Equilibrium shape and interface roughening of small liquid Pb inclusions in solid Al

The shape of liquid Pb inclusions embedded in a solid At matrix was investi gated at temperatures between 300 and 500 degreesC using in-situ electron m icroscopy. Inclusion shapes in the size rang, from a few nanometers to abou t 150 nm were found to depend on size, temperature and thermal history. Dur ing isothermal annealing after melting, small inclusions rounded off while larger inclusions remained faceted until the temperature was raised to abou t 500 degreesC. During subsequent cooling, inclusions refaceted, although l ess strongly than during heating. The shape hysteresis between heating and cooling cycles was found to be due to the barrier of ledge nucleation neces sary to advance the faceted interfaces. It is shown that this kinetic barri er can explain the observed dependence on size and temperature, and that th e (111) interface undergoes a roughening transition at about 550 degreesC. Even under conditions of kinetic limitation it was possible to measure loca l equilibrium by modeling kinetically limited inclusions as a droplet in a crevice. For this type of measurement, the hysteresis between heating and c ooling cycles disappeared, and the true equilibrium shape could be derived. The anisotropy of interfacial energy was shown to be significantly smaller than previously reported, and at about 2%, similar to the anisotropy of th e surface energy for fcc metals, From the width of facets on the equilibriu m shape, the step energy was determined to be epsilon = 1.9(.)10(-11) J/m a t 350 degreesC. Published by Elsevier Science Ltd on behalf of Acta Materia lia Inc.