Liquidus Surface and Spinodal of Fe-B-C Alloys

doi:10.26565/2312-4334-2020-1-06

Natalia Yu. Filonenko State Institution “Dnipropetrovsk Medical Academy of the Ministry of Health of Ukraine”, Dnipro, Ukraine; Z.I. Nekrasov Iron and Steel Institute of National Academy of Sciences of Ukraine https://orcid.org/0000-0003-1219-348X
Alexandra N. Galdina Oles Honchar Dnipro National University https://orcid.org/0000-0003-2311-8506

DOI: https://doi.org/10.26565/2312-4334-2020-1-06

Keywords: Fe2B boride, eutectics, thermodynamic stability of a melt, Fe3(CB) boron cementite, eutectics

Abstract

In this work the study is performed for the specimens of Fe-B-C alloys with boron content of 0.005–7.0 wt. % and carbon content of 0.4–6.67 wt. %, the rest is iron. According to the findings of microstructure analysis, XRD and differential thermal analyses, the primary phases and the temperatures of their formation are determined. Depending on boron content (in the range of 1.5–8.80 wt. %) and carbon content (0.5–6.67 wt. %) in the Fe-B-C alloys, the primary phases in the process of crystallization are γ-Fe, boron cementite Fe₃(CB) and boride Fe₂В. The outcomes of the experiment carried out in this work determine the phase composition and phase transformations occurring in the alloys and the liquidus surface is constructed. The findings show that the liquidus temperature for Fe-B-C system alloys is low compared to binary Fe-B and Fe-C alloys. At the liquidus surface of the Fe-B-C alloys, there is a point at boron content of 2.9 wt. % and carbon content of 1.3 wt. % with the lowest temperature of 1375 K and it is the point of intersection of monovariant eutectics. This fact is in a good agreement with the results of other authors. The microstructure of alloys located at the curves of monovariant eutectics is represented by the γ–Fe+Fe₂B and γ–Fe+Fe₃(CB) eutectics and the primary crystals of Fe₂B iron boride in the shell of Fe₃(BC) boron cementite. In this paper it is shown experimentally the existence of a quasi-binary section and the coordinates of the peritectic point are fixed: the boron content is 5.0 wt. %, carbon content is 3.0 wt. % and the temperature is 1515 K. The free energy of the Fe-B-C melt is calculated for the first time by the quasi-chemical method and the surface of thermodynamic stability of the Fe-B-C melt is plotted, depending on temperature and boron and carbon content in the alloy. The results obtained in the paper show that in order to obtain a homogeneous Fe-B-C melt, which does not contain any microheterogeneous structure in the form of short-order microregions, it is necessary to perform the overheating more than to 180 K for the region where the primary phase is iron, and no less than to 200 K for the regions with boron cementite and boride.

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