Physical Metallurgy

• Crystallization


  Transition from liquid to solid state
• Crystallization


  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