IN-SITU OBSERVATIONS OF THE INTERACTION OF LIQUID LEAD INCLUSIONS WITH GRAIN-BOUNDARIES IN ALUMINUM

The evolution of liquid lead (Pb) inclusions at grain boundaries in al uminum (Al) was investigated by direct in situ TEM observation in the temperature range from 330 degrees-643 degrees C. In agreement with ea rlier reports on quenched alloys, the characteristic contact angle of the lens-shaped grain boundary inclusions was found to be near 120 deg rees. This angle remained approximately constant over the entire tempe rature range, ruling out the possibility of a wetting transition. Coar sening of grain boundary inclusions tvas observed to proceed mainly by Ostwald ripening, although coalescence could also be observed. Inclus ions at grain boundaries, at triple junctions, and at the intersection of grain boundaries with the foil surfaces adopted characteristic sha pes that were shown to be equilibrium forms. At the highest temperatur es, the grain boundaries were observed to detach from the inclusions a nd the interaction of a migrating grain boundary with inclusions could be observed.