1. Nucleation - formation of stable solid particles/aggregates from the liquid
2. Crystal growth - addition of atoms to the formed nuclei
Nucleation
Occurs below the freezing point
Types of nucleation
Heterogeneous - nuclei form on a solid surface (form the wall, or particulates in a liquid)
Homogeneous - nuclei form on their own (within the liquid)
For nucleation to occur, latent heat of fusion is required to establish surfaces between liquid and solid
Latent heat of fusion
Difference in potential energy
Solidification
1. Atoms become closer to each other resulting in reduction in potential energy
2. Reduction in potential energy leads to release in energy which is responsible for establishment of surfaces and thus initiating nucleation
Nucleation is temperature dependent
For pure materials, insufficient energy is released to create stable nuclei and some degree of undercooling is required
The amount of undercooling can be reduced by presence of solid impurities (insoluble) which reduce the surface energy
Types of interfacial growth in a liquid
Planar or linear - when the liquid ahead of liquid-solid interface has a positive temperature gradient, heat is removed by conduction through the growing solid
Cellular/Dendritic - when there is a temperature inversion and the temperature decreases ahead of the liquid-solid interface
Cellular growth
Occurs as result of small undercooling (supercooling)
Dendritic growth
Occurs as result of large undercooling (Constitutional supercooling)
In pure metals - undercooling results from thermal supercooling
In alloys - undercooling results from both thermal and constitutional supercooling
Effects of undercooling of solidification structures
