Solidificação direcional, microestrutura e dureza de ligas ZnSb

Abstract

High-temperature soldering alloys (270 °C to 350 °C) are used in sectors such as the automotive, aerospace and energy industries and are in increasing demand due to their higher performance and smaller size. The alloys used in these applications are based on lead (Pb), a potentially harmful metal due to its toxicity levels. Thus, Zn-Sb alloys are emerging as potential candidates to replace lead-based alloys, due to their low cost, mechanical properties and transformation temperatures close to these applications. This work investigates the effect of antimony (Sb) on macrostructural and microstructural evolution, thermal parameters (cooling rate - Ṫ, growth rate for liquidus isotherm - VL and eutectic front - VE), macrosegregation, and hardness in binary Zn-1.0wt.%Sb, Zn-2.0wt.%Sb and Zn-4.0wt.%Sb alloys solidified directionally non-equilibrium. The samples have been characterized by Optical Microscopy (OM), Scanning Electron Microscopy (SEM), X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD), as well as mechanical Vickers hardness (HV) tests. Thermodynamic calculations have been carried out using the CALPHAD method via the Thermo-calc software, in order to obtain information such as transformation temperatures, solidification paths, phases and their fractions. The increase in Sb content from 1% to 2% did not cause changes in the macrostructures, which had a completely columnar growth, while the Zn-4.0wt.%Sb alloy exhibited a macrostructure with equiaxed growth due to the grain refining effect of Sb. The microstructure of the Zn-1.0wt.%Sb and Zn-2.0wt.%Sb alloys is composed of Zn-rich dendrites surrounded by a eutectic mixture composed of Zn + Zn4Sb3 (the latter with a fiber morphology, with Zn4Sb3 plates appearing for Ṫ<2.80 °C/s and 2.25 °C/s, respectively). The Zn-4.0%Sb alloy exhibited a microstructure composed of a eutectic Zn + Zn4Sb3 mixture (with mostly fibrous intermetallics, forming plates for Ṫ<2.90 °C/s) with primary Zn4Sb3 particles of idiomorphic morphology, indicating a hypereutectic composition. Increasing the Sb content led to changes in the macrossegregation profiles, following the inverse, constant and normal sequence for the Zn-1.0wt.%Sb, Zn-2.0wt.%Sb and Zn-4.0wt.%Sb alloys, respectively. The increase from 1% to 2%Sb did not lead to changes in the scale of the dendritic arrangement, but with the 4%Sb content, there was a refinement of the eutectic fibrous spacing compared to the other two compositions, as well as a decrease in the liquidus and solidus temperatures. The increase in Sb content caused an increase in Vickers hardness, associated with the mechanisms of solid solution hardening, microstructural refinement (eutectic arrangement) and the largest fraction of the hardening Zn4Sb3 phase in the eutectic. The literature indicates divergent compositions of eutectic concentration for the Zn-Sb system (1wt.%Sb to 2wt.%Sb), and this work indicates the Zn-4.0wt.%Sb alloy as quasi-eutectic due to the microstructure found appearing to be hypereutectic in contrast to the eutectic concentration of 4.0wt.%Sb found in the thermodynamic simulations obtained via Thermo-Calc.