Directional solidification in a AgCuSn eutectic alloy
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I. INTRODUCTION AND MOTIVATION
THE Ag-Cu-Sn ternary eutectic has recently become of great interest because of its potential use as a lead-free solder. Several microstructures have been observed in this alloy, some of them containing large primary phase crystals and others indicative of coupled eutectic growth in which all phases grow at a common interface in close proximity, without large crystals of a primary phase.[1] Because large primary phase crystals can have a significant effect on the mechanical properties, it is useful to know the conditions under which the more desirable coupled eutectic microstructure forms, and how this microstructure varies with processing conditions. Primary phase crystals can of course be present due to a deviation of the alloy from the true eutectic composition. They can also arise, however, from several effects in alloys which are at the equilibrium eutectic composition. Studies of directional solidification of eutectic systems have frequently focused on binary systems containing periodic arrays of lamellae[2] or rods,[3,4] mostly because such systems can most readily be compared to theoretical models of eutectic solidification, such as that of Jackson and Hunt.[5] Ternary eutectics display a wide range of structures, some of them rather complex even when not highly irregular.[6] McCartney et al.[7] have discussed the general factors describing the conditions under which coupled growth can be expected, and Rutter and collaborators[8,9,10] have analyzed several ternary systems including measurements of phase spacings and their dependence on solidification velocity. The Ag-Cu-Sn eutectic differs from most of the ternary eutectics that have been studied in detail in that it contains very small volume fractions of two faceted intermetallic phases, Ag3Sn and Cu6Sn5, in a matrix of nonfaceting Sn. On the basis of the phase compositions[11] and densities,[12] we expect the volume fractions of the phases in the eutectic to be Sn, 0.948; Cu6Sn5, 0.017; and Ag3Sn, 0.035. In this system, it is therefore of interest to evaluate the extent of the coupled zone and the interaction, or lack thereof, between the two intermetallic phases. In this article, we will investigate the solidification velocity range of the coupled zone in an alloy very close to the equilibrium eutectic composition. Directional solidification ROBERT J. SCHAEFER, Physicist, is with the Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. DANIEL J. LEWIS, Materials Scientist, is with General Electric Global Research, Niskayuna, NY 12309. Manuscript submitted February 28, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
experiments were performed using a Sn-rich alloy with 3.82 pct mass fraction Ag and 0.90 pct mass fraction Cu. Directional solidification experiments are normally designed to attain a steady-state solidification velocity and to relate the observed microstructures to this velocity and, to a lesser extent, to the temperature gradients in the vicinity of the solid-liquid in
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