Effects of Yb Additions on Refinement of Eutectic Si in Al-5Si Alloys

Abstract

Modification of eutectic Si from platelet to fibrous morphology can be interpreted using impurity-induced twinning (IIT) growth mechanism and twin plane re-entrant edge (TPRE) growth mechanism. According to IIT mechanism, Yb has an exactly fitting radius ratio (1.646). However, to date, the effects of Yb additions on eutectic Si is still very limited. In this contribution, a series of Al-5 wt% Si alloys with Yb additions have been investigated using thermal analysis and multi-scale micro structure characterization techniques. Only a refined plate-like eutectic Si structure was observed. However, in contrast to Sr additions, no fibrous morphology and no significant Si twinning was observed, which is not consistent with the generally accepted IIT mechanisms. We proposed that the morphology change from platelet to fibrous, caused by stronger Si twinning, can be defined as modification; while, the decrease in size, caused by higher nucleation and growth undercooling, should be defined as refinement.