1. Eutectic Solidification

2. Lα+
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.

21. Thank you