Nucleation in undercooled liquids

During solidification the amount of undercooling is an important factor in determining microstructural development by controlling phase selection during nucleation. Usually crystallization is initiated at a heterogeneous catalytic site at a relatively small undercooling. The use of fine (10–20 μm) droplet samples is an effective method of obtaining a substantial increase in undercooling to the range (0.3−0.4)Tm (where Tm is the melting temperature) during slow cooling. Droplet undercooling behavior can be controlled by size variation and coating treatments, but the undercooling limit often appears to be set by a surface catalysis consistent with classical nucleation theory. The operative solidification reaction in droplets is determined by a competition between the equilibrium and possible metastable phase products which can be modified by the effect of cooling rate and undercooling on nucleation kinetics. With a controlled heterogeneous catalysis, specific nucleation reactions and product structures may be selected and metastable phase equilibria may be determined experimentally.