Microscopic Observation of the Solidification of Small Metal Droplets

Abstract

The solidification behavior of small metal (10 to 100 micron diameter) droplets has been observed on a high temperature microscope stage. An abrupt change in surface appearance and in the case of high melting metals a sudden brightening (``blick'') accompanies solidification. The solidification temperatures observed for a collection of droplets may be widely distributed, but a significant, usually the major, fraction of the droplets supercool some maximum amount (ΔT-)max that is reproducible and characteristic of the metal. For many metals (ΔT-)max≈0.18 times the absolute melting temperature. (ΔT-)max is not much changed by wide variations in the cooling rate and droplet size. The growth rate of metal crystals is very great so that the solidification rate of the droplets is controlled by the nucleation frequency.