Experimental results and analysis in terms of classical thermodynamics of variations in temperature and melting enthalpies in eutectic alloys upon changes in mutual phase orientation

Experiments were carried out to validate the hypothesis of a possible rise in temperature of eutectic melting when mutual phase orientation becomes disarranged. Mutual disorientation of phases has to convert an eutectic alloy from the thermodynamic system state into a mechanical blend of phases. To disturb mutual phase orientation seven eutectic aluminium base alloys were rolled to different deformation degrees. Using differential thermal analysis method (DTA) temperature and enthalpy of melting were determined on the same samples in equilibrium and disorientation phase state. Increase of eutectic melting temperature up to 22 ºC and 7-14% reduce in enthalpy of melting were obtained. It was for the first time that the alloy was overheated to such a high temperature without a phase transformation at low heating rates (80 deg/min). This effect might be explained by the fact that free energy values of phases comprising eutectic are lower than in initial state when they are separated and does not depend on interphase surface energy. Thermodynamic equilibrium state of eutectics with lower free energy retains only at specific inter-phase orientation, which allow to provide valent electron correlations and metallic bond between eutectic phases. This in its turn lowers down the system energy and in terms of thermodynamics distinguishes an eutectic alloy from a mechanical blend of phases.