Microstructural evolution of a hypoeutectic Al–Si–Mg alloy via the synergistic effect of heat treatment and yttrium-containing nanoscale precipitates

The microstructure is the key factor determining the strength and ductility of aluminum alloys. Although adding the rare-earth element yttrium (Y) is an effective microalloying strategy for significantly improving the mechanical properties of materials, the underlying microstructural evolution and potential strengthening mechanisms remain to be elucidated. In this study, varying amounts of Y were added to a hypoeutectic Al-Si-Mg alloy, and the evolution of the alloy microstructure was systematically analyzed using advanced characterization techniques, including aberration-corrected transmission electron microscopy (AC-TEM) and electron backscatter diffraction (EBSD). The results show that adding Y effectively refines the primary α-Al grains and eutectic Si particles. A high-density dislocation network within the α-Al grains extends into the eutectic Si phase at grain boundaries. This dislocation structure impedes dislocation motion, induces slip on crystal planes, and deflects crack propagation paths during loading, thereby improving mechanical properties. In addition, introducing Y alters the nucleation behavior of eutectic Si, increases the thermodynamic energy barrier for nucleation, raises the phase transition temperature, and makes the eutectic reaction more difficult. As strain increases, the density of geometrically necessary dislocations (GNDs) increases continuously, storing large amounts of energy at low-angle grain boundaries (LAGBs), which drive their transformation into medium-angle grain boundaries (MAGBs) and even high-angle grain boundaries (HAGBs). Meanwhile, the dislocation distribution also diffuses from the vicinity of grain boundaries into the grain interior. After T6 heat treatment, the precipitation of Al 2 Si 2 Y and Mg 2 Si phases was observed on the eutectic Si particles. Some crystal planes of the two phases exhibit a coherent relationship, characteristics of specific precipitation. Furthermore, the study found that the orientation distribution of eutectic Si grains within and outside grain boundaries is significantly affected by the addition of the rare-earth element Y, and that the arrangement of Si atoms on the outer side exhibits pronounced lattice rotation relative to that on the inner side. The above research results provide a theoretical and experimental basis for the design and manufacture of lightweight and high-performance aluminum alloys.