Eutectic Solidification
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The document discusses eutectic solidification, where a liquid transforms into two solid phases at a single temperature. It describes how one phase will form via diffusion in the liquid, depleting the local area of one constituent and pushing the composition into the solid phase range. This causes the second phase to form adjacent to the first. The two phases then grow side-by-side in a laminated microstructure. There are two types of eutectic solidification: normal, where the phases form alternate lamellae via diffusion between the phases; and anomalous, where one phase is capable of faceting and forms irregular microstructures sensitive to growth conditions.
Eutectic Solidification
- 2. Lα+ β In eutectic reaction : liquid transforms into to two solid phases. Eutectic reaction
- 3. Eutectic composition liquid Liquid, L + L + L + Eutectic temperature Eutectic composition
- 4. Crystal of one phase forms (via diffusion in the liquid) Local area becomes “depleted” of one constituent. The depleted area near the solidification pushes the local composition into a solid forming range on the phase diagram. This causes the second solid phase to form adjacent to the first phase. Eutectic reaction Liquid, L + L + L + Eutectic Reaction (at eutectic temperature and composition)
- 5. Second solid phase forms Local area becomes Depleted in the second phase constituent Eutectic reaction Liquid, L + L + L +
- 6. The 2 phases grow side-by-side creating a laminated microstructure Eutectic reaction Liquid, L + L + L +
- 7. Eutectic reaction: Characteristic microstructure: laminates of each of the 2 phases Eutectic reaction Liquid, L + L + L +
- 8. Types of Eutectic solidification: 1.Normal 2.Anomalous Normal: • The two phases appear either as alternate lamellae or as rods of minor phase embedded in the other phase. • Involves two non-faceting phases. • This occurs when both phases have low entropy of fusion. • E.g.: Pb-Sn
- 9. The dark layers are Pb-rich α phase, the light layers are the Sn-rich β phase.
- 10. Anomalous: • This occurs when one component is capable of faceting i.e. has a high entropy of melting. • The phases appear as broken lamellar/irregular/complex regular or quasi regular. • Sensitive to solidification conditions and impurities and displace the much wider range of microstructures. • Non isothermal growth • E.g.: Al-Si alloy
- 11. Al-Si alloy
- 13. Growth Mechanism Cooperative growth: the two phases of the eutectic grow together as a diffusion couple. Divorced growth: the two phases of the eutectic grow separately. There is no direct exchange of solute between the two solid phases and no trijunction.
- 16. • As the A-rich α phase solidifies excess B diffuses a short distance laterally where it is incorporated in the B-rich α phase. Similarly the A atoms rejected ahead of the β diffuse to the tips of the adjacent α lamellae. • Rate of growth depends on rate of diffusion which in turn depends on interlamellar spacing λ. As λ increases rate of diffusion decreases and thus rate of growth of lamellae decreases. Figure 1. Growth of lamellar eutectics
- 17. Limit to λ Figure 2. Figure 3. Figure 4.
- 18. Calculation of λ* and growth rate Figure 5.
- 19. Calculation of λ* and growth rate Figure 6.
- 20. Tri-dependence of ΔT0, λ and v This equation shows that by varying the interface undercooling (ΔT0) it is possible to vary the growth rate (v) and spacing (λ) independently. It is therefore impossible to predict the spacing that will be observed for a given growth rate. However, controlled growth experiments show that a specific value of λ is always associated with a given growth rate.
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