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On the example of linear substructures of Mn modifications, we show two types of spirals formed by tetrablocks assembled along their sides, namely, seven-vertex unions of four tetrahedra. Experimental data that confirm the existence of spiral packings of this kind in the structures of metallic nanowires are discussed. The noncrystallographic symmetry of the spiral packing of tetrablocks enables us to adequately describe the crystalline and noncrystalline tetrahedral structures that are also present in densely packed metals. [ABSTRACT FROM AUTHOR]

A structural model of the transformation of cubic α-Mn into high-pressure hexagonal close-packed ε -Mn phase is suggested. The model is based on the transformation of a triple of tetrahedrons into an octahedron, implemented through a shift of the diagonal of a nonplanar "rhombus" formed by the faces of neighboring tetrahedrons. Asystem of such transformations transfers a fragment of a tetrahedral α -Mn spiral into a fragment of a rod composed of octahedrons in hexagonal closest packing. The model takes into account the noncrystallographic symmetry of the considered fragments of the crystalline phases. [ABSTRACT FROM AUTHOR]

A structural model of metallic melts is suggested on the basis of the structures of all modifications of Mn. The model is based on spirals of tetrablocks, i.e., unions over faces of four regular tetrahedrons. The helices composed of tetrablocks have a high density and a non-translational long-range ordering, which explains the higher experimental values of the melt density as compared to the random packing model and the blurring of the diffraction pattern of the melt. When the melt crystallizes into one of the Mn modifications, such a spiral is inherited in the form of its periodic approximant. The model makes it possible to describe adequately the changes in the structure-sensitive properties of metals during heat treatment. [ABSTRACT FROM AUTHOR]

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]

A single structural model for the experimentally determined succession of phase transitions between cubic α-, β-, γ- and δ-modifications of Mn, in which the lattice constants of the modifications decrease with the temperature growth is suggested. The model is based on the definition of the Mn phases as successive periodic approximants of a linear substructure of a four-dimensional analog of an icosahedron. The model seems to be appropriate for an adequate description of structural transformations in close-packed metals and of variation of their structure-sensitive properties under heat treatment. [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]

A model of intergrowth of graphite and diamond in a metallic matrix based on existence of generating clusters of tetracoordinated structures representing structural implementations of mathematical constructions is suggested. The model demonstrates the displacements of atoms, as a result of which one structure transforms into another. The choice of such "mathematically defined molecules" is determined unambiguously by mathematical constructions rather than by physical (interatomic) interactions just like spatial groups are defined purely analytically without supposition of existence of atoms. [ABSTRACT FROM AUTHOR]

We propose an atomistic description of polymorphic transformations in metal alloys, according to which an elementary act of transformation is reduced to the transfer of diagonals of a rhombus formed by two triangles. Metallic atoms are located in the corners of these triangles, and their common edge is transferred (i.e., turns into the long diagonal of rhombus). We estimated the energy barrier of polymorphic transformation (transfer of the diagonals) for various types of filling of the corners with iron, chromium, and manganese atoms in different combinations. The energy threshold is computed by using the Morse pair potential. The numerical parameters of the function approximating the potential are calibrated by the experimental values of the energy of sublimation and the temperature dependences of the elastic constants of iron, chromium, and manganese. [ABSTRACT FROM AUTHOR]

A mechanism is proposed for the nucleus formation at the mutual transformation of austenite and cementite crystals. The mechanism is founded on the interpretation of the considered structures as crystallographic tiling onto non‐intersecting rods of triangulated polyhedra. A 15‐vertex fragment of this linear substructure of austenite (cementite) can be transformed by diagonal flipping in a rhombus consisting of two adjacent triangular faces into a 15‐vertex fragment of cementite (austenite). In the case of the mutual austenite–cementite transformation, the mutual orientation of the initial and final fragments coincides with the Thomson–Howell orientation relationships which are experimentally observed [Thompson & Howell (1988). Scr. Metall.22, 229–233] in steels. The observed orientation relationship between f.c.c. austenite and cementite is determined by a crystallographic group–subgroup relationship between transformation participants and noncrystallographic symmetry which determines the transformation of triangulated clusters of transformation participants. Sequential fulfillment of diagonal flipping in the 15‐vertex fragments of linear substructure (these fragments are equivalent by translation) ensures the austenite–cementite transformation in the whole infinite crystal. The energy barrier for diagonal flipping in the rhombus with iron atoms in its vertices has been calculated using the Morse interatomic potential and is found to be equal to 162 kJ mol−1 at the face‐centered cubic–body‐centered cubic transformation temperature in iron. [ABSTRACT FROM AUTHOR]

The possibility of the use of the effect of magnetic impedance as a criterion for estimating the homogeneity of potentially amorphous melt-quenched alloys and for optimizing of the heat treatment mode of nanocrystalline soft magnetic alloy 5BDSR is considered. The measurements of the effect of magnetic impedance on small-section specimens cut from amorphous ribbons show the possibility of getting additional information on the variation of magnetic properties under nanocrystallization. The magnetic characteristics of alloy 5BDSR are shown to be unsteady after single-stage annealing by the modes standardized for this material. It is shown that double-stage heat treatment stabilizes the magnetic properties of the alloy and elevates these properties. [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]

A model of the structure of Fe – Fe3 C melts in the temperature range 1300 – 1700°C is developed on the basis of recent experimental data. In accordance with the model, the solubility of carbon in liquid iron is limited (about 2%). Stable dispersed particles of the second phase (cementite) form in the melt at a higher carbon content. [ABSTRACT FROM AUTHOR]

A geometrical model of transformation of a body-centered cubic lattice of α-phase into a hexagonal close-packed lattice of α-phase is developed with the aim of explaining the special features of the crystal geometry of formation of martensite phases in titanium and zirconium and in alloys based on them. The transformation is described as mutual reconstruction of coordination polyhedra of the cubic and hexagonal lattices through an intermediate configuration of the crystal structure of ω-phase. In the language of algebraic geometry the transition is implemented as reconstruction of an 11-atom cluster that represents a union of three octahedra around a common edge into an 11-atom cluster composed of 11 tetrahedra united over faces. Experimentally observed orientation relations and habit planes at α → ω and β → α transformations are describable by elements of the structure of the mentioned clusters. [ABSTRACT FROM AUTHOR]

The structural model of formation of the ω-phase in titanium alloys is refined by considering coordination polyhedra of the matrix b.c.c. phase. It is established that fragments of the crystal lattice of the ω-phase can unite with fragments of the lattice of the intermediate Ti2Fe phase in Ti – 20 at.% Fe alloy after quenching from the liquid state. The structural model makes it possible to explain the anomalies of diffusion scattering on electron diffraction patterns of quenched alloys of the Ti – Fe system. [ABSTRACT FROM AUTHOR]

The article considers linear substructures of close-packed metals represented by helical packings of universal building units (tetrablocks) united over faces, i.e. seven-vertex unions of four tetrahedra over faces. The implementation of such helical packings is demonstrated for cubic crystals of α- and β-manganese as an example. [ABSTRACT FROM AUTHOR]

The structural types of copper, diamond and tungsten are represented by a linear union of four tetrahedrons over faces within solution of a general problem of representation of a crystal structure as a union of special atomic clusters. This union (a tetrablock) plays the role of a universal building unit, because it determines the generating clusters (building units) of the basic structural types considered. Spiral unions of these generating clusters are implemented in other structural types too, as well as in metallic liquids and glasses. Since symmetry is a geometrical equivalent for the physical requirement of minimal free energy in a system, the choice of such clusters is determined by purely symmetry (mathematical) considerations. [ABSTRACT FROM AUTHOR]

An approach to description of the structural types of phases is suggested, in accordance with which the structural types A15, C15 and C14 of Frank–Kasper phases are described as the densest packing of chains of special unions of tetrahedrons-tetrablocks. A tetrablock is a seven-vertex union of four tetrahedrons over faces. For the packing to be the densest, the ideal tetrahedrons in the tetrablocks should undergo a minimum possible deformation. Since a tetrablock is a natural structural unit of one-dimensional chains, the approach developed may be used for plotting structural models of phase transitions where the transformation develops within linear substructures. Chains of tetrablocks may also be structural components of metallic melts and glasses. [ABSTRACT FROM AUTHOR]

The problem of formation of properties in microalloyed sheet steels is considered. The cause and the possible mechanism of precipitation of carbides of refractory metals on the boundary of the γ → α phase transition are suggested. A possible model of formation of a Fe3 C nucleus from austenite is presented. [ABSTRACT FROM AUTHOR]

Main problems arising in installation of novel processes of production of permanent magnets based on the Fe – Cr – Co and Sm – Co systems are presented. The technological possibilities of production of permanent magnets from an alloy of the Fe – Cr – Co system by methods of MIM technology and powder metallurgy are described. Methods of MIM-technology with the use of domestic granulate are used to fabricate permanent magnets based on alloy 25Kh15KA (Fe – Co – Cr system) with residual induction Br = 1.12 T, maximum energy product (BH)max = 23.0 kJ/m3 and coercivity HcB = 39.7 kA/m. [ABSTRACT FROM AUTHOR]

The possibility of fabrication of granulate based on an alloy of the Fe – Cr – Co system with reduced (7 – 10%) cobalt content for synthesizing permanent magnets by the MIM process is considered. A multicomponent granulate composition (polyformaldehyde, high-pressure polyethylene, stearic acid, beeswax) of spherical powders of the Fe – Cr – Co alloy with reduced cobalt content is produced by gas sputtering (average particle diameter 4.2 μm, degree of polydispersity 0.75). The microstructure of the granulate and its granulometric composition are studied. The characteristics of the granulate (the mean diameter of the particles, their morphology and degree of polydispersity) exceed qualitatively the characteristics of the Catamold 42CrMo4 high-quality granulate produced by the BAFS Company. [ABSTRACT FROM AUTHOR]