Abstract: This paper presents a systematic investigation of the microstructural evolution and mechanical properties of Al-7Si-3Cu alloys prepared under high pressure of 5 GPa. It is found that the dendritic structure disappears due to the enhanced stability of the growth interface under high pressure. Meanwhile, a considerable solute-induced modification effect is achieved in the eutectic Si. Besides, unconventional larger eutectic colonies and ultrafine nanocrystalline eutectics are observed within the intragranular area. Combined with detailed microstructural characterization, the crucial role of high pressure on the evolution of these isolated phases is discussed. The study also explores the precipitation behavior and strengthening mechanisms of alloys prepared under direct aging and solution-aging treatments. Upon the direct aging treatment, θ' phases, coarse Al₂Cu particles, and Si particles form simultaneously in the matrix owing to the increased solid solubility of alloying elements, which results in the highest mechanical properties with a microhardness of 132.8 HV and compressive stress of 715 MPa. Solution-aging treatment leads to the segregation of Si on θ' phases, which proved to be energetically favorable according to first-principles calculations. This work provides new insights into the microstructural optimization of aluminum-silicon-based alloys through high-pressure manipulation and paves the way for further industrial applications.

Key words: Al-Si-Cu alloy, High pressure, Nanocrystalline eutectics, Precipitates, Mechanical properties