チルプレート上で急冷凝固させたAl-17%Si合金における共晶Si形態

抄録

  Hyper eutectic Al-Si alloys containing no P are expected to possess excellent fluidity upon under-cooling solidification. The mechanical properties of these alloy castings are affected by the eutectic Si phase structure. In this research, an Al-17% Si alloy molten metal containing no P was pelted on a chill plate to obtain thinned casting plate. This microstructure was observed to have a plate-like, rod-like, peanuts-like and spheroidal eutectic Si phase. Therefore, we investigated the mechanism responsible for changing eutectic Si phase structure as just described. In the initial stage of solidification, primary Si of the order of 5μm generated nuclei on the chill plate surface. This solidification is attributed to rapid cooling, and it is likely that the small (approximately 100μm) pseudo plate-like eutectic Si phase initiated cell growth within the primary Si phase. It is assumed that during solidification growth, the castings were in an adiabatic state because of separation from the chill plate, and therefore, the heat of the remaining liquid diffused into the pseudo plate-like eutectic Si phase, which seems to be comprised of rod-like and spheroidal shapes. Owing to the slow cooling, a relatively large eutectic plate-like Si phase grew.