Melts of the eutectic alloy Ag-39.9 at. %Cu were cooled below the liquidus temperature by the glass flux method. The solidification behavior was followed by measuring the temperature change (recalescence), and the resulting microstructures were investigated by optical and electron microscopy. Up to a certain critical undercooling ΔT*≊76 K, the recalescence rate is less than 100 K s−1, and a feathery, lamellar microstructure is formed, with typically one or two growth centers. When the undercooling exceeds ΔT*, a sudden increase in the recalescence rate (up to 104 K s−1) is observed, and the microstructure consists of a dendritic, irregular two-phase component surrounded by a lamellar eutectic. The critical undercooling temperature coincides with the temperature (T0) at which the Gibbs free energies of the liquid and the supersaturated Ag-Cu solid solution are equal. The critical behavior is therefore thought to be due to the formation of this metastable phase. A model is proposed to explain the observed behavior and microstructures.
Melts of Ag, Cu, and four different Ag-Cu alloys, including the eutectic composition, with masses between 1 and 2.5 g were undercooled by the glass flux method, and the temperature of the specimen was measured with a pyrometer during cooling and recalescence. For each composition the maximum recalescence rate was determined as a function of the undercooling. Comparison of the results for Ag and Cu with published measurements of the dendrite growth velocities in these metals showed that the maximum recalescence rate provides a good semiquantitative measure of the growth rate. For the Ag-Cu alloys there is a critical solidification temperature—close to T0—at which the maximum recalescence rate rises abruptly by about two orders of magnitude, accompanied by significant changes in the microstructure. It is shown that this behavior can be explained theoretically on the basis of a transition from diffusion-controlled growth of the eutectic or the primary dendrites to dendritic growth of a supersaturated phase, provided that interface kinetics and growth-rate-dependent partition coefficients are taken into account.
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