Studying mechanosynthesized Haegg carbide ( chi -Fe sub(5)C sub(2))

• Published in: Physics of metals and metallography 2015-08, Vol.116 (8), p.791-801
• Main Authors: Barinov, V A, Protasov, A V, Surikov, V T
• Format: Article
• Language: English
• Subjects: Carbides; Carbon; Cementite; Crystallography; Formations; Iron; Mossbauer effect; Unit cell
• ISSN: 0031-918X; 1555-6190
• DOI: 10.1134/S0031918X15080025

Methods of thermomagnetic analysis and Mossbauer experiments ( super(57)Fe) were used to investigate the formation of Haegg carbide ( chi -Fe sub(5)C sub(2)) under the conditions of mechanical milling of alpha -Fe in a medium of liquid hydrocarbons. It has been established that, with the employed parameters of milling, the synthesis of chi carbide begins after the completion of the stage of the formation of cementite ([thetas] phase). The borderline of temperature stability of the monophase state of the chi carbide has been determined to be no more than 800 K. At T > 800 K, chi carbide decomposes into cementite and free carbon. The optimum temperature of heating of the synthesized Haegg carbide at which the population of the crystallographically nonequivalent positions of the Fe atoms is close to the ideal (0.2: 0.4: 0.4) is 775 K; the Curie temperature is T sub(C) = 520 K. The analysis of the Mossbauer data and of the results of a geometrical simulation of the configurations of Fe atoms in the the chi carbide unit cell made it possible to establish that the above relationship between the populations of positions is satisfied with the allowance for the anisotropic component h sub(an) of the field of hyperfine interaction. Under the effect of h sub(an), the crystallographically equivalent atoms Fe(4e) become nonequivalent (Fe(e sub(1)) and Fe(e sub(2))) in the magnetic sense. This specific feature manifests in the appearance in the presence of the distribution of hyperfine fields P(H) of two Mossbauer contributions, i.e., p(e sub(1)) and p(e sub(2)) with equal fractions of iron atoms in each of the contributions f sub(Fe)(e sub(1)) = = f sub(Fe)(e sub(2)) = 0.1 with the magnitudes of the fields H approximately 11 and 16 T, respectively.