Studying mechanosynthesized Hägg carbide (χ-Fe5C2)

• Published in: Physics of metals and metallography Vol. 116; no. 8; pp. 791 - 801
• Main Authors: Barinov, V. A., Protasov, A. V., Surikov, V. T.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.08.2015; Springer; Springer Nature B.V
• Subjects: Analysis; Chemistry and Materials Science; Diffusion; Intermetallic compounds; Iron compounds; Materials Science; Metallic Materials; Phase Transformations; Structure; mechanosynthesis; Hägg carbide (χ-Fe; C; parameters of hyperfine interactions; magnetic susceptibility; carbide; Curie temperature
• ISSN: 0031-918X; 1555-6190
• DOI: 10.1134/S0031918X15080025

Methods of thermomagnetic analysis and Mössbauer experiments ( 57 Fe) were used to investigate the formation of Hägg carbide (χ-Fe 5 C 2 ) under the conditions of mechanical milling of α-Fe in a medium of liquid hydrocarbons. It has been established that, with the employed parameters of milling, the synthesis of χ carbide begins after the completion of the stage of the formation of cementite (θ phase). The borderline of temperature stability of the monophase state of the χ carbide has been determined to be no more than 800 K. At T > 800 K, χ carbide decomposes into cementite and free carbon. The optimum temperature of heating of the synthesized Hägg 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 C = 520 K. The analysis of the Mössbauer data and of the results of a geometrical simulation of the configurations of Fe atoms in the the χ 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 an of the field of hyperfine interaction. Under the effect of h an , the crystallographically equivalent atoms Fe(4 e ) become nonequivalent (Fe( e 1 ) and Fe( e 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 Mössbauer contributions, i.e., p ( e 1 ) and p ( e 2 ) with equal fractions of iron atoms in each of the contributions f Fe ( e 1 ) = = f Fe ( e 2 ) = 0.1 with the magnitudes of the fields H ≈ 11 and 16 T, respectively.