Chill modification in AI–Si–Sb eutectic alloys

Abstract

Interface undercooling and Si interparticle spacing measurements and observations of the Si phase structure are presented for Al–12·7Si–0·2Sb (wt-%) alloys directionally solidified over the growth velocity range 20–820 μm S−1 with a temperature gradient in the liquid of 32 K cm−1. The undercooling measurements show that the undercooling decreases over the growth velocity range where chill modification occurs. Some cozonal twinning was observed in the fibres but the density of twinning decreased with increasing growth velocity and many fibres were observed to be twin free. The measured undercoolings and spacings are related to growth curves derived from the Jackson and Hunt analysis of eutectic growth, after allowance for a constitutional undercooling term due to Sb buildup at the growth interface during solidification. It is shown that the present measurements can be reconciled with a chill modification mechanism that assumes a faceted–non-faceted transition in the Si phase, but not with a mechanism that considers that the chill modified structure is a rapidly solidified flake structure.MST/3164