Directional Solidification of Sn-3.5wt.%Ag-xZn alloys
Directional solidification; Sn-Ag-Zn alloys; microstructure; cell/dendritic transition; microhardness.
Alloys of the Sn-Ag system are of interest to the electronic microcomponent industry, as they have superior mechanical properties to the alloys of the Sn-Pb system, as well as good wetting in copper substrates and great fluency resistance and thermal fatigue. The microstructure of the eutectic Sn-3,5% Ag alloy solidified non-equilibrium consists of tin-rich dendrites (β-Sn) surrounded by an eutectic mixture Sn+Ag3Sn, where the morphology of the Ag3Sn intermetallic is controlled by the cooling rate. In this sense, this work aims to study the influence of the addition of zinc (Zn) in the thermal parameters such as cooling and growth rates (ṪL/VL-liquidus isotherm and ṪE/VE-eutectic front), microstructure and microhardness of the eutectic directionally solidified Sn-3.5wt.% Ag alloy under unsteady state conditions. The samples were characterized by Optical Microscopy (MO), Scanning Electron Microscopy (SEM), X-Ray Fluorescence (FRX), X-Ray Diffraction (DRX) and Vickers Microhardness. The microstructure of the Sn-3.5wt.% Ag alloy is completely dendritic, composed of a Sn-rich matrix (β-Sn) surrounded by a eutectic mixture of the Sn and Ag3Sn phases. On the other hand, the Zn-modified alloy with 1wt.% Zn exhibited a fully cellular eutectic microstructure, where its interior is formed by a mixture of the β-Sn, ε-Ag3Sn and ζ-AgZn phases. The Sn-3.5wt.% Ag-1wt.% Zn alloy showed a cellular/dendritic transition to VL= 1.15 mm/s and ṪL= 8.55 °C/s. The decrease in the Ṫ and V values caused a coarsening of the dendritic and cellular eutectic arrangements. Additions of 0.5wt% and 1wt.% Zn promoted increases of 42.6% and 47.5% in hardness of the Sn-3.5wt.% Ag alloy, respectively.