Effect of Trace La on Microstructure and Thermal Conductivity of Hypoeutectic Al-7Si Alloy

Al-Si phase change materials are widely used in solar thermal power generation and industrial waste heat reclamation due to their high heat storage density, high phase transition temperature, and low cost. Hypoeutectic Al-7Si phase change thermal storage alloys with trace La additions were produced through smelting and casting to examine how La affects their microstructural characteristics and thermophysical performance. The findings show that La is adsorbed at the eutectic Si growth interface. Due to the difference in atomic radii, it alters the stacking sequence of Si atoms, generating numerous high-density staggered twins on the {111}Si planes of eutectic Si. La additions modify the morphology of eutectic Si, leading to a morphological transition from lamellar to short rods structures with reduced dimensions. The optimal eutectic Si modification is achieved with 0.06 wt.% La addition. The altered morphology and reduced size of the eutectic Si phase enhance the continuity of the α-Al matrix. This reduces the scattering of free electrons by eutectic Si, increases their mean free path, and ultimately improves the thermal conductivity of the alloy. With 0.06 wt.% La addition, the Al-7Si alloy achieved a peak thermal conductivity of 179.3 W·m−1·K−1, representing a 15.36% enhancement over the unmodified alloy. After 100 thermal cycles, the alloy maintained its phase transition temperature, but the modification effect of La diminished, as evidenced by increased formation of lamellar eutectic Si. Consequently, the latent heat of the Al-7Si-0.06 alloy decreased from 340.4 J/g to 328.6 J/g.