Ultrasonic solidification and microstructural evolution of binary Al-12.6%Si eutectic alloy

Abstract

Three-dimensional (3D) ultrasounds were introduced into liquid Al-12.6%Si eutectic alloy to explore its dynamic solidification and structural evolution mechanisms. Under static solidification, there was a typical eutectic microstructure composed of irregular flake (Si) phase and dendritic matrix α(Al) phase. Once 3D ultrasounds were applied, the undercooling increase caused by ultrasonic cavitation promoted the independent nucleation of eutectic (Si) and α(Al) phases, whereas the uniform temperature field and solute field secured by acoustic streaming made these two eutectic phases grow isotropically. As a result, a divorced eutectic structrue formed, which consisted of blocky (Si) phase and globular α(Al) phase. The high frequency vibration effect of ultrasound increased the twinning probability of (Si) phase, leading to lots of multiple twin (Si) phase with intercrossing interfaces. The intensive ultrasound reduced interfacial energy between (Si) and α(Al) phases, and consequently enhanced the quantity of parallel growth oriented crystalline planes, which finally improved the microhardness of ultrasonicated binary Al-12.6%Si eutectic alloy.