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Eutectic diagram with partial solubility of the components in solid state

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This kind of phase diagram is a “hybrid” ofthe diagram with complete solid and liquid solution and the eutectic diagram (the metals are completely soluble in liquid state and entirely insoluble in solid state).
Consider solidification of an alloy with concentration C. When the alloy temperature is higher than TL, single liquid phase exists (point M on the diagram).
When the temperature reaches the value TL (point M1 on the liquidus curve) solidification starts. According to solidus curve the first solid crystals (primary crystals) of the α-phase have composition C1.
Further cooling of the alloy causes changing of the liquid phase composition according to the liquidus curve and when the alloy temperature reaches a certain intermediate value T (position MT ), liquid phase of composition Cy and solid α-phase of composition Cx are in equilibrium.
At the temperature equal to TE (eutectic temperature) formation of the primary crystals stops and the remainding liquid phase , having composition CE (eutectic composition), transforms to a finely devided mixture of small solid crystals of α-phase and β-phase (eutectic phase transformation).
At this temperature all α-phase crystals have composition Cα and all crystals of β-phase have composition Cβ.
Relative amounts of the α-phase primary crystals and the eutectic mixture may be calculated by the “lever rule” :

WP / WE = M2E / M2N

or

WP / WE = (CE-C) / (C- Cα)

Where:

WP – weight of the α-phase primary crystals;

WE – weight of the eutectic mixture;

Just below the eutectic temperature TE the alloy consists of two solid phase: α-phase and β-phase, relative amounts of whichis determined by the “lever rule” :

Wα / Wβ = M2F / M2N

or

Wα / Wβ = (Cβ-C) / (C- Cα)

Where:

Wα – weight of the α-phase;

Wβ – weight of theβ-phase;

During further cooling solid solution phases (α-phase and β-phase) change their compositions according to the solvus curves NN3 and FF3 .

Solvus curve determins formation of solid solution phase from another solid solution phase – similar to liquidus curve.

At the temperature T3 α-phase crystals have composition Cα and all crystals of β-phase have composition Cβ.

Hypo-eutectic alloys

If an alloy composition C is lower, than eutectic composition CE , solidification of the alloy starts from formation of the primary crystals of α-phase according to the left branch of the liquidus curve. These alloys are called hypo-eutectic.

Hyper-eutectic alloys

If an alloy composition C is higher, than eutectic composition CE , solidification of the alloy starts from formation of the primary crystals of β-phase according to the right branch of the liquidus curve. These alloys are called hyper-eutectic.

Eutectoid phase transformation is analogous to the eutectic transformation, however it occurs with a solid solution phase, breaking up into a mixture of two finely divided phases of different compositions.
 
 
 
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