquid phase. Then, a double eutectic crystallizes, consisting of a solid solution based on aluminum-doped ferrite and graphite (α + CGr). With an increase of silicon content to 2%, a triple eutectic appears, consisting of aluminum alloyed with ferrite, graphite and aluminum carbide Al4C3 (α + CGr + Al4C3). After that, the maximum alloyed with aluminum ferrite is released from the remainder of the liquid phase. Upon subsequent cooling of cast iron due to a decrease in the solubility of aluminum and carbon in the α phase, Al4C3 carbide is released from it. These data are consistent with the results of thermographic studies. The study of the structure of sand cast iron castings, showed that it generally corresponds to expectations of polythermal sections and thermographic analysis. But different in details. So, in cast iron, instead of the expected Al4C3 carbide, iron - aluminum carbide is formed, which is close with composition to Fe1.5AlCx.Keywords: Aluminum cast iron, crystallization, phase transformations, structure
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