Luming Shuai

Mechanical properties and corrosion resistance of La₂O₃/A356 composites with different contents of La₂O₃ were investigated by optical microscopy, X-ray diffractometry, scanning electron microscopy, electrochemical tests, and immersion corrosion tests. The results show that the addition of La₂O₃ refined the α-Al phase of the A356 matrix, and the long stripe-like Si phase and β-Al₅FeSi phase were transformed into short rod-like forms. The La₂O₃/A356 composites with 1.0 wt.% La₂O₃ exhibited the most optimal mechanical properties and corrosion resistance. The yield strength, ultimate tensile strength, and elongation of La₂O₃/A356 composites with 1.0 wt.% La₂O₃ were higher than those of the matrix. The results of electrochemical experiments and the immersion corrosion test show that the corrosion potential of La₂O₃/A356 composites with 1.0 wt.% La₂O₃ was 72 mV higher than that of the matrix, the corrosion current density was 84.8% lower than that of the matrix, and the impedance Z was improved by 59.1% compared to the matrix. The addition of La₂O₃ improved the mechanical properties of the A356 matrix by refining the grains, inhibiting the nucleation of eutectic Si, and promoting the twinning growth mechanism. Moreover, the effect of La₂O₃ on the micro-galvanic corrosion behavior of A356 was discussed.

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.