Features of crystallization of cast glass-ceramic spinel-pyroxene materials under non-equilibrium conditions

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Morphometrical parameters of the structural components of the glass-ceramic materials obtained from the melts are critical for the desired properties; these parameters largely depend upon the crystallization sequence. The objective of the present work is to study the features of crystallization of the glass-ceramic spinel-pyroxene materials from the melt under non-equilibrium conditions. The research uses a melt of the following composition, weight %: SiO2 – 45; MgO – 15.5; CaO – 12.5; Al2O3 – 15.5; FeO – 6.5; Fe2O3 – 3.5; TiO2 – 1; Na2O+K2O – 3; CaF2 – 1.5; Cr2O3 – 1.5, and the material obtained through its crystallization and solidification. Two types of samples were obtained; both were subjected to solidification in sand-clay molds but at different process parameters of crystallization, the cooling rate of the molds right after casting being 100 and 200 °С/h, respectively.The study of peculiarities of formation of the structure of cast glass-ceramic spinel-pyroxene materials under non-equilibrium conditions has established that: (1) the morphological parameters of structural components, in particular, spinels, under non-equilibrium crystallization are influenced not only by the cooling rate, but also by the conditions of solidification (contact with air, with mold, no contact); the milder is the mode of crystallization, that is the lower is the cooling rate, the higher is the density of spinel distribution and the size of the largest components; (2) when crystallizing the casts in the mold, five typical zones are distinguished: a zone of the direct air contact; a transition zone between air cooling and cast center; a zone without contact with external environment; a transition zone between the cast center and the zone of contact with the mold’s wall; a zone of direct contact with the mold; (3) the length of some of the zones depends on the mode of crystallization: the less equilibrium it has, the shorter is the no-contact zone and the longer are the zones of contact with air and mold; (4) there is a mechanism that causes microzoning of the structure: during crystallization, a specific critical size of spinel is achieved when the spinel acquires its own “zone of influence”; that is, a particular area of the melt proportional to the size of the spinel becomes a material for formation of a shell around the spinel by way of epitaxial growth.