Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation...

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Solidification and Solidification and Strengthening Strengthening Chapter 9 Chapter 9

Transcript of Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation...

Page 1: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

Solidification and StrengtheningSolidification and StrengtheningChapter 9Chapter 9

Page 2: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

SolidificationSolidification• Occurs by:Occurs by:

– NucleationNucleation• Formation of tiny

stable solid in liquid

– GrowthGrowth• Nucleus or tiny

stable solid increases in size

Page 3: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

Cooling Cooling CurvesCurves

***These will become important again when we talk about phase diagrams***

Page 4: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

NucleationNucleation

Page 5: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

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

Page 6: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

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

Page 7: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

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?

Page 8: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

Brainstorm with another student – 3 minutes

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

Page 9: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

• 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?

Page 10: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

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

Page 11: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

Growth Mechanisms Growth Mechanisms Planar GrowthPlanar Growth

Page 12: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

Planar Growth

• Well innoculated (many sites for heterogeneous nucleation)

• Cooled under equilibrium conditions (very, very slowly)

• Thermal arrest on cooling curve

Page 13: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

Growth Mechanisms Growth Mechanisms Dendritic GrowthDendritic Growth

Page 14: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

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.

Page 15: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

Have you ever seen a dendrite?

http://www.tc.gov.yk.ca/digitization/images_web/007332.jpg

Page 16: Solidification and Strengthening Chapter 9. Solidification Occurs by:Occurs by: –Nucleation Formation of tiny stable solid in liquid –Growth Nucleus or.

Casting Casting StructuresStructures