風結ぶ言葉たち

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 phase states and types of phases present 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 satisfy 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 various 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 various critical points of the alloys on the cooling curves (the temperature points at which 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: $ Pb-Sn, Al-Ag, Al-Si, Pb-Bi $, etc.

Binary Peritectic Phase Diagram#

Peritectic Reaction (Transformation)#

A reaction in which a solid phase crystallizing from the liquid phase interacts with the liquid phase to generate a new solid phase. $ L + \alpha \Rightarrow \beta $

Peritectic Phase Diagram#

A phase diagram in which two components are infinitely soluble in the liquid state, have limited solubility or are completely insoluble in the solid state, and undergo peritectic reactions during cooling.

Binary alloy systems with peritectic reactions: $ Pt-Ag, Sn-Sb, Cu-Sn, Cu-Zn $, etc.

Binary Phase Diagram of Stable Compounds#

Stable Compounds#

Compounds that have a melting point and maintain their inherent structure without decomposition below the melting point.

Binary alloy systems with stable compounds: $ Mg-Si, Mn-Si, Fe-P, Cu-Sb $, etc.

1

Binary Phase Diagram with Eutectoid Reaction#

Eutectoid Reaction (Transformation)#

A reaction (transformation) in which two new solid phases with completely different chemical compositions and structures precipitate simultaneously from a solid phase at a certain composition and 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 Alloys: Alloys based on iron and carbon as the main 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 a free state (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: Peritectic Line ($ 1495^\circ C $)

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

Eutectoid 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 Different Forms of Cementite

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

• Eutectic Cementite: Cementite generated during the eutectic reaction.

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

• Eutectoid Cementite: Cementite generated during the eutectoid reaction.

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

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

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

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

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

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

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

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

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

Effects of Carbon Content on the Structure and Properties of Iron-Carbon Alloys#

1. Effects of Carbon Content on the Equilibrium Structure of Iron-Carbon Alloys

   • Effects 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.

   • Effects 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, and the relative amounts of other structures reach their maximum at their characteristic composition points.

2. Effects of Carbon Content on the Mechanical Properties of Iron-Carbon Alloys

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

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

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

   • Effects on Plasticity and Toughness: Plasticity and toughness decrease with increasing $ C% $.

3. Effects of Carbon Content on the Processability of Iron-Carbon Alloys

   • Effects on Machinability: Medium carbon steel has the best machinability.

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

   • Effects on Castability: Castability is good for cast iron near the eutectic point.

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

This article is synchronized to xLog by Mix Space
The original link is https://nishikori.tech/posts/prose/Binary-phase-diagram-and-its-application