Spheroidization mechanism of primary silicon in hypereutectic Al–Si alloys with Eu addition

In this study, the effects of Eu addition on the microstructures and mechanical properties of A390 alloy were investigated, along with the spheroidization mechanism of primary silicon with Eu. The results showed that when the Eu addition reached 0.6 wt%, both refinement and spheroidization of primary silicon, alongside fibrous eutectic silicon can be obtained. Owing to such a microstructural modification, the ultimate tensile strength and elongation were increased by 24.9% and 101.2%, respectively. While in the unmodified alloy, the octahedral, plate‐like, and five‐star primary silicon particles exhibited few Σ3 twin boundaries, whereas in the Eu‐modified alloy, spheroidal primary silicon displayed curved Σ3 twin boundaries, sub‐grain boundaries, and special Σ9 twin boundaries. Atom probe tomography analysis indicated that Al and Eu atoms were present in the spheroidal primary silicon as spheroidal, T‐shaped, straight rod‐like, and curved rod‐like clusters, with the Al:Eu ratio of 2.92 ± 0.46. The spheroidization mechanism of primary silicon was proposed through five aspects: (i) spheroidal clusters inducing parallel and intersecting Σ3 twins, (ii) straight rod‐like clusters causing poisoning of twin plane re‐entrant edge, (iii) T‐shaped clusters leading to multi‐thickness intersecting Σ3 twins, (iv) curved rod‐like clusters resulting in curved Σ3 twins, and (v) curved rod‐like clusters induce special Σ9 twins. These mechanisms facilitate a transition from initially anisotropic to isotropic growth of primary silicon with Eu addition. The present work provides an important insight for the development of a novel way to achieve the primary Si spheroidization and the improvement of mechanical properties of hypereutectic Al–Si alloys.