The effects of La and Y on the microstructure and mechanical properties of cast Al-Si-Cu alloys were investigated by X-ray diffractometer (XRD), optical microscope (OM), and scanning electron microscope (SEM). The results indicated that the addition of La and Y had a great effect on the refinement of α-Al grains, the modification of eutectic Si phase, and the reduction of β-Al5FeSi length in Al-Si-Cu alloys. The A380 + 0.6 wt.% La/Y alloy exhibited the best microstructure and mechanical properties. The UTS and EI of the A380 + 0.6 wt.% La/Y alloy were 215.3 MPa and 5.1%, which were 22.9% and 37.8% higher than those of the matrix alloy, respectively. In addition, neither Al11La3 nor Al3Y generated by the addition of La and Y could not serve as the nucleation core of α-Al grains, so the grain refinement of α-Al originated from the growth limitation and constitutional supercooling. Since La and Y promote twinning generation and constitutional supercooling, the eutectic Si phase also changed from stripe-like to short fibrous or even granular and was significantly refined. Furthermore, thermodynamic calculations indicated that the Al11La3 phase was formed first and the Al3Y phase was generated on the Al11La3 phase.

Microstructure and mechanical properties of 7075 aluminum matrix composites (AMCs) reinforced with TiO2-coated carbon nanotubes (TiO2@CNTs) prepared by high energy ultrasonic assisted casting were investigated. In this paper, a novel method was developed to increase interfacial bonding by coating TiO2 on the surface of CNTs. The results showed that high-energy ultrasound can effectively promote the uniform dispersion of TiO2@CNTs in the melt, and the TiO2@CNTs can refine the α-Al phase. The optimal addition of TiO2@CNTs was 0.9wt.%, and the optimal ultrasonic time was 10 min. The yield strength, ultimate tensile strength, and microhardness of the as-cast composites prepared under the optimal parameters were 201.22 MPa, 261.79 MPa, and 126.02 HV, respectively, which increased by 40.9%, 32%, and 32.2% compared with those of the matrix, respectively. The semi-coherent interface was formed at the TiO2/α-Al interface and well-bonded interfaces between the CNTs and the matrix were formed. The fine grain strengthening, thermal mismatch strengthening, Orowan strengthening, and direct bearing strengthening were considered as four strengthening mechanisms that occurred simultaneously, leading to excellent mechanical properties of as-cast 7075-TiO2@CNTs composites.