Formation and segregation of primary silicon in Bridgman solidified Al-18.3 wt% Si alloy

Abstract

The effect of withdrawal velocity V and temperature gradient G on the formation and segregation characteristics of primary silicon on Bridgman-solidified Al-18.3 wt% Si is reported. The morphology of primary silicon changes from plate-like to polyhedral with increase of GV over the range 1.0–3.4K/s. The polyhedral primary silicon forms as linked clusters by successive branching on edges following attainment of a critical size in each polyhedral element of the cluster. Primary silicon macrosegregates at lower velocities towards the bottom of samples and around the embedded thermocouple attributable to heterogenous nucleation on refractoryware followed by localized growth as found previously in isothermal holding studies. After correction of local silicon alloy concentration for such effects of macrosegregation, local undercooling for the first formation of primary silicon shows no significant dependence on V, G or GV. Corresponding measurements of number density of polyhedral clusters indicate an operative contact angle for heterogeneous nucleation of primary silicon of 30 ± 4° for the range of conditions studied.