- 70 Views
- Uploaded on

Download Presentation
## PowerPoint Slideshow about ' MSE 440/540: Processing of Metallic Materials' - igor-higgins

**An Image/Link below is provided (as is) to download presentation**

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript

### MSE 440/540: Processing of Metallic Materials

Instructors: Yuntian Zhu

Office: 308 RBII

Ph: 513-0559

Lecture 2: Solidification Theory

Homework: Problem from notes,

1

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?

2

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?

3

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)

4

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?

5

Homogeneous Nucleation

Note that r* and ΔGv* decrease with increasing undercooling

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

6

Heterogeneous Nucleation Rate

Innoculants (seeds):

Nucleation of Melting:

Low undercooling is needed for heterogeneous nucleation

gSL + gLV < gSV

8

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 of 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

9

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

10

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

11

Download Presentation

Connecting to Server..