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Solidification and StrengtheningSolidification and StrengtheningChapter 9Chapter 9
SolidificationSolidification• Occurs by:Occurs by:
– NucleationNucleation• Formation of tiny
stable solid in liquid
– GrowthGrowth• Nucleus or tiny
stable solid increases in size
Cooling Cooling CurvesCurves
***These will become important again when we talk about phase diagrams***
NucleationNucleation
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
Types of NucleationTypes of Nucleation• Homogeneous – formation of critically sized solid Homogeneous – formation of critically sized solid
from the liquid by the clustering together of a large from the liquid by the clustering together of a large number of atoms at a high undercooling (without an number of atoms at a high undercooling (without an external surface) ; rarely occurs except under extreme external surface) ; rarely occurs except under extreme laboratory conditionslaboratory conditions
• Heterogeneous- Formation of a critically sized solid Heterogeneous- Formation of a critically sized solid on an impurity surface; most commonon an impurity surface; most common
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 grains…now why would we want to do that????? Any ideas?
Brainstorm with another student – 3 minutes
• Given the following cooling curve what can we say for sure about the type of nucleation?
• We can’t 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?
• Since there is no undercooling, then nucleation had to occur by heterogeneous nucleation. Homogeneous nucleation requires undercooling.
Growth Mechanisms Growth Mechanisms Planar GrowthPlanar Growth
Planar Growth
• Well innoculated (many sites for heterogeneous nucleation)
• Cooled under equilibrium conditions (very, very slowly)
• Thermal arrest on cooling curve
Growth Mechanisms Growth Mechanisms Dendritic GrowthDendritic Growth
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.
Have you ever seen a dendrite?
http://www.tc.gov.yk.ca/digitization/images_web/007332.jpg
Casting Casting StructuresStructures