Particle-dendrite interactions during the solidification of undercooled metal-matrix composites

The interactions of particles with high-velocity dendrites growing into an undercooled liquid have been studied for the first time. The initial examination of these interactions began with several model systems that could be studied without extensive chemical reactions between the particles and the matrix. Examination of the microstructures of the differential thermal analyzer (DTA) processed samples shows no clear evidence that the particles were either pushed or engulfed. In every case it appears that the particles were trapped at the grain boundaries of the matrix metal. The spatial distributions of the particles in the microstructures were quantitatively characterized using a technique based on Minkowski functionals.; Models that predict particle incorporation behavior are generally developed for low-velocity planar interfaces. Thermodynamic models predict pushing for all of the current experiments. For the particles used in the current experiments, kinetic models predict engulfment in all of the current experiments. The wide range of critical velocities predicted by the kinetic models and the contradiction of the predictions of the kinetic and thermodynamic models illustrate that theoretical descriptions of the particle incorporation process need additional development.; To analyze the discrepancy between the predictions of the particle incorporation models and the current experiments, several factors were examined. Thermal interactions between the particle and dendrite tip were found to enhance particle incorporation. Fluid flow due to shrinkage or convection inhibits incorporation and is the most likely cause of disagreement between theoretical predictions and the current experiments.; A model to simulate DTA signals was developed for the melting and dendritic solidification of an undercooled pure material. The model calculations are in qualitative agreement with experimentally measured melting and freezing DTA signals. With a more sensitive DTA apparatus, it may be possible to use this DTA model to determine solidification velocities by fitting the experimental data to a simulated curve.; It has long been observed in practice that annealing gold in air can purify the metal and increase its undercooling. Recent experiments have given some insight into the mechanism behind this phenomenon. The model for this phenomenon is based on the precipitation of impurity oxides within the gold.