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A model of the arrangement of atoms of interstitial impurity in iron austenite is developed, according to which an introduction of an interstitial impurity atom causes transformation of a regular close packing (CP or FCC) composed of octahedrons and tetrahedrons into a cementite-type local prismatic packing by diagonal flipping. Carbon dissolves in the crystalline structure of iron austenite as a centered five-atom tetrahedral cluster, which represents a fragment of the carbon packing in the crystalline structure of diamond. The diffusion of the impurity atom is accompanied by a transformation into a prism in the neighboring lattice period, while the prismatic packing in the initial location of the impurity atom is reconstructed back into a conventional interstitial void. To check the adequacy of the proposed model, theoretical calculations of the activation enthalpy of carbon and nitrogen diffusion in austenite were performed, according to which the activation enthalpy values are 149 ± 20 and 155 ± 20 kJ/mol, respectively, which is fairly consistent with the experimental data reported in the literature (153 and 169 kJ/mol). [ABSTRACT FROM AUTHOR]

To develop their previous results, the authors demonstrated that the structural-type phases Th2Ni17 and Th2Zn17 may be represented as the densest helical packing of tetrablock chains as linear 7-vertex unions of four tetrahedrons over faces. It was concluded that the tetrablock can be used as a universal building unit, including more complex structural types of Frank–Kasper phases than those considered in our previous works. [ABSTRACT FROM AUTHOR]

Published data on the effect of boron on the processing behavior of steels, hardenability in particular, are reviewed. A model of the arrangement of boron atoms in the crystal structure of γ-Fe not stipulating a uniform distribution of individual boron atoms in the fcc iron lattice is suggested. The model should explain the observed special features of the influence of low concentrations of boron on the hardenability of steels. [ABSTRACT FROM AUTHOR]