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Most models that describe the interaction of an insoluble particle with an advancing solid-liquid interface are based on the assumption of steady state. However, as demonstrated by experimental work, the process does not reach steady state until the particle is pushed for a while by the interface. In this work, a dynamic mathematical model was developed. The dynamic model demonstrates that this interaction is essentially non-steady state and that steady state eventually occurs only when solidification is conducted at subcritical velocities. The model was tested for three systems: aluminumzirconia particles, succinonitrile-polystyrene particles, and biphenyl-glass particles. The calculated values for critical velocity of the pushing/engulfment transition were in the same range with the experimental ones.

Results of the directional solidification (DS) experiments on particle engulfment and pushing by solidifying interfaces (PEP), conducted on the space shuttle Columbia during the Life and Microgravity Science (LMS) Mission, are reported. Two pure aluminum (99.999 pct) 9 mm cylindrical rods, loaded with about 2 vol pct 500-m-diameter zirconia particles, were melted and resolidified in the microgravity (g) environment of the shuttle. One sample was processed at a stepwise increased solidification velocity and the other at a stepwise decreased velocity. It was found that a pushing/engulfment transition (PET) occurred in the velocity range of 0.5 to 1 m/s. This is smaller than the ground PET velocity of 1.9 to 2.4 m/s. This demonstrates that natural convection increases the critical velocity. A previously proposed analytical model for PEP was further developed. A major effort to identify and produce data for the surface energy of various interfaces required for calculation was undertaken. The predicted critical velocity for PET was 0.775 m/s.

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