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Important Concepts

Phase Diagram: A graphical representation that describes the relationships between the state, temperature, pressure, and composition of a system.

State: Refers to the condensation state and type of phases in the system.

Phase Transition: The process in which a phase in an alloy changes from one type to another.

Special Note

Phase diagrams are established under thermodynamic equilibrium conditions. The most commonly used method for determining phase diagrams is thermal analysis, which requires the alloy to cool very slowly during cooling to meet the conditions of thermodynamic equilibrium. Therefore, phase diagrams are also known as equilibrium phase diagrams or equilibrium diagrams.

Functions of Phase Diagrams

Using phase diagrams, one can understand the following about materials with different compositions under different conditions:

1. What phases exist;
2. The relative amounts of each phase;
3. The phase transitions that occur in the material when composition and temperature change.

Establishment of Binary Phase Diagrams#

Thermal Analysis Method#

Taking the $ Cu-Ni $ alloy as an example

1. Prepare a series of different $ Cu-Ni $ alloys (for example, $ 100% Cu, 80% Cu-20% Ni, 60% Cu-40% Ni, 40% Cu-60% Ni, 20% Cu-80% Ni, 100% Ni $ and other 6 alloys);

2. Measure the cooling curves of the above alloys separately;

3. Identify the critical points of the alloys on the cooling curves (the temperature points where solidification begins and ends);

4. Mark each critical point on the coordinate plane of the phase diagram (the coordinate plane of the binary phase diagram, with the horizontal axis as composition and the vertical axis as temperature);

5. Connect the critical points with the same properties on the phase diagram plane to establish the phase diagram.

Basic Types and Analysis of Binary Phase Diagrams#

Binary Eutectic Phase Diagram#

Eutectic Reaction (Transformation)#

The reaction (transformation) in which two different solid phases crystallize simultaneously from the liquid phase. $ L \Rightarrow \alpha+\beta $

Binary alloy systems with eutectic phase diagrams: $Cu-Ni, Au-Ag, Fe-Ni, Cu-Au, Cr-Mo$, etc.

Lever Rule: Used for calculating the relative amounts of two phases when a binary alloy is in two-phase equilibrium.

$Q_L=\frac{x_2-x}{x_2-x_1}\times100%$

$Q_\alpha=\frac{x-x_1}{x_2-x_1}\times100%$

Dendritic Segregation: Alloys typically grow in a branched form during crystallization, resulting in compositional differences between the trunk and branches, known as dendritic segregation, which is a metallurgical defect.

Treatment Method: Generally can be alleviated or eliminated through forging and homogenization annealing (also known as diffusion annealing).

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Binary Eutectoid Phase Diagram#

Eutectoid Reaction (Transformation)#

The reaction (transformation) in which a solid phase of a certain composition simultaneously precipitates two new solid phases with completely different chemical compositions and structures at a certain temperature. $ \alpha \Rightarrow \beta_1+\beta_2 $

Eutectoid phase diagrams and eutectic phase diagrams are similar in shape, but the reactions that occur are completely different. The analysis methods for eutectoid phase diagrams are similar to those for eutectic phase diagrams.

Iron-Carbon Alloy Phase Diagram#

Iron-Carbon Alloy: An alloy with iron and carbon as the basic components.

Two Major Categories: Carbon Steel ($ C% < 2.11% $), Cast Iron ($ C% > 2.11% $)

Forms of Carbon in Iron-Carbon Alloys:

1. C dissolves in the lattice of Fe to form interstitial solid solutions (ferrite, austenite).
2. C interacts with Fe to form compounds ($Fe_3C$).
3. Exists in free form (graphite).

Basic Phases in Iron-Carbon Alloys#

Ferrite#

Symbol: $ \alpha $ or $ F $

Definition: An interstitial solid solution formed by carbon dissolving in the body-centered cubic lattice of $ \alpha-Fe $.

The interstitial solid solution formed by carbon dissolving in the body-centered cubic lattice of $ \delta-Fe $ is also ferrite, and to distinguish it, it is referred to as $ \delta $-ferrite or high-temperature ferrite.

Properties: Low strength and hardness, high plasticity and toughness.

$HB=50-80, \delta = 30-50% $

Austenite#

Symbol: $ \gamma $ or $ A $

Definition: An interstitial solid solution formed by carbon dissolving in the face-centered cubic lattice of $ \gamma-Fe $.

Properties: Low strength and hardness, high plasticity and toughness.

$HB=170-220, \delta = 30-50% $

Compared to ferrite, austenite can dissolve more carbon and has higher strength and hardness.

Cementite#

Symbol: $ C_m $ or $ Fe_3C $

Definition: An interstitial compound formed by the interaction of carbon and iron.

Properties: High melting point, high hardness, high brittleness, and almost zero plasticity.

$HB=800, \delta\approx 0% $

Analysis of Iron-Carbon Alloy Phase Diagram#

Characteristic Points#

Characteristic Lines#

Liquid and Solid Phase Lines#

ABCD: Liquid Phase Line

AHJECF: Solid Phase Line

Three Horizontal Lines#

HJB: Eutectoid Line ($1495^\circ C$)

Eutectoid Reaction: $L_{0.53} +\delta_{0.09} \leftarrow^{1495^\circ C}\rightarrow \gamma_{0.17}$

ECF: Eutectic Line ($1148^\circ C$)

Eutectic Reaction: $L_{4.3} \leftarrow^{1148^\circ C}\rightarrow \gamma_{2.11}+Fe_3C$ , forming ledeburite ** $ L_d = \gamma_{2.11}+Fe_3C $ **

PSK: Peritectic Line ($727^\circ C$)

Peritectic Reaction: $\gamma_{4.3} \leftarrow^{727^\circ C}\rightarrow \alpha_{0.0218}+Fe_3C$ , forming pearlite ** $ P = \alpha_{0.0218}+Fe_3C $ **

Three Solid-State Transformation Lines#

GS: $\gamma \leftarrow^{Heating}_{Cooling} \rightarrow \alpha $ Transformation temperature line, also known as the $ A_3 $ line

ES: $\gamma \leftarrow^{Heating}{Cooling} \rightarrow Fe_3C{II} $ Carbon solubility curve in austenite ($\gamma$), also known as the $ A_{cm} $ line

PQ: $\alpha \leftarrow^{Heating}{Cooling} \rightarrow Fe_3C{III} $ Carbon solubility curve in ferrite ($\alpha$)

Five types of cementite with different forms

• Primary Cementite ($Fe_3C_I$): Cementite precipitated from the liquid phase.

• Eutectic Cementite: Cementite generated in the eutectic reaction.

• Secondary Cementite ($Fe_3C_{II}$): Cementite precipitated from austenite.

• Eutectoid Cementite: Cementite generated in the eutectoid reaction.

• Tertiary Cementite ($Fe_3C_{III}$): Cementite precipitated from ferrite.

Analysis of the Equilibrium Crystallization Process of Typical Iron-Carbon Alloys#

The microstructure of industrial pure iron ($ \omega_c=0-0.0218% $) at room temperature: $F+Fe_3C_{III}$

The microstructure of sub-eutectoid steel ($ \omega_c=0.0218-0.77% $) at room temperature: $F+P$

The microstructure of eutectoid steel ($ \omega_c=0.77% $) at room temperature: $P$

The microstructure of super-eutectoid steel ($ \omega_c=0.77-2.11% $) at room temperature: $P+Fe_3C_{II}$

The microstructure of sub-eutectic white cast iron ($ \omega_c=2.11-4.3% $) at room temperature: $P+Fe_3C_{II}+L_d'$

The microstructure of eutectic white cast iron ($ \omega_c=0-4.3% $) at room temperature: $L_d'$

The microstructure of super-eutectic white cast iron ($ \omega_c=4.3-6.69% $) at room temperature: $Fe_3C_{I}+L_d'$

The Effect of Carbon Content on the Microstructure and Properties of Iron-Carbon Alloys#

1. The effect of carbon content on the equilibrium microstructure of iron-carbon alloys

   • Effect on phase composition

   The phase composition of iron-carbon alloys at room temperature: $ F $ and $ Fe_3C $.

   As $ C% $ increases, the relative amount of $ F $ decreases, while the relative amount of $ Fe_3C $ increases.

   • Effect on microstructure composition

   The microstructure composition of iron-carbon alloys at room temperature: $ F, Fe_3C_{III}, P, Fe_3C_{II}, L_d', Fe3C_I $.

   As $ C% $ increases, the relative amount of $ F $ decreases, while the relative amount of $ Fe_3C_I $ increases, with the relative amounts of other structures reaching their maximum at their characteristic composition points.

2. The effect of carbon content on the mechanical properties of iron-carbon alloys

   Ferrite ($ F $): Soft and ductile phase; Cementite ($ Fe_3C $): Hard and brittle phase.

   • Effect on hardness: Hardness gradually increases with increasing $ C% $.

   • Effect on strength: Strength first increases and then decreases with increasing $ C% $.

   • Effect on plasticity and toughness: Plasticity and toughness decrease with increasing $ C% $.

3. The effect of carbon content on the processability of iron-carbon alloys

   • Effect on machinability: Medium carbon steel has the best machinability.

   • Effect on forgeability: Low carbon steel has better forgeability than high carbon steel.

   • Effect on castability: Cast iron near the eutectic point has good castability.

   • Effect on weldability: Low carbon steel has better weldability than high carbon steel.

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