Abstract:An optimized combination of gamma alumina (4 wt.%) and strontium (0.1 wt.%) was incorporated in cast Al-20Si alloy to obtain fine form of silicon. During casting process, the amount of γ-Al₂O₃ was varied from 0.5-6 wt.% to refine primary Si and Sr was varied from 0.05-0.1 wt.% to modify eutectic Si. The results showed that the average size of primary Si is 24 μm for addition of 4 wt.% γ-Al₂O₃ to the alloy whereas 0.1 wt.% Sr resulted in sphericity of eutectic Si to ~0.6 and average length of ~1.2 μm. The thermal analysis revealed that γ-Al₂O₃ can act as potential heterogeneous nucleation sites. Moreover, simultaneous addition of γ-Al₂O₃ and Sr does not poison γ-Al₂O₃ particles and inhibit their nucleation efficiency as in the case of combined addition of phosphorous and strontium to Al-20Si alloy. Therefore, it was concluded that enhanced tensile strength, i.e., ultimate tensile strength (increase by 20%) and elongation (increase by 23%) in Al-20Si-4γ-Al₂O₃-0.1wt.%Sr alloy as compared to as-cast Al-20Si alloy can be attributed to refinement of primary Si, modification of eutectic Si and the presence of α(Al) in the alloy as evident from eutectic shift.

Key words: Al-Si alloy; γ-Al₂O₃; refinement; modification; primary Si