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 hypereutectic alloy Al-30% Si, directionally solidified as a bulk (non-composite) alloy, and also as the matrix of a fibrous metal matrix composite. Over 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 value, the presence of significant kinetic undercooling. This is in contrast with solidification of the composite, in which the primary Si tip undercooling decreases markedly with increasing tip velocity and is in quantitative agreement with theory for cellular solidification with no kinetic undercooling. These results, supported by metallographic observations, indicate that "wetting" of the alumina fibers by the growing 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 solid/fiber/liquid juncture. This juncture is shown to be significantly more efficient in nucleating new facet planes than is are-entrant twin plane corner. (C) 1997 Acta Metallurgica Inc.