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EQUIAXED dendritic growth is a frequently observed phenomenon during the solidification process in an undercooled alloy.[1] During this mode of solidification, the dendritic crystals are separate and can move easily in the initial stage of the solidification process. With the continuous growing of the dendrites, the dendrite tips begin to impinge upon each other, so that a dendritic skeleton network is established throughout the solidifying volume.[2,3] The term dendrite coherency point (DCP) refers to this point. For a period of time, the DCP has been recognized as an important characteristic in aluminum alloys, because it marks the transition from mass feeding to inter-dendritic feeding in the solidification process.[4,5] This means that, to compensate solidification shrinkage, the melt must flow through the solid network of the dendrites. Casting defects during equiaxed dendritic growth, e.g., macrosegregation, shrinkage, porosity, and hot tearing, start to develop at the point where the dendrites impinge and a continuous network becomes coherent.[6] M.H. GHONCHEH, M.Sc Graduate, and S.G. SHABESTARI, Professor, are with the Center of Excellence for High Strength Alloys Technology (CEHSAT), School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), 1684613114 Narmak, Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted July 18, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A

Experimental methods for measuring dendrite coherency are based on the notion that a sharp change in some property should occur when the crystals form a crystal network.[4] At first, DCP was investigated by Singer and Cottrell.[7] They reported a correlation between this solidification characteristic and tensile property of materials. Spencer et al.[8] studied the effect of DCP on the rheological behavior of Pb-Sn alloys. The point of dendritic coherency can be determined by rheological means or by thermal analysis technique.[9–11] In the rheological technique, a rotating impeller is immersed into a solidifying alloy and the torque is recorded as a function of the temperature of the melt. A sudden increase in torque is associated with the point at which the dendrites start to touch, impeding the free rotation of the impeller. This method is used by some researchers such as, Claxton and Arnberg et al.[12,13] The application of thermal analysis to study the evolution of the samples microstructure was reported in earlier publications by Cibula and Crossley et al.[14,15] The thermal analysis method uses the two-thermocouple technique developed by Backerud et al.[16] One thermocouple is located at the center of a crucible and the other one at the inner wall. This technique is based on the assumption that the established dendritic network at the DCP will result in a rapid decrease in the temperature difference between the wall and the central regions, due to the higher thermal conductivity of the solid material

compared with the liquid.[13,16] The DCP is then determined by measuring the maxim