Phase Diagrams in Chemistry
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Questions and Answers

Define the term 'Component' in the context of metallurgy.

Component refers to the pure metals and/or compounds in which an alloy is composed of.

What is meant by 'Solubility Limit'?

The solubility limit is the maximum concentration of atoms that may dissolve in a matrix to form a solution and is temperature dependent.

What is a 'Phase' in the context of metallurgy?

  • A portion of a system with uniform physical and chemical characteristics (correct)
  • A specialized tool used in metallurgical testing
  • A pathway in a phase diagram
  • A type of alloy composition

    • Phase Equilibrium is achieved when a system's free energy is at a maximum under specific conditions.

    <p>False</p> Signup and view all the answers

    The ___ is a graphical representation showing the stable phases and their compositions over specific ranges of conditions.

    <p>phase diagram</p> Signup and view all the answers

    What is the maximum solubility of carbon in alpha ferrite at 727°C?

    <p>0.022 wt%</p> Signup and view all the answers

    What is the maximum solubility of carbon in austenite at 1147°C?

    <p>2.14 wt%</p> Signup and view all the answers

    Delta ferrite is stable only at high temperatures.

    <p>True</p> Signup and view all the answers

    At what temperature does the iron carbide Fe3C decompose into alpha iron and graphite?

    <p>650 - 700 °C</p> Signup and view all the answers

    The invariant reaction that occurs at 1147°C is the ______ reaction.

    <p>eutectic</p> Signup and view all the answers

    At 727°C, the invariant reaction involves the transformation of gamma into alpha and cementite. This is known as the ______ reaction.

    <p>eutectoid</p> Signup and view all the answers

    Match the following compositions with their respective phases:

    <p>99.65 wt% Fe–0.35 wt% C = hypoeutectoid 98.90 wt% Fe–1.10 wt% C = hypereutectoid</p> Signup and view all the answers

    What is the fraction of eutectoid cementite (𝑊𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 𝐹𝑒3 𝐶) in a hypereutectoid composition?

    <p>0.104</p> Signup and view all the answers

    What is the amount of pearlite (𝑊𝑝𝑒𝑎𝑟𝑙𝑖𝑡𝑒) in a hypereutectoid composition?

    <p>0.943</p> Signup and view all the answers

    Which type of cooling condition allows the prediction of microstructures after continuous cooling treatments?

    <p>CCT curve</p> Signup and view all the answers

    Match the following terms with their descriptions:

    <p>TTT curve = Used for very slow cooling or heating to predict phase transformations CCT curve = Used for rapid cooling or heating to predict microstructures after continuous cooling treatments Isothermal Transformation Curve = Allows prediction of microstructure and corresponding amount after some period of constant temperature</p> Signup and view all the answers

    What is the critical cooling rate?

    <p>the minimum quenching rate that will form 100% martensite</p> Signup and view all the answers

    What is a phase in the context of metallurgy?

    <p>A homogeneous portion of a system that has uniform physical and chemical characteristics</p> Signup and view all the answers

    What is phase equilibrium in metallurgy?

    <p>A system that is at equilibrium, with a minimum free energy under specific conditions</p> Signup and view all the answers

    What is a phase diagram?

    <p>A graphical representation of the phases present and their ranges in composition, temperature, and pressure</p> Signup and view all the answers

    What are the types of phase diagrams?

    <p>Unary, Binary, and Ternary</p> Signup and view all the answers

    What is a unary phase diagram?

    <p>A phase diagram with one component</p> Signup and view all the answers

    What is the purpose of a phase diagram?

    <p>All of the above</p> Signup and view all the answers

    What does a phase diagram show?

    <p>The phases present and their ranges in composition, temperature, and pressure</p> Signup and view all the answers

    What is an equilibrium phase diagram?

    <p>A phase diagram that shows all the reactions in a system in equilibrium</p> Signup and view all the answers

    What is the value of 𝑊𝛼 in the given hypoeutectoid composition?

    <p>0.95</p> Signup and view all the answers

    What is the value of 𝑊𝐹𝑒3 𝐶 in the given hypoeutectoid composition?

    <p>0.05</p> Signup and view all the answers

    What is the value of 𝑊𝑝𝑟𝑜−𝛼 in the given hypoeutectoid composition?

    <p>0.56</p> Signup and view all the answers

    What is the composition range of the given hypoeutectoid composition?

    <p>0.022 to 0.35</p> Signup and view all the answers

    What is the significance of the temperature 727°C in the iron-carbon phase diagram?

    <p>It is the temperature at which austenite transforms to alpha ferrite and cementite.</p> Signup and view all the answers

    What is the significance of the temperature 1147°C in the iron-carbon phase diagram?

    <p>It is the eutectoid temperature.</p> Signup and view all the answers

    What is the type of composition that has a carbon content less than 0.022?

    <p>Pure iron</p> Signup and view all the answers

    What is the type of composition that has a carbon content between 0.022 and 0.76?

    <p>Hypoeutectoid</p> Signup and view all the answers

    What is the value of Wproeut−Fe3C in a hypereutectoid composition?

    <p>0.057</p> Signup and view all the answers

    What is the value of Wpearlite in a hypereutectoid composition?

    <p>0.943</p> Signup and view all the answers

    What is the expression for the fraction of eutectoid cementite (Weutectoid Fe3C) in a hypereutectoid composition?

    <p>Weutectoid Fe3C = Wtotal - Wproeut−Fe3C - Wpearlite</p> Signup and view all the answers

    What is the value of Weutectoid Fe3C in a hypereutectoid composition?

    <p>0.104</p> Signup and view all the answers

    What is the expression for Wproeut−Fe3C?

    <p>(1.10 - 0.76) / (6.70 - 0.76)</p> Signup and view all the answers

    What is the expression for Wpearlite?

    <p>(6.70 - 0.76) / (6.70 - 1.10)</p> Signup and view all the answers

    What is the value of Wproeut−Fe3C in a hypereutectoid composition?

    <p>0.057</p> Signup and view all the answers

    What is the expression for calculating the fraction of eutectoid cementite (Weutectoid Fe3C) in a hypereutectoid composition?

    <p>Weutectoid Fe3C = Wtotal - Wproeut−Fe3C - Wpearlite</p> Signup and view all the answers

    What is the formula for the amount of proeutectoid cementite (𝑾𝒑𝒓𝒐−𝑭𝒆𝟑 𝑪) in a hypereutectoid composition?

    <p>𝑺𝒑𝒓𝒐−𝑭𝒆𝟑 𝑪 = 𝑽+𝑿 / 𝑪′𝟏 − 𝟎.𝟕𝟔</p> Signup and view all the answers

    What is the formula for the amount of proeutectoid ferrite (𝑾𝒑𝒆𝒂𝒓𝒊𝒕𝒆) in a hypereutectoid composition?

    <p>𝑾𝒑𝒆𝒂𝒓𝒊𝒕𝒆 = 𝑽+𝑿 / 𝟔.𝟕𝟎 − 𝟎.𝟕𝟔</p> Signup and view all the answers

    What is the variable 𝑽 in the formulas for proeutectoid cementite and proeutectoid ferrite?

    <p>The carbon content of the composition</p> Signup and view all the answers

    What is the purpose of determining the amounts of proeutectoid cementite and proeutectoid ferrite in a hypereutectoid composition?

    <p>To determine the microstructure of the composition</p> Signup and view all the answers

    What type of composition is characterized by the presence of proeutectoid cementite and proeutectoid ferrite?

    <p>Hypereutectoid composition</p> Signup and view all the answers

    What is the significance of the 𝑪′𝟏 value in the formulas for proeutectoid cementite and proeutectoid ferrite?

    <p>It represents the solubility limit of carbon in alpha iron</p> Signup and view all the answers

    Why is it important to accurately determine the amounts of proeutectoid cementite and proeutectoid ferrite in a hypereutectoid composition?

    <p>To understand the microstructure and properties of the composition</p> Signup and view all the answers

    What is the relationship between the amounts of proeutectoid cementite and proeutectoid ferrite in a hypereutectoid composition?

    <p>They are inversely proportional</p> Signup and view all the answers

    What is the value of 𝑊𝑝𝑒𝑎𝑟𝑙𝑖𝑡𝑒 in a hypoeutectoid composition?

    <p>0.44</p> Signup and view all the answers

    What is the value of 𝑊𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 𝛼 in a hypoeutectoid composition?

    <p>0.39</p> Signup and view all the answers

    What is the value of 𝑊𝑡𝑜𝑡𝑎𝑙 𝛼 in a hypoeutectoid composition?

    <p>0.95</p> Signup and view all the answers

    What is the relationship between 𝑊𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 𝛼 and 𝑊𝑡𝑜𝑡𝑎𝑙 𝛼 in a hypoeutectoid composition?

    <p>𝑊𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 𝛼 = 𝑊𝑡𝑜𝑡𝑎𝑙 𝛼 - 𝑊𝑝𝑟𝑜−𝛼</p> Signup and view all the answers

    What is the fraction of eutectoid cementite in a hypereutectoid composition?

    <p>0.95</p> Signup and view all the answers

    What is the amount of pearlite in a hypereutectoid composition?

    <p>0.76</p> Signup and view all the answers

    What is the relationship between the fractions of eutectoid alpha and cementite in a hypoeutectoid composition?

    <p>𝑊𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 𝛼 = 1 - 𝑊𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 𝐹𝑒3𝐶</p> Signup and view all the answers

    What is the significance of the eutectoid composition in a binary phase diagram?

    <p>It represents the composition of a mixture of two phases at a specific temperature.</p> Signup and view all the answers

    Study Notes

    Phase Diagrams

    • A phase diagram is a graphical representation of the phases present and the ranges in composition, temperature, and pressure over which the phases are stable.
    • It is a collection of curves showing solubility limits and is also called an equilibrium phase diagram because all reactions represented in the diagram are allowed to go to equilibrium.

    Types of Phase Diagrams

    • Unary phase diagram: one component
    • Binary phase diagram: two components
    • Ternary phase diagram: three components

    Features in a Phase Diagram

    • Axes:
      • y-axis: temperature (°C or °F)
      • x-axis: composition (wt% or atom%)
    • Lower-case Greek letters (α, β, γ, etc.): solid solutions
    • L or Lx: denotes a liquid phase
    • Phase field/region: defined by the phase or phases that exist over a range of compositions and temperatures
    • Phase boundary: lines that separate different phase fields
    • Solidus: locus of temperature below which only solids are present
    • Liquidus: locus of temperatures above which only liquids are present
    • Solvus: boundary between a single solid phase field and a two solid phase field

    Phase Diagram: Interpretation

    • Phases present: locate the temperature-composition point on the diagram and note the phase(s) that are present
    • Composition of phases: expressed in terms of the concentrations of the components
    • Quantity of phases in equilibriated mixtures: can be determined using the Lever Rule

    Lever Rule

    • STEPS:

      1. Define alloy composition and temperature

      2. Set tie line and define compositions at the ends of the tie line

      3. To find the amount of one phase, use the equation:

        % amount of one phase = (difference in composition between the two ends of the tie line) / (difference in composition between the two phases)

    Invariant Reactions

    • Eutectic: L1 → S1 + S2
    • Eutectoid: S1 → S2 + S3
    • Peritectic: S 1 + L 1 → S2
    • Peritectoid: S 1 + S 2 → S3
    • Monotectic: L1 → S1 + L2
    • Syntectic: L 1 + L 2 → S1

    Fe-Fe3C Phase Diagram

    • Phases:
      • 𝝰-Fe / alpha Ferrite: BCC crystal structure, stable form of iron at room temperature, maximum solubility of carbon is 0.022 wt% at 727 °C
      • 𝝲-Fe / Austenite: FCC crystal structure, maximum solubility of carbon is 2.14 wt% at 1147 °C
      • 𝜹-Fe / delta Ferrite: BCC crystal structure, stable only at high temperature
      • Fe3C / iron carbide / cementite: metastable intermetallic compound, remains as a compound indefinitely at room temperature, but decomposes (very slowly, within several years) into α-Fe and C (graphite) at 650-700 °C

    Invariant Reactions in Fe-Fe3C Phase Diagram

    • Peritectic, 1493 °C: L 0.53 wt% C + 𝜹 0.09 wt% C ⇌ 𝜸(0.41 wt% C)
    • Eutectic, 1147 °C: L 4.30 wt% C ⇌ 𝜸 2.14 wt% C + Fe3C(6.70 wt% C)
    • Eutectoid, 727 °C: 𝜸 0.76 wt% C ⇌ 𝜶 0.022 wt% C + Fe3C(6.70 wt% C)

    Hypoeutectoid Composition

    • Amount of phases can be determined using the Lever Rule
    • Example: For a 99.65 wt% Fe–0.35 wt% C alloy at a temperature just below the eutectoid, determine the fractions of total ferrite and cementite phases, the fractions of the proeutectoid ferrite and pearlite, and the fraction of eutectoid ferrite.### Hypereutectoid Composition
    • The fraction of eutectoid ferrite (α) can be calculated using the formula: 𝑊𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 𝛼 = 𝑊𝑡𝑜𝑡𝑎𝑙 𝛼 − 𝑊𝑝𝑟𝑜−𝛼
    • For example, if the total amount of α is 0.95 and the amount of pro-eutectoid α is 0.56, then the fraction of eutectoid α is 0.39 (0.95 - 0.56)

    Phase Amount

    • The amount of pro-eutectoid cementite (𝑊𝑝𝑟𝑜−𝐹𝑒𝐶) can be calculated using the formula: 𝑊𝑝𝑟𝑜−𝐹𝑒𝐶 = (𝑪′𝟏 − 𝟎.𝟕𝟔) / (𝑽+𝑿)
    • The amount of pearlite (𝑊𝑝𝑒𝑎𝑟𝑙𝑖𝑡𝑒) can be calculated using the formula: 𝑊𝑝𝑒𝑎𝑟𝑙𝑖𝑡𝑒 = (6.70 − 𝐶1′) / (𝑽+𝑿)

    Hypereutectoid Composition: Phase Amount

    • For a 98.90 wt% Fe–1.10 wt% C alloy at a temperature just below the eutectoid, the fractions of total ferrite and cementite phases can be calculated as follows:
      • The fraction of total ferrite (𝑊𝛼) is 0.839
      • The fraction of total cementite (𝑊𝐹𝑒3𝐶) is 0.161
    • The fractions of the pro-eutectoid cementite and pearlite can be calculated as follows:
      • The fraction of pro-eutectoid cementite (𝑊𝑝𝑟𝑜−𝐹𝑒𝐶) is 0.057
      • The fraction of pearlite (𝑊𝑝𝑒𝑎𝑟𝑙𝑖𝑡𝑒) is 0.943
    • The fraction of eutectoid cementite (𝑊𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 𝐹𝑒3 𝐶) is 0.104

    Phase Transformation

    • Equilibrium conditions occur when there is very slow cooling or heating, and the phase diagram is used to predict phase transformations
    • Non-equilibrium conditions occur when there is rapid cooling or heating, and the TTT curve and/or CCT curve is used to predict phase transformations

    Time-Temperature-Transformation Curve

    • The TTT curve is used to predict the microstructure and corresponding amount after some period of constant temperature
    • The TTT curve is also known as the Isothermal Transformation Curve (IT)

    Continuous-Cooling Transformation Curve

    • The CCT curve is used to predict the microstructure after continuous cooling treatments
    • The CCT curve shows the effect of cooling rate on the microstructure

    TTT vs CCT

    • The TTT curve is used for isothermal treatments, while the CCT curve is used for continuous cooling treatments
    • The TTT curve shows the effect of time on the microstructure, while the CCT curve shows the effect of cooling rate on the microstructure

    Example Problems

    • Example 1: Eutectoid Steel - Microstructures and/or phases present: Pearlite (50%), Bainite (50%)
    • Example 2: AISI 1045 - Microstructures and/or phases present: Bainite (50%), Martensite (50%) with some retained austenite
    • Example 3: AISI 1045 - Microstructures and/or phases present: Pro-Ferrite (42%), Pearlite (29%), Bainite (29%)
    • Example 4: AISI 1045 - Microstructures and/or phases present: Pro-Ferrite, Pearlite, Bainite, Martensite
    • Example 5: AISI 4340 - Microstructures and/or phases present: Bainite, Martensite at a cooling rate of 5°C/s
    • Example 6: AISI 4340 - Microstructures and/or phases present: Pro-Ferrite, Pearlite, Bainite, Martensite at a cooling rate of 0.01°C/s

    Phase and Phase Equilibrium

    • A phase is a homogeneous portion of a system that has uniform physical and chemical characteristics.
    • Phase equilibrium refers to systems where more than one phase may exist, with the system at equilibrium if its free energy is at a minimum under specific temperature, pressure, and composition combinations.

    Phase Diagrams

    • A phase diagram is a graphical representation of phases present and the ranges in composition, temperature, and pressure over which the phases are stable.
    • Phase diagrams are used to predict phase transformations and their products, understand microstructure formation, predict existing phases for selected alloy compositions at desired temperatures, determine the chemical composition of each phase, and calculate the quantity of each phase present.

    Types of Phase Diagrams

    • There are three types of phase diagrams: unary, binary, and ternary.

    Unary Phase Diagram

    • A unary phase diagram shows the phases present in a one-component system.

    Hypoeutectoid Composition: Phase Amount

    • The fraction of proeutectoid ferrite can be calculated using the lever rule.
    • The fraction of pearlite can be calculated by subtracting the fraction of proeutectoid ferrite from 1.
    • The fraction of eutectoid ferrite can be calculated by subtracting the fraction of proeutectoid ferrite from the total ferrite composition.

    Hypereutectoid Composition: Phase Amount

    • The fraction of proeutectoid cementite can be calculated using the lever rule.
    • The fraction of pearlite can be calculated by subtracting the fraction of proeutectoid cementite from 1.
    • The fraction of eutectoid cementite can be calculated by subtracting the fraction of proeutectoid cementite from the total cementite composition.

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