National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Moscow, Russia:

P. V. Morokhov, Leading Engineer, e-mail: morokhov@mail.ru
V. M. Ananin, Leading Electronics Engineer
A. A. Ivannikov, Assistant
O. N. Sevryukov, Assistant Professor
A. N. Suchkov, Assistant Professor

Metallographic and XR phase analysis reveal gravity-induced macrosegregation of melt in the time of melting of near eutectic alloys under uniform temperature condition. Cu-alloys' samples show that gravityinduced concentration of denser phase at crucible bottom leads to increase in local melting temperature and increase in the time, required for entire volume for thermodynamic equilibrium achievement. Undissolved phases were observed at the melt bottom both inside the melting interval, and far beyond the liquidus temperature. Gravity-induced macro-segregation of the melt is carried out only through melting. It was not observed in the time of homogenized melt crystallization. Experimental data, obtained by oscillating-cup technique, shows macrosegregation in temperature behavior of viscosity curves. Setting of undissolved phases on crucible bottom leads to viscosity hysteresis versus temperature curves, measured through heating and cooling. Branching point on viscosity temperature dependence (after which behavior of heating and cooling curves match each other) indicates that the sample was transitioned into a fully liquid state, while its composition along its height can still stay inhomogeneous. Macro-segregation helps to explain the difference of heating and cooling curves in differential thermal analysis, as well as hysteresis of temperature dependence of melt viscosity. Macro-segregation slows the melting, which leads to smaller differential thermal analysis signal and enlarged temperature interval between solidus and liquidus. According to this, cooling differential thermal analysis signal depends upon previous time-temperature treatment as long as complete homogenization is reached.

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