Critical solidification behavior of undercooled Ag-Cu alloys

Abstract

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.

The origin of anomalous eutectic structures in undercooled Ag–Cu alloy

“…This critical undercooling has been reported as occurring at 70 K [11], 75 K [10] or 76 K [9,12], close to the calculated T 0 temperature, at which the Gibbs free energies of the liquid and solid phases of the same composition are equal [9,10].…”

“…Other techniques have produced a range of equilibrium and non-equilibrium microstructures including eutectic [e.g. 5,8,[9][10][11][12][13][14], dendritic [e.g. 5,6,9,12], banded [e.g.…”

“…The dendritic region consists of a fine-grained, two-phase mixture, the formation of which is ascribed to the decomposition of the supersaturated solid solution during cooling to room temperature [10].…”

“…Consequently Ag-Cu is regularly selected as a suitable system for undercooling studies. Various processing methods have been employed to investigate solidification of this alloy from its melt, including splat-quenching [1], glass slag melting [2], laser-quenching [3,4], electron beam melting [5], mushy-state quenching [6], melt spinning [7,8], levitation techniques [8], melt fluxing [9][10][11][12][13] and droptube processing [14].…”

Effect of Sb Addition on the Solidification of Deeply Undercooled Ag-28.1 wt. % Cu Eutectic Alloy

“…Table 1. Physical parameters of Ag-Cu eutectic alloy [21,22]. The bulk undercooling ∆T at the dendrite tip can be expressed as four terms:…”