MSE 440/540: Processing of Metallic Materials

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MSE 440/540: Processing of Metallic Materials

Instructors: Yuntian ZhuOffice: 308 RBII

Ph: 513-0559

[email protected]

Lecture 2: Solidification Theory

Homework: Problem from notes,

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Field Trip #1

• When: 2pm, Friday, Sept. 4, 2015 (drive = 1.5 h)

• Where: Biscoe Foundry, 530 East Main St, P.O. Box 748, Biscoe, NC 27209

• Dress code: • Volunteers: 1% credit

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Solidification

Nearly every metal product started as a liquid at some point in time. -What is Solidification

- What is the Melting Point?- What’s composition?

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Nucleation and Growth

During solidification, solid nuclei form in the liquid and subsequently grow until the entire volume is a solid.

-What is the driving force for nucleation and growth?

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Homogeneous Nucleation

Consider free energy change when small spherical nuclei of solid is formed in a liquid

LS

G = H – TS

ΔG = ΔH – TΔS

At Tm, ΔG = 0, ΔH = L

ΔS = L/Tm

ΔGv = L – T(L/Tm) = L(1-T/Tm)

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Homogeneous Nucleation

There is a critical radius r*, at a given undercooling ΔT where embryos can grow and reduce the free energy.

r* occurs at

HOMEWORK: Show that:

r* =-2/ΔGv

ΔG at r* ,

If ΔGv = LΔT/Tm, then

= thermodynamic barrier to nucleation or work of nucleation

Quiz: What is the physical origin of energy barrier for nucleation?

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Homogeneous Nucleation

Note that r* and ΔGv* decrease with increasing undercooling

Quiz: The effect of T on solidification speed and microstructure.

https://www.youtube.com/watch?v=-kIOXusTgoU Freeze beer/2:22

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Heterogeneous Nucleation

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Heterogeneous Nucleation Rate

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Innoculants (seeds):

Nucleation of Melting:

SL + LV < SV

Low undercooling is needed for heterogeneous nucleation

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Commercial Practice

Dynamic Nucleation

- Vibrate melt to collapse internal cavities - Fragmentation of existing solids; breaking of dendrite arms (crystal multiplication) - Electromagnetic mixing and stirring

Nucleation during Melting

- Why does melting usually occur at Tm, even at high heating rates?

Most liquid metals wet their own solid, so the wetting angle Θ = 0 no energy barrier for nucleation

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Commercial Practice

Characteristics of a good inoculant - low interfacial energy between nucleant and growing solid

- γSP decreases with decreasing lattice mismatch between solids and nucleant with increasing chemical affinity (coherent interface)

- Should be as stable as possible in the liquid melt Tminoc > Tmmelt

- possess a high surface area (rough or pitted)

-Smaller particles

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Growth: Practical Results

Crystals grow in two ways after nucleating

-Planar growth – heat extraction through the solid phase and a smooth solid/liquid interface -Dendritic growth – formation of branched skeleton structures.

http://www.youtube.com/watch?v=S07fPo45BvM

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Reading Assignment

• Read Chapter 5

• HW:

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