On the Nature of Tetragonality of a Martensite of Carbon Steel

Yu. Ya. Meshkov, V. A. Lobodyuk

G.V. Kurdyumov Institute for Metal Physics, NAS of Ukraine, 36 Academician Vernadsky Blvd., UA-03142 Kyiv, Ukraine

Analysis of the values of tetragonality of the carbon martensite presented earlier in Ref. [1] is developed taking into account the factor of limited solubility of carbon in austenite, which causes the inhomogeneity of distribution of the local tetragonal distortions of the $\alpha$-phase lattice in the whole volume of the martensite crystal, and so the experimentally fixed value of tetragonality $c/a$ is the averaged value for the steel with a given content of carbon. A model of numerical estimation of the local tetragonality value of the $\alpha$-phase unit cell containing the C atom is proposed. As shown on the basis of analysis of the interatomic-distances’ changing in lattices of $\gamma$- and $\alpha$-phases as a result of the martensitic transformation, the location of carbon atom in the middle of edge $c$ of martensite b.c.c.-lattice unit cell causes to a strong local distortion of the lattice shape. As concluded, as a result, the martensite crystal of the carbon steel consists of mainly b.c.c. unit cells of $\alpha$-phase with rare (respectively to C concentration) inclusions of the ‘pseudo-cubic’ unit cells with one edge increased to 1.41$a$. Obviously, the aggregate of such enlarged carbon-containing edges of the ‘pseudo-cube’ oriented along the $c$ axis leads to a splitting of martensite diffraction lines {110} (that is most often used to determine tetragonality), which gives a reason for treating that as the tetragonality of the whole lattice with the ratio of its cells’ axes $c/a$ averaged over the whole crystal of the $\alpha$-phase.

Key words: carbon steel, martensite, austenite, lattice parameter, tetragonality, distortion, inhomogeneity.

URL: http://mfint.imp.kiev.ua/en/abstract/v39/i10/1423.html

DOI: https://doi.org/10.15407/mfint.39.10.1423

PACS: 61.50.Ks, 61.66.Dk, 61.72.Bb, 61.72.Dd, 61.72.S-, 81.30.Kf, 81.40.Ef

Citation: Yu. Ya. Meshkov and V. A. Lobodyuk, On the Nature of Tetragonality of a Martensite of Carbon Steel, Metallofiz. Noveishie Tekhnol., 39, No. 10: 1423—1433 (2017) (in Russian)

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