FORMATION AND SEGREGATION OF PRIMARY SILICON IN BRIDGMAN SOLIDIFIED AL-18.3 WT-PERCENT SI ALLOY

The effect of withdrawal velocity V and temperature gradient G on the formation and segregation characteristics of primary silicon on Bridgm an-solidified Al-18.3 wt% Si is reported. The morphology of primary si licon changes from plate-like to polyhedral with increase of GV over t he range 1.0-3.4 K/s. The polyhedral primary silicon forms as linked c lusters by successive branching on edges following attainment of a cri tical 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 nucleati on 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 undercoolin g for the first formation of primary silicon shows no significant depe ndence on V, G or GV. Corresponding measurements of number density of polyhedral clusters indicate an operative contact angle for heterogene ous nucleation of primary silicon of 30 +/- 4 degrees for the range of conditions studied.