Abstract:The modification effect of La addition on the microstructural characteristics of hypereutectic Al-17%Si alloys was studied. Microstructures were examined using conventional optical microscopy and electron probe X-ray microanalyzer. The morphology of Si crystal was evaluated by quantitative metallographical analysis. The results indicate that simultaneous modification to both primary and eutectic Si crystals is obtained by the addition of pure rare earth metal La in the hypereutectic Al-17%Si alloys while the optimal effect of La is obtained at a rather large scale. La begins to form intermetallic compound when its concentration exceeds approximately 1%, which appears as dark platelets in etched specimens. Electron probe X-ray microanalysis experiment shows that La-rich platelet could be represented as Al₁Si₂La₂ consisting of LaSi₂ and some unknown ternary Al_xSi_yLa phase.

Key words:   hypereutectic Al-Si alloy; casting; La; modification