Improvement of impact properties of Al-Si-Mg alloy via solution treatment and joint modification with Sr and La

An effort to obtain superior impact properties for Al-7Si-0.35 Mg alloy is presented,where modification with 0.02 wt% Sr and 0.1 wt% La as well as solution treatment was jointly employed.The samples were solution treated at 535℃ for 15 min to 12 h.The microstructure,fracture mechanism,and their correlation with the impact properties of the alloy were studied in detail mainly through optical microscopy（OM）,scanning electron microscopy（SEM） and oscillography impact test.The results show that the addition of Sr and La refined the eutectic Si particles significantly from～ 2.05 μm（modified with Sr alone） to～ 0.75 μm in as-cast microstructure,leading to a very homogeneous distribution of spheroidized Si particles in the alloy solution treated at 535℃ for 8 h.The alloy exhibits excellent impact toughness up to 75 J·cm-2,which is much higher than the maximum impact toughness of the alloys modified by Sr alone(～ 46 J·cm-2).The major reason for this remarkable increase in the impact property is the dramatic increase in crack initiation energy.The dispersoid-free zones（DFZs）near the eutectic regions mainly consist of the ductile Al-matrix,which exhibits excellent ductility.The ductile Al-matrix of the DFZs hinders the crack propagation,resulting in a significant increase in crack propagation energy.