DOI: https://doi.org/10.30970/jps.02.54
THE STRUCTURE OF LIQUID CO–SN ALLOYS
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Journal of Physical Studies 2(1), 54–61 (1998)
DOI: https://doi.org/10.30970/jps.02.54 THE STRUCTURE OF LIQUID CO–SN ALLOYS |
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M. S. Komarnytsky, S. I. Mudry, V. P. Halchak
Ivan Franko Lviv State University, Chair of Physics of Metals
8 Kyrylo i Mefodii Str., Lviv, UA–290005, Ukraine
Liquid alloys which contain transition metals and semimetals such as Sn, Ga are interesting both from a theoretical and a practical point of view. Liquid 3$d$-transition metals have a structure with a close packed short range order and liquid semimetals with a looser atomic arrangement. On the other hand, these kinds of liquid metals also show specific features in electronic structure. Incomplete $d$-shell is the property of liquid 3$d$-transition metals, while the existense of covalent bondings is a major characteristic of liquid semimetals.
One can observe structural distinctions on the structure factors which have significantly different maximum positions. For liquid 3$d$-transition elements they are about 3 \AA$^{-1}$, and for liquid semimetals this value is near $S=2$ \AA$^{-1}$. Hence, the mixing of 3$d$-elements with semimetals suggests these interesting changes of structure after alloying.
The structure of liquid Co-Sn alloys has been studied by means of the $X$-ray diffraction method. This structural investigation was carried out by using the $Θ$–$Θ$ difractometer. The $X$-ray scattered intensity from the open surface of the melt was measured in helium atmosphere at different temperatures. The FeK$_α$–radiation was used which provided a high resolution in the Ds range 1 to 8 \AA$^{-1}$. A graphite monochromator was placed in a diffracted beam. The intensity observation was done by the constant $s$ step equal to 0.01 \AA$^{-1}$. The scattered intensity was measured with a fixed count method to insure a constant statistical error. The measured intensities were corrected for the background scattering, absorption and polaryzation. The corrected intensities were scaled to the coherent scattering intensity in electron units by considering the Compton scattering.
Special attention has been paid to the concentration dependence of the structure factors. Liquid alloys containing 20.5; 33.3; 40; 50; 66.7; 77.7 and 90 at.$\%$ Sn were investigated. Fig.~1 shows the obtained experimental structure factors $a(s)$ at different temperatures above the liquidus curve. The analysis of the concentration dependences of main structural parameters has shown that complexes of Co$_3$Sn$_2$ represent the main structural units in the structure of Co-Sn liquid alloys. A model of associates was used for the interpretation of the $X$-ray data.
Equation (1) may be used for the calculation of $a_{A_xB_y}(s)$ which describes the structure of the associated groups of atoms. However, we must take into account that a part of associates is decomposed upon melting and heating. Thus, a fraction of associates will decrease with the rise of temperature. For the quantative estimation of the fraction of associates we used thermodynamic data for the investigated melts [5]. Using for molten alloy entalpy and entropy of mixing we have estimated the fraction of associates in melt of stoichiometric concentration. Having determined this value one may calculate the fraction of associates at other concentrations. Fig.~5 shows a comparision of experimental structure factors with the calculated ones using the associated solution model. As can be seen from fig.~5, the experimental and calculated data are in agreement.
Thus, the $X$-ray investigation of liquid Co-Sn alloys shows that their atomic arrangement is determined by a tendency to associated groups of atoms located between indentical atoms. A 3$d$-transition metal like structure dominantly determines the atomic arrangement in a wide concentration range.