On the transition from pushing to engulfment during directional solidification of the particle-reinforced aluminum-based metal-matrix composite 2014+10 vol pct Al2O3

Microstructural evolution in the commercial aluminum-based metal matrix composite 2014+10 vol pct Al₂O₃ was investigated during directional solidification with planar interface within the initial transient. Investigations were directed toward phase formation and phase distribution, with special emphasis on the critical conditions for the transition from particle pushing to engulfment. In situ nucleation of intermetallic particles, identified as being the complex silicides (Fe, Mn)₃Si₂Al₁₅, their growth, pushing, and subsequent engulfment are shown to be interactively coupled to the pushing and engulfment of the inert alumina particles. The experimental conditions for engulfment of the inert particles are in good agreement with predictions according to the “critical velocity” model of Pötschke and Rogge, the critical velocities ranging from 0.3 to 1.0 μm/s, due to the effect of the solutal field. This indicates that in castings with equiaxed dendritic solidification patterns, the radial growth velocities are not necessarily responsible for pushing the particles into the interdendritic spaces. For the intermetallic particles, the dependence of the critical velocity on particle size is not linear as for inert particles, but deflected upward for increasing size. This is probably due to the fact that they act as a sink for certain species of segregated solute atoms, meaning that size and solutal distortions are reactively coupled.