Eutectic Solidification

Lα+β

In eutectic reaction : liquid transforms into to two solid phases.

Eutectic reaction

Eutectic composition liquid

Liquid, L

a + L b + La b

a + b

Eutectictemperature

Eutectic composition

Crystal of onephase forms(via diffusion in the liquid)

Local area becomes “depleted”of one constituent.

The depleted area near the solidification pushes the local compositioninto a solid forming range on the phase diagram. This causes the secondsolid phase to form adjacent to the first phase.

Eutectic reaction

Liquid, L

a + L b + La b

a + b

Eutectic Reaction(at eutectictemperature andcomposition)

Second solid phase forms

Local area becomesDepleted in the secondphase constituent

Eutectic reaction

Liquid, L

a + L b + La b

a + b

The 2 phases growside-by-side creating alaminated microstructure

Eutectic reaction

Liquid, L

a + L b + La b

a + b

Eutectic reaction:Characteristic microstructure: laminates of each of the 2 phases

Eutectic reaction

Liquid, L

a + L b + La b

a + b

Types of Eutectic solidification:1. Normal2. 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

The dark layers are Pb-rich α phase, the light layers are the Sn-rich β phase.

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

Al-Si alloy

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.

• 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

Limit to λ

Figure 2.

Figure 3.

Figure 4.

Calculation of λ* and growth rate

Figure 5.

Calculation of λ* and growth rate

Figure 6.

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.

Thank you