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MSE 440/540: Processing of Metallic Materials

MSE 440/540: Processing of Metallic Materials. Instructors: Yuntian Zhu Office: 308 RBII Ph: 513-0559 ytzhu@ncsu.edu Lecture 2: Solidification Theory Homework: Problem from notes,. Solidification. Nearly every metal product started as a liquid at some point in time.

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MSE 440/540: Processing of Metallic Materials

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  1. MSE 440/540: Processing of Metallic Materials Instructors: Yuntian Zhu Office: 308 RBII Ph: 513-0559 ytzhu@ncsu.edu Lecture 2: Solidification Theory Homework: Problem from notes, 1

  2. 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

  3. 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

  4. 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

  5. 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

  6. Homogeneous Nucleation Note that r* and ΔGv* decrease with increasing undercooling Quiz: The effect of DT on solidification speed and microstructure. 6

  7. Heterogeneous Nucleation 7

  8. Heterogeneous Nucleation Rate Innoculants (seeds): Nucleation of Melting: Low undercooling is needed for heterogeneous nucleation gSL + gLV < gSV 8

  9. 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

  10. 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

  11. 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. 11

  12. Reading Assignment • Read Chapter 5 • HW: 12

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