5.4.3 Other five types of phase diagrams of binary systems在线视频

Hello everyone

In this lesson let’s study

the other five types of binary system phase diagrams

First let ’s look at the fourth type

the binary system phase diagram with a compound which can form or decompose at some temperature

In the figure on the left

when the compound C is heated to TD below the eutectic temperature

it decomposes into crystals A and B

No liquid occurs

There is no liquidus in equilibrium with compound C on the phase diagram

It is impossible to crystalize C directly from the liquid phase

C can only be generated by the solid phase reaction between crystals A and B

The compound C in the right figure only exists in a certain temperature range

and it also decomposes at low temperature

With the knowledge of this

the crystallization paths of melts 1 and 2 are relatively easy to be understood

Let's learn this phase diagram

In the figure, crystal C only exists in a certain temperature range

it is also an incongruent melting compound

when you analyze the crystallization path of melt 3

on the basis of the previous knowledge point

you need to pay attention that

when cooled to a low temperature stage

C will decompose into crystals A and B

The following is the fifth type of phase diagram

a binary system phase diagram with polycrystalline transition

This type of phase diagram includes two different ones

one is that the crystal transition occurs below the eutectic point

and the other is that the crystal transition occurs above the eutectic point

We will introduce them in detail below

In the figure on the left

at the crystal transition point P phase A

will have a crystal transition between Aα and Aβ

The crystal transition temperature is lower than the eutectic point

the isotherm of the crystal transition is below point E

In the phase region above this curve

crystal A exists in the form of Aα

and the phase region below this curve

crystal A exists in the form of Aβ

In the figure on the right

the crystal transition temperature TP is higher than the eutectic temperature TE

At point P

a crystal transition occurs between Aα and Aβ

and there is a liquid phase in the system

At this same time, three phases Aα

Aβ and liquid coexist in equilibrium

point P is also an invariant point

which is the crystal transition point

It is easy to understand that

this crystal transition point cannot be the end point of crystallization

This is the binary phase diagram of the CaO-SiO2 system

The transition temperature of α-cristobalite and α-tridymite in the figure is 1470 ℃

which is above the eutectic point 1436 ℃

The transition temperature of α-CS and β-CS is 1125 ℃

that is, the two crystal forms of wollastonite

transform at a temperature lower than the eutectic point

The crystallization route of melt 1 in this phase diagram is easy to analyze

Above TP temperature it is the simplest phase diagram

Pay attention to TP

crystal transition occurs between Aα and Aβ

and Aα will transform into Aβ completely

The crystallization products are B and Aβ

In this phase diagram crystal Aα will crystallize from melt 2 firstly

When it is cooled to TP

the crystal transition will occur

and three phases coexist

The liquidus point remains at P

until the crystal transition is completed

The liquid continues to cool with continuous crystallization of Aβ

the following analysis of the crystallization path is easier

This is the sixth type of phase diagram we will learn

a binary system phase diagram with liquid phase stratification

In the phase diagrams of the five binary systems discussed earlier

the liquid phases are completely miscible

In some practical systems

the two components are not completely miscible in the liquid state

and can only be miscible finitely

At this time

the liquid separates into two parts

one is the saturated solution of component B in component A (L1)

and the other is the saturated solution of component A in component B (L2)

The cap-shaped area CKD in the figure is a liquid-liquid phase separation area

As the temperature rises

the solubility of the two liquids increases

their compositions are getting closer and closer

Reaching the highest point K in the hat-shaped region

the compositions of the two liquids are the same

and the stratification phenomenon disappears

Point K is a critical point

The temperature at point K is called critical temperature

Point E is the eutectic point of A and B

and there is another invariant point F in the system

The phase change that occurs at point F is

LC → LD + A

that is when cooled rystal A will crystallize from liquid C

and LC liquid is transformed into liquid LD with a low content of crystal A

Next, let's take a look at the crystallization path of melt 1

Cooled down to L1 temperature

the liquid begins to stratify

and is divided into two liquid phases of different compositions, L1 and L1'

Then with the decrease of temperature

and the melt will be divided into two liquid phases of different compositions, L2 and L2'

When cooled to TD

liquid LC continues to decompose into liquid LD and crystal A

until liquid LC is exhausted

After liquid LC disappears

the temperature of the system continues to fall

The liquidus point changes from point D to E along the liquidus DE of crystal A

and the crystallization ends at point E

The crystallization products are crystals A and B

In the phase diagrams of the CaO-SiO2 and MgO-SiO2 systems

there is a liquid-liquid stratification zone at the region of high SiO2 content.

We will give you a detailed analysis when we talk about professional phase diagrams

This is the seventh type

a binary system diagram with a complete solid solution

Above the liquidus a4b is the high-temperature liquid region

below the solidus a5b is the single-phase region of the solid solution

and the middle is the solid-liquid two-phase region where the liquid and solid solution are in equilibrium

During the crystallization of Melt 1

the liquidus point goes from point 2 to 6

and the solidus point goes from point 3 to 7

Attention this crystallization end point is a point on the liquidus.

Moreover

the solid solution is a kind of crystal

and the diffusion speed of atoms is very slow

It is not easy to adjust the composition like a liquid solution

As long as the cooling rate is not slow enough during the crystallization process

unbalanced crystallization is likely to occur

This is the eighth type

a binary phase diagram with a limit solid solution

Solid solution can be formed between A and B

but the solubility is limited

and cannot be dissolved in any ratio

In the figure, SA (B) represents

a solid solution formed by dissolving B into A

SB (A) represents

a solid solution formed by dissolving A into B

aE is the liquidus of SA (B)

and bE is the liquidus of SB (A)

aC and bD are solidus

Point E is the eutectic point of SA (B) and SB (A)

Point C represents the maximum solid solubility of B in A

point D represents the maximum solid solubility of A in B

CF is the solvus line of SA (B)

DG is the solvus lines of SB (A)

According to the trend of these two solvus curves

the solubility of the two components A and B

decreases with the dropping of temperature

The crystallization paths of melt 123 in the picture

is basically the same as the one in the simplest phase diagram

But it should be noted that

the change of the initial solid point is moved on the solidus

and can no longer correspond to pure A and B

The crystalline product is two solid solutions of SA(B) and SB (A)

This figure is

a binary phase diagram of a discontinuous solid solution without eutectic point

There is no eutectic point between SA (B) and SA(B)

but there is a peritectic point J

During cooling

when the liquid point moves to point J

a peritectic process will occur

Liquid L reacts with SA(B) to form SA(B)

We analyze the crystallization path of melt 3

First of all when cooled to a point on SA (B)

SA(B) will begin to crystalize from the melt

When the temperature drops to point J peritectic reaction occurs

SA (B) transfers to SA(B)

The solid point moved from point C to F to G

Then SA(B) disappears

and the liquidus point moves away from point J

and moves to point F

SA(B) crystalizes from the melt continuously

When the solid point reaches point E

the solid point and the system composition point coincide

and the crystallization have finished

It is worth noting that

the end point of crystallization of melt 3

is point F on the liquidus BJ

The crystallization end point is a point on a liquidus

most crystallization end points are invariant points

this is different

This table summarizes the crystallization rules of different composition points

on the binary phase diagram with the transition-discontinuous solid solution

You can learn from the phase diagram

with an inconsistent molten compound

No more details here

everyone

here we have learned

the basic 8 types of binary systems

We hope you can grasp the key points of each phase diagram

and analyze the property of invariant points,

lay the foundation for the application of professional phase diagram

Class is over

See you