Role of Mg2Si precipitates size in determining the ductility of A357 cast alloy

This study evaluates the heat treatment effects on microstructural and mechanical properties of A357 alloy. Solution treatments performed at 540 °C for 6, 8, and 10 h represent identical microstructures and show that the eutectic phase is partially dissolved into the matrix, resulting in Si particles of similar shape and size. Contrarily, heat treatment parameters influence Mg2Si precipitates length rather considerably; 8 h solution treated-peak aged condition produces the smallest sized needle-shaped βʺ precipitates and contributes principally in the achievement of higher ductility. It is identified that over-aging not only enhances the length of the precipitates; however, it also causes the partial transformation of needle-shaped βʺ into rod type βʹ precipitates. This phenomenon is the core reason for the reduction in hardness and ductility simultaneously, while enhancement in strength characteristics slightly upon over-aging. Entropy values of differential scanning calorimetry thermograms and calculated dimples diameter of fractured surfaces are fairly aligned with mechanical properties and agree with microstructural characterizations of nano-sized precipitates analyses. Developed relationships among precipitates size (length), ductility, and quality index indicate that precipitation mechanism does not merely contribute to the achievement of superior strength properties; nevertheless, it also plays a crucial role in determining the ductility of A357 cast alloy. [Display omitted] •Various heating conditions alter the length of Mg2Si precipitates while producing identical shape & size of Si particles.•Peak-aging generates βʺ precipitates whereas; over-aging causes partial transformation of βʺ into βʹ precipitates.•Variations in precipitates length do not affect strength properties however; the ductility is influenced rather noticeably.•Relationships among precipitates size, ductility, and Q. I exhibit the vital role of precipitation kinetics for A357 alloy.