Development of Spherical Eutectic Si Particles through M-SIMA Process and Its Effects on the Tensile Properties of Al-7Si-0.05Sr Alloy

This paper investigates the microstructure and mechanical behavior of modified strain-induced melt activation (M-SIMA) processed Al-7Si-0.05Sr alloy. The M-SIMA process involves multiple warm rolling at 150 °C (below recrystallization temperature) to reduce its thickness by 60% to its original dimension, followed by semi-solid isothermal treatment at 585 °C for 30 min. The microstructure was characterized by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, atomic force microscopy, and transmission electron microscopy. During the early stage of the experiment, Sr-containing as-cast alloy showed a better hardness value than that of Sr-free alloy; the former was attributed to the formation of the SrSi 2 Al 2 phase and uniform distribution of fine fibrous Si particles, which was triggered by the Sr addition. The mechanical properties of SIMA-treated samples in a semi-solid state were studied by tensile test. The higher ultimate tensile strength (UTS) of the Sr-containing M-SIMA processed alloy (267 MPa) than that of the Sr-free M-SIMA processed alloy (204 MPa) was associated with the highly dense and uniformly distributed Si particles (with Feret diameter of ~ 300 nm) in the former one. However, both specimens had the nearly equal size of primary α-Al grains. The UTS was ~ 31% higher while the elongation (%) was ~ 53% higher than that of the Sr-free alloy, manifesting the benefits of thermally stable SrSi 2 Al 2 phase at the grain boundaries, and the interdendritic regions and reduction in porosities, blow holes and shrinkage cracks.