Solidification of silver–germanium alloys under microgravity conditions

SOLIDIFICATION of any melt occurs by the process of nucleation and growth. The driving force arising from cooling the melt below its equilibrium freezing temperature (undercooling) drives the process of nucleation. Depending
upon the magnitude of the driving force, equilibrium phases or competing metastable phases get nucleated. In case nucleation is completely prevented, the melt freezes into a non-crystalline phase – the glass. The presence of insoluble foreign material, the container wall and impurities promote nucleation by providing the substrates.
Therefore, any attempt to achieve high degrees of undercooling must remove such nucleation sites and several
methods like the glass-slag techniques are available to
facilitate the removal of extraneous nucleants.