04 - Eutectic transformation

How do real metals mostly solidify?

Eutectically or peritectically

What does a complete solubility in the liquid state mean?

both components are mixed together in a single homogeneous phase

What does a complete insolubility in the solid state mean?

Nothing of the solute can dissolve in the solvent. Components can’t mix together to form a homogeneous solid mixture. Instead they separate and from 2 distinct phases

What does a limited solubility in solid state mean?

The components can dissolve in each other up to a certain limit (solubility limit), above which the solute starts to form a new phase

What’s a eutectic alloy?

Alloy that solidifies directly at one single temperature (not a temperature range) and doesn’t pass through a liquid+solid state.

What’s the eutectic composition?

The precise composition for which both components of the alloy solidify simultaneously into eutectic mixture at the eutectic temperature (they occur at the same time)

What’s the eutectic tempersture?

The temperature at which an alloy with eutectic composition solidifies into the eutectic mixture (A and B solidify simultaneously)

Draw a binary eutectic phase diagram with complete solubility in liquid state and complete insolubility in solid state

Draw a binary eutectic phase diagram with complete solubility in liquid state and limited solubility in solid state

Explain all the stages and regions of a binary eutectic phase diagram with complete solubility in liquid state and complete insolubility in solid state

• liquid: material fully liquid, homogeneous, uniform phase

• L+S: component A starts to solidify into crystals, the remaining liquid becomes richer in B component because A is used to form crystals. When the eutectic temperature is reached the remaining liquid reaches the eutectic composition and A & B solidify simultaneously into eutectic mixture

  No B atoms are dissolved in A at all

• solid: final solid phase = A crystals + eutectic mixture (layers of A and B)

Explain all the stages and regions of a binary eutectic phase diagram with complete solubility in liquid state and limited insolubility in solid state for a hypoeutectic (or hyper) composition, WITH crossing of the pure alpha-crystals region

• liquid: homogeneous mixture of A and B

• L+alpha: A starts to solidify into crystals, the remaining liquid becomes richer in B as A is used to form the crystals.

  The solid phase (alpha-crystals) has a small amount of B dissolved in it.

• Pure alpha-crystals: The remaining liquid will never reach the eutectic composition, and thus will solidify into pure alpha-crystals when reaching a certain temperature

• alpha + beta seg: as the temperature decreases, the solubility limit of B in A also decreases.

  The alpha-crystals cannot hold as much as B atoms as before → the excess of B atoms precipitates out of the alpha-crystals and forms a new phase, which segregates at the grains boundaries of the crystals

Explain all the stages and regions of a binary eutectic phase diagram with complete solubility in liquid state and limited insolubility in solid state for a hypoeutectic (or hyper) composition, WITHOUT crossing of the pure alpha-crystals region

• liquid: homogeneous mixture of A and B

• L+alpha: A starts to solidify into crystals, the remaining liquid becomes richer in B as A is used to form the crystals.

  The solid phase (alpha-crystals) has a small amount of B dissolved in it.

  The remaining liquid reaches the eutectic composition when the eutectic temperature is reached, and solidifies into the eutectic solid mixture (layers of alpha & beta)

• alpha + beta seg: as the temperature decreases, the solubility limit of B in A also decreases.

  The alpha-crystals cannot hold as much as B atoms as before → the excess of B atoms precipitates out of the alpha-crystals and forms a