High Performance of T6‐Treated Al–15Mg₂Si–3Cu Composite Reinforced with Spherical Primary Mg₂Si after the Co‐Modification of Bi + Sr

In this work, the effect of the addition of Bi combined with T6 treatment on microstructure and tensile properties of Sr‐modified Al–15Mg₂Si–3Cu composite is investigated. The results show that the refinement effect of Sr on primary Mg₂Si is further enhanced via the combined addition of Bi. Interestingly, the truncated octahedral primary Mg₂Si with the smallest size (11.5 μm) is obtained by adding 0.1% Sr and 1% Bi, which is distinctly different from the cubic shape modified by single 0.1% Sr. Consequently, as a precursor with strong diffusion of Mg and Si atoms during T6 treatment, the truncated octahedral primary Mg₂Si is completely transformed into spherical morphology with the highest sphericity (0.92) solution treated at 500 °C for 6 h. Additionally, further results reveal that the co‐modification of Sr + Bi is more efficient not only in promoting the spheroidization of primary Mg₂Si, but also obtaining the fine dot‐like eutectic Mg₂Si than single modification of Sr or Bi. As a result, the tensile strength and elongation are significantly enhanced to 374 MPa and 4.75%, respectively. Thereby, this work offers an alternative co‐modification system combined with T6 treatment to synthesize the spherical primary Mg₂Si, which is helpful to prepare the high performance Al–Mg₂Si composite.