Transcript Lecture2-Aug.25

MSE 440/540: Processing of Metallic Materials
Instructors: Yuntian Zhu
Office: 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
G = H – TS
L
S
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* =-2g/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 DT on
solidification speed and microstructure.
https://www.youtube.com/watch?v=-kIOXusTgoU
Freeze beer/2:22
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Heterogeneous Nucleation
g ML - g SM
cosq =
g SL
S(q ) =
(2 + cosq )(1 - cosq ) 2
4
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Heterogeneous Nucleation Rate
Innoculants (seeds):
Nucleation of Melting:
Low undercooling is needed
for heterogeneous nucleation
gSL + gLV < gSV
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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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