Solidification and Strengthening Chapter 9

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Solidification and StrengtheningChapter 9
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Solidification

• Occurs by
• Nucleation
• Formation of tiny stable solid in liquid
• Growth
• Nucleus or tiny stable solid increases in size

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Cooling Curves
These will become important again when we talk
about phase diagrams
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Nucleation
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Important terms

• Embryo A tiny particle of solid that forms from
  the liquid as the atoms cluster together. The
  embryo may grow into a stable solid or may remelt
  .
• Nuclei Tiny particles of solid that form from
  the liquid as atoms cluster together. Because
  these atoms are large enough to be stable ,
  nucleation has occurred and growth of the stable
  solid can begin

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Types of Nucleation

• Homogeneous formation of critically sized solid
  from the liquid by the clustering together of a
  large number of atoms at a high undercooling
  (without an external surface) rarely occurs
  except under extreme laboratory conditions
• Heterogeneous- Formation of a critically sized
  solid on an impurity surface most common

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Some important terms

• Innoculation The addition of heterogeneous
  nuclei (impurities) in a controlled matter to
  increase the number of sites for heterogeneous
  nucleation, hence the number of grainsnow why
  would we want to do that????? Any ideas?

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Brainstorm with another student 3 minutes

• Given the following cooling curve what can we say
  for sure about the type of nucleation?

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• We cant say for sure what type of nucleation
  occurred here. Homogeneous requires significant
  undercooling, but heterogeneous nucleation, in
  many situations, may also require undercooling.
• How about this cooling curve?

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• Since there is no undercooling, then nucleation
  had to occur by heterogeneous nucleation.
  Homogeneous nucleation requires undercooling.

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Growth Mechanisms Planar Growth
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Planar Growth

• Well innoculated (many sites for heterogeneous
  nucleation)
• Cooled under equilibrium conditions (very, very
  slowly)
• Thermal arrest on cooling curve

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Growth Mechanisms Dendritic Growth
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Dendritic Growth

• Poor nucleation
• Requires undercooling (recalescence part of
  cooling curve)
• Dendritic growth continues until the undercooled
  liquid is warmed to the freezing temp, any
  remaining liquid solidifies by planar growth.

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Have you ever seen a dendrite?
http//www.tc.gov.yk.ca/digitization/images_web/00
7332.jpg
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Casting Structures