Effect of Ge and Mg additions on the aging response behavior and mechanical properties of Al-Si-Cu alloy

The influence of Ge and Mg additions on the aging response behavior, microstructural evolution and mechanical properties of Al-Si-Cu alloy was investigated. The addition of Ge significantly accelerates the aging response of Al-Si-Cu alloy with shorter aging time to reach peak-hardness. The enhanced aging response is caused by the formation of Si-Ge particles serving as heterogeneous nucleation sites for θ′-Al2Cu phases due to their high interfacial energy. The strength of Al-Si-Cu alloy slightly increases because of the accelerated precipitation of θ′ phases. The addition of Mg results in the early precipitation of Q′-Al5Cu2Mg8Si6 phases in Al-Si-Cu alloy due to the strong interaction between Cu and Mg, and both θ′ phases and Q′ phases dominate in the peak-aging state, which leads to the ultimate tensile strength of 396.3 MPa. The addition of both Ge and Mg leads to the formation of Mg2Ge phases which are undissolving in solution treatment and the quick peak-aging time (20 h) is also presented in Al-Si-Cu-Mg-Ge alloy due to the effect of Ge. Besides, the additional strengthening contribution of Q′ phases leads to a high peak-aging yield strength (300.3 MPa) and ultimate tensile strength (342.1 MPa). The results indicate that adding both Mg and Ge to Al-Si-Cu alloy has a synergetic advantage to introduce the strengthening effect and obtain the accelerated aging response. •The enhanced aging response by Ge addition results from the formation of Si-Ge particles, which act as heterogeneous nucleation sites for θ′ phases.•The co-existence of Q′ phases and θ′ phases dominating in the peak-aging state results in high strength in 0.4 Mg-added Al-7Si-3Cu alloy.•A possible mechanism of fast coarsening behavior of θ′ phase by Ge addition was demonstrated.•The addition of Mg and Ge presents combined advantages of enhanced aging response and excellent mechanical properties.