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,. Solidification. Nearly every metal product started as a liquid at some point in time.
Nearly every metal product started as a liquid at some point in time.
During solidification, solid nuclei form in the liquid and subsequently grow until the entire volume is a solid.
Consider free energy change when small spherical nuclei of solid is formed in a liquid
G = H – TS
ΔG = ΔH – TΔS
At Tm, ΔG = 0, ΔH = L
ΔS = L/Tm
ΔGv = L– T(L/Tm)
There is a critical radius r*, at a given undercooling ΔT where embryos can grow and reduce the free energy.
Quiz: What is the physical origin of energy barrier for nucleation?
Note that r* and ΔGv* decrease with increasing undercooling
Quiz: The effect of DT on solidification speed and microstructure.
Nucleation of Melting:
Low undercooling is needed for heterogeneous nucleation
gSL + gLV < gSV
- 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