KINETIC UNDERCOOLING IN SOLIDIFICATION OF A HYPEREUTECTIC AL-SI ALLOY- EFFECT OF SOLIDIFYING WITHIN A CERAMIC PREFORM COMPOSITE

Dendrite tip temperature measurements are reported for the hypereutect ic alloy Al-30% Si, directionally solidified as a bulk (non-composite) alloy, and also as the matrix of a fibrous metal matrix composite. Ov er the range of tip velocities studied (10-1000 mu m s) the primary Si tip undercooling in the directionally solidified bulk alloy increases slightly with increasing tip velocity, and indicates, by its large va lue, the presence of significant kinetic undercooling. This is in cont rast with solidification of the composite, in which the primary Si tip undercooling decreases markedly with increasing tip velocity and is i n quantitative agreement with theory for cellular solidification with no kinetic undercooling. These results, supported by metallographic ob servations, indicate that ''wetting'' of the alumina fibers by the gro wing silicon phase in the composite essentially eliminates the kinetic barrier to growth of primary Si crystals. The underlying mechanism is rationalized on the basis of macroscopic capillaric analysis at the s olid/fiber/liquid juncture. This juncture is shown to be significantly more efficient in nucleating new facet planes than is are-entrant twi n plane corner. (C) 1997 Acta Metallurgica Inc.