Lecture note 9 (10-04-2011)

Lecture note 9 (10-04-2011) - MAE 581 Advanced Materials...

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1 MAE 581 Advanced Materials Science Lecture note 9 Phase equilibrium and structure Definition of heterogeneous nucleation Nucleation normally occurs at nucleation sites on surfaces contacting the liquid or vapor. Suspended particles or minute bubbles also provide nucleation sites. This is called heterogeneous nucleation. Definition of homogeneous nucleation Nucleation without preferential nucleation sites is homogeneous nucleation. Homogeneous nucleation occurs spontaneously and randomly, but it requires superheating or supercooling of the medium. Why are the heterogeneous nucleation and the homogeneous nucleation so different? Explanations are given as follows: Liquid Solid losing energy gaining energy
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Homogeneous nucleation usually creates larger size grains Heterogeneous nucleation can create smaller size grains The differences between two nucleation mechanisms can be seen in a chill-cast ingot manufacturing method shown below. Figure 2.5 Chill-cast ingot structure.
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Lecture note 9 (10-04-2011) - MAE 581 Advanced Materials...

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