Solidificação direcional, microestrutura e microdureza da liga eutética Sn-3,5%Ag modificada por Zn

Abstract

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 metallic substrates and great fluency resistance and thermal fatigue. Due to the lead toxicity, restrictions were imposed, banning the use of traditional Sn-Pb alloys. 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 Zn additions (0.5 and 1.0 wt%) 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 alloy modified with 1wt.% Zn exhibited a microstructure completely formed by cell growth, 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 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.