The origin of predominance of cementite among iron carbides in steel at elevated temperature
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The origin of predominance of cementite among iron carbides in steel at elevated temperature C.M. Fang1,2, M.H.F. Sluiter3, M.A. van Huis1,4, C.K. Ande2,3, H.W. Zandbergen1 1 Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands. 2 Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands. 3 Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. 4 EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium.
ABSTRACT A systematic first-principles study was conducted on the stability of binary iron carbides. The calculations showed that all the binary iron carbides are unstable relative to the elemental solids (α-Fe and graphite). Apart from a cubic Fe23C 6 phase, the energetically most favorable carbides exhibit hexagonal close-packed (hcp) Fesublattices. Structural relaxation of the hcp iron carbides was analyzed and discussed together with their relative thermodynamically stability. Finite-temperature analysis showed that contributions from lattice vibration and anomalous magnetic ordering (Curie-Weiss behavior), rather than from the conventional lattice mismatch with the matrix, are the origin of the high stability and predominance of cementite among the iron carbides in steels.
INTRODUCTION In spite of numerous efforts in the last decades, the physical understanding of the formation and stability of iron carbide phases is still in a premature state [1-10]. In the early 1950s, Hofer and Cohn investigated formation of iron carbide precipitates in quenched steels [1,2]. They observed different stages of carbide formation starting at temperatures of about 370 K. They also observed that the final precipitate is -Fe3C, cementite. Slow-cooling steels contain mainly ferrite and cementite [1-4]. Furthermore, θ-Fe3C was also found during carburization of iron metal/iron oxides [5]. Recent experiments showed that cementite, as a weak magnet, also shows almost no thermal expansion (Invar behavior) at the temperature below its Curie temperature [6]. Despite of many theoretical efforts, very basic questions on iron-ironcarbide phase equilibrium remain unanswered [3-5,7,8]. One prominent question is why the cementite phase is observed much more frequently than other carbide phases in ferritic low-carbon steel. Iron carbides exhibit a rich variety of crystal structures and physical properties. About a dozen structural models for binary iron carbides have been obtained or proposed by experimentalists, covering a wide variety of crystal structures [2,8-10]. To explain the
predominance of cementite in steel, theoretical approaches, especially the parameter-free state-of-the-art electronic density-functional theory (DFT) techniques, can provide essential insights. Here we review our recent study on the structures and stability of binary iron carbides using DFT methods [11-15]. Various structural models and magnetic orderings are examined for in total 17 carbide phases,
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