Cast heat-resistant Al-Si-Cu-Ni-Mg alloys serve as crucial foundational materials for fabricating heat-resistant, lightweight components, such as pistons, which are used in high-power-density engines. A key strategy to increase the temperature resistance of these alloys involves incorporating Sc elements that possess high melting points and low thermal diffusion coefficients to regulate the phase structure and morphology of the alloys. Therefore, the effects of the addition and content of Sc on the microstructure and evolution of the AlCuNi phase of heat-resistant Al-Si-Cu-Ni-Mg piston alloys were investigated. The results reveals that with increasing Sc content, a new AlSiCuNiSc phase forms in the piston alloy, and the number and size of the AlSiCuNiSc phases increases with increasing Sc content. In addition, with the formation of the

AlSiCuNiSc phase, the δ-Al3CuNi phase undergoes a transformation from a typical eutectic lamellar to a rod-like structure. During thermal exposure, the Cu and Ni atoms in the δ-Al3CuNi phase diffuse into the α-Al matrix, and a nanoscale γ-Al7Cu4Ni phase is formed at the phase interface of δ-Al3CuNi. However, the morphology and composition of the AlSiCuNiSc phase almost did not change before and after heat exposure, indicating extremely high thermal stability. The presence of atoms such as Si and Sc with high melting points and low thermal diffusivity in the AlSiCuNiSc phase greatly improves its thermal stability.