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Preventing Falls in Housing Construction (Word, 3.9 MB) 53 Pages

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Preventing Falls in Housing Construction (Word, 3.9 MB) 53 Pages

    METAL MELTS STRUCTURAL RESEARCHES

    N.A. Vatolin

    Institute of Metallurgy, Ural’s Division of Russian Academy of Sciences,

    101, Amundsen Str., 620016, Ekaterinburg, Russia

    Introduction

    Structure of phases is very important for study of speeds and mechanism of phase transitions, and also chemical reactions. The most simple state of substance in this relation is gaseous one, where the substance as separate molecules is regularly distributed in space and at small pressure is simply described by the law of ideal gases. However, at increase of pressure up to tens or hundreds atmospheres, it is hardly possible to speak about completely uniform distribution of particles in volume. Density fluctuations of gas appear, the laws for ideal gas are not available in such systems. The structure of already considerably influences the majority of its properties. It is natural, that the structure of solid state substances for many years is investigated by various methods and this information is concentrated in the numerous reference book.

    It is much more difficult to obtaine structure of a liquid, since the space arrangement of atoms is not fixed, as in a solid state body, however atoms are not completely free as in gas. Therefore properties of liquid can approach to gas at overheating or to solid state body close to melting temperatures. In this case it is possible with confidence to speak about the short order existing around of each moving atom that revils in a regular arrangement of fading concentric spheres with alternating density behind the ionic centers of atoms, got in an environment of given atom. It can be defined from experiment by curves of function of radial distribution of atoms (RDF), obtained from diffraction researches, in particular, from x-ray measurements of electron scattering intensity I (S) depending on a wave vector S

    S = 4 sin / , (1)

    where - half of angle of scattering; - length of a radiation wave.

    RDF (~(R)) is the probability of a presence of any atom at

    distance R from atom accepted as the centre, is obtaned by Fourier-transformation of curve intensity of scattering.

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    0Experimental dependence I(S) for molten iron at 1550С and 001750С as well as RDF at 1550С calculated from I(S) are given in a

    fig. 1. Distance up to the first maximum on this curve gives average distance between atoms in metal, and the area under this peak determines coordination number, i.e. number of atoms in the first coordination sphere around of atom accepted for central.

    The numerous data show, that coordination of atoms in a liquid and the distances between the nearest atoms are close to those of crystal. Therefore many researchers follow J.I.Frenkel concepts about quasicrystal nature of liquid (especially, liquid metals), stated by them at 30-th years [1].

     The liquid metals are usually regarded as simple liquids such as the liquefied inert gases, for which the theory is most full developed.

     The liquid differs from crystal state bodies by absence of the long order, i.e. uncorrelated arrangement of any particle concerned as the center. As noted by J.I.Frenkel, character of thermal movement of particles in liquid has dual nature, that unites in itself property attributed to gases (fluidity), and crystal bodies (shift elasticity). The shift elasticity of liquids is usually hidden by their small viscosity. However, if a liquid is quickly cooled, i.e. amorfized, it becomes hard, but not crystal and gets shift elasticity.

    Pure metals

     The conclusions about structure of pure liquid metals began to occur from experimental researches of temperature dependencies of those or other properties of melts (viscosity, electric conductivity, density, solubility of gases, x-ray structure analysis, overcooling at crystallization etc.).

     So, at of aluminum by 130 K above melting temperature T all m

    diffraction maximums on curve intensity of electron scattering are moved to the large angles of scattering, the side maximum on the main peak of -1-1the structural factor disappears at S = 31 nm, and at S = 40 nm there

    is an additional maximum [2].

    This variations in structure of aluminum accompanied by

    changes of viscosity at 1040-1120 K [3] and density at 1070-1170 K [4]. The solubility of hydrogen in pure aluminum with growth of temperature

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is maximal at 1070 K [5].

    Overheating of iron by 130 K above T gets displacement of m

    scattering curves maximums, but to the smaller angles of scattering, asymmetry on the part of the large angles of scattering and increases width of the main peak [6].

     -2 I(10, e. un.

     -1-1 S, nm S, nm-1S, nm

     RDF b

     R, nm R, nm

    Fig.1. Intensity change of electron scattering from a surface of liquid

    iron (a) and radial distribution