Structural evolution of Al-8%Si hypoeutectic alloy by ultrasonic processing

High intensity power ultrasound was respectively introduced into three different solidification stages of Al-8%Si hypoeutectic alloy, including the fully liquid state before nucleation, the nucleation and growth process of primary α(Al) phase and L → (Al) + (Si) eutectic transformation period. It is found that both the primary α(Al) phase and (Al + Si) eutectic structure were refined by different degrees with various growth morphologies depending on the ultrasonic treatment stage. Based on the experimental results, the cavitation-induced nucleation due to the high undercooling caused by the collapse of tiny cavities was proposed as the major reason for refining the primary α(Al) phase. Meanwhile, obvious eutectic morphological change was observed only when ultrasound was directly introduced in the eutectic transformation stage, in which typical divorced eutectics and (Al + Si) eutectic cells with symmetrical flower shape were formed at the top of the alloy sample. The introduction of ultrasound in each solidification stage also improves the yield strength of Al-8% Si alloy to a diverse extent.