<p>Liquid-quenching rapid solidification experiments, combined with three-dimensional reconstruction techniques, were employed to investigate the morphology and growth behavior of primary Si and eutectic Si under the influence of ultrasonic vibration in an Al-11.5Si-4Cu-2Ni-1Mg-0.45Fe (wt.%) alloy designed for piston applications. The results indicated that applying ultrasonic vibration below the liquidus temperature decreases the size of primary Si, enhances its precipitation, and refines its morphology. Furthermore, ultrasonic vibration facilitates the precipitation of eutectic Si, transforming its morphology from elongated needle-like structures to shorter, rod-like forms. These morphological changes in primary and eutectic Si under ultrasonic vibration are primarily attributed to the cavitation effect, which reduces the size of Si-Si clusters, increases their population, and lowers the nucleation barrier during the solidification process.</p>