Shtablavyi I. Correlations of free volume and short-range order structure in metallic melts with different degrees of microheterogeneity of atomic distribution

The work is devoted to solving fundamental and practical problems related to the establishment of the relationship between the structure of the short-range order and the free volume in metal melts. The work contains the results of X-ray diffraction research, computer modeling and free volume calculation of metal melts as well as two- and three-component alloys and composites based on them. In addition, the structure and free volume of liquid-phase composites with a metal matrix were studied. In order to study the atomic structure of melts, we used the analysis of temperature and concentration dependences of full and partial structural parameters (interatomic distances and coordination numbers). The free volume of the alloys in the liquid state within the short order was calculated using the values of the radii of the first coordination spheres and the number of atoms contained in them. On the basis of computer models of melts, the volume not occupied by atoms was calculated within the framework of the theory of free volume and the hole model of melts. It is shown that the free volume within the first coordination sphere has smaller values compared to the free volume calculated at a greater distance, as well as the average free volume within the alloy. As a result of the study of liquid metals, which are characterized by dense packing of atoms in the crystalline state and in a small range of temperatures after melting, it was found that the change in main structure parameters is mainly due to an increase in the size of the intercluster region, the degree of ordering of which is smaller compared to clusters, and the change in free volume within the first two coordination spheres is determined mainly by changing the boundaries of the first and second coordination spheres without changing the coordination number. For the eutectic systems Sn-Bi and Sn-Pb, it was shown that the dynamics of the change in free volume with the temperature of the melts as well as the temperature dependences of the structural parameters, indicates the transition from cluster to atomic solution. In the case of eutectic systems with a predominant content of one of the components, the microhomogeneity of the atomic structure is due to the formation of clusters that form around atoms whose concentration in the eutectic is lower. Analysis of the results obtained by both experimental methods and methods of structure reconstruction makes it possible to explain the mechanism of structure formation and free volume transformation of melts with a tendency to segregation of components. Results of the free volume calculation for Bi100-xZnx alloys indicate a significant difference between the free volume calculated within the first coordination sphere and the average free volume per atom. This difference in the values of free volume indicates a significant degree of heterogeneity of the studied melts, which is due to their cluster structure, which leads to the formation of two types of atomic ordering. Using the method of reconstruction of the structure of the Ga0,7Bi0,3 melt for different temperatures, the destruction of large cavities during the transition of the liquid from the microheterogeneous to the homogeneous state was established. In the case of systems which are characterized by the formation of chemical compounds in the crystalline state it was found that the main influence on the value of free volume after melting is the interaction of atoms of different types resulting in the formation of clusters with chemical ordering. For the Co-Sn system, a nonmonotonic dependence of structural parameters and free volume within the first coordination sphere on the content of components is established. The inflection points of the temperature dependences of the free volume in the frame of short-range order correspond to the concentrations of alloys whose clusters form the structure of the melts of the Co-Sn system. It has been found that the addition of nickel to the Al-Si melt reduces the free volume by placing nickel atoms in the cavities of the base melt. The reason for this is the predominant interaction between nickel atoms and simultaneously aluminum and silicon. In this case, we can talk about the so-called crosslinking effect of nickel on the structure of the melts of the Al-Si system. Studies of the structure of composites with a liquid metal matrix made it possible to conclude that the degree of ordering of atoms increases due to the effect of surface compaction of the liquid structure observed on the free surface of the liquid or at the liquid-crystal interface. The compaction of the composite structure, we have found, is accompanied by a decrease in free volume within the short-range order. Despite the fact that the introduction of nano- and microparticles promote the increasing of the degree of microinhomogeneity at the mesoscale, at the atomic level there is a decrease in free volume with increasing content of crystalline particles.