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These are the Lecture Slides of Science and Engineering of Materials which includes Point Defects, Types of Defects, Equilibrium Number, Thermal Vibrations, Boltzmann Constant, Regular Lattice Sites, Substitutional Solid Solutions, Composition Conversions etc. Key important points are: Total Amount of Phase, Intermediate Phases, Solid Solution Phases, Intermetallic Compounds, Eutectoid Reactions, Peritectic Reactions, Congruent Phase Transformations, Iron–Iron Carbide
Typology: Slides
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Fraction of α phase determined by application of the lever rule across the entire α + β phase field:
Wα = (Q+R) / (P+Q+R) (α phase)
Wβ = P / (P+Q+R) (β phase)
Cu-Zn: α and η are terminal solid solutions, β, β’, γ, δ, ε are intermediate solid solutions.
Eutectoid Reactions (I)
Eutectoid
Cu-Zn
Eutectoid Reactions (II)
Congruent Phase Transformations
(e.g, allotropic transformation such as α-Fe to γ-Fe or melting transitions in pure solids)
Incongruent transformation, at least one phase changes composition (eutectic, eutectoid, peritectic).
Congruent melting of γ
Ni-Ti
The Iron–Iron Carbide (Fe–Fe 3 C) Phase Diagram
Steels: alloys of Iron (Fe) and Carbon (C).
Fe-C phase diagram is complex. Will only consider the steel part of the diagram, up to around 7% Carbon.
Comments on Fe–Fe 3 C system
C is an interstitial impurity in Fe. It forms a solid solution with α, γ, δ phases of iron
Maximum solubility in BCC α-ferrite is 0.022 wt% at727 °C. BCC:relatively small interstitial positions
Maximum solubility in FCC austenite is 2.14 wt% at 1147 °C - FCC has larger interstitial positions
Mechanical properties: Cementite (Fe 3 C is hard and brittle: strengthens steels. Mechanical properties also depend on microstructure: how ferrite and cementite are mixed.
Magnetic properties: α -ferrite is magnetic below 768 °C, austenite is non-magnetic
Classification. Three types of ferrous alloys:
Iron: < 0.008 wt % C in α−ferrite at room T
Steels: 0.008 - 2.14 wt % C (usually < 1 wt % ) α-ferrite + Fe 3 C at room T (Chapter 12)
Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)
Eutectic and eutectoid reactions in Fe–Fe 3 C
Eutectoid: 0.76 wt%C, 727 °C γ(0.76 wt% C) ↔ α (0.022 wt% C) + Fe 3 C
Eutectic: 4.30 wt% C, 1147 °C L ↔ γ + Fe 3 C
Eutectic and Eutectoid reactions are important in heat treatment of steels Docsity.com
Pearlite, layered structure of two phases: α-ferrite
and cementite (Fe 3 C)
Alloy of eutectoid composition (0.76 wt % C)
Layers formed for same reason as in eutectic:
Atomic diffusion of C atoms between ferrite (0.
wt%) and cementite (6.7 wt%)
Mechanically, properties intermediate to
soft, ductile ferrite and hard, brittle cementite.
Microstructure of eutectoid steel (II)
In the micrograph, the dark areas are Fe 3 C layers, the light phase is α-ferrite
Compositions to the left of eutectoid (0.022 - 0.
wt % C) hypoeutectoid ( less than eutectoid -Greek)
alloys.
γ → α + γ → α + Fe 3 C
Compositions to right of eutectoid (0.76 - 2.14 wt
% C) hypereutectoid ( more than eutectoid -Greek)
alloys. γ → γ + Fe 3 C → α + Fe 3 C
Microstructure of hypereutectoid steel (I)
Hypereutectoid contains proeutectoid cementite
(formed above eutectoid temperature) plus perlite
that contains eutectoid ferrite and cementite.
Microstructure of hypereutectoid steel (II)
Example: hypereutectoid alloy, composition C (^1)
Fraction of pearlite:
WP = X / (V+X) = (6.7 – C 1 ) / (6.7 – 0.76)
Fraction of proeutectoid cementite:
WFe3C = V / (V+X) = (C 1 – 0.76) / (6.7 – 0.76)
Summary
Austenite Cementite Component Congruent transformation Equilibrium Eutectic phase Eutectic reaction Eutectic structure Eutectoid reaction Ferrite Hypereutectoid alloy Hypoeutectoid alloy Intermediate solid solution Intermetallic compound Invariant point Isomorphous Lever rule Liquidus line Metastable
Make sure you understand language and concepts:
Microconstituent Pearlite Peritectic reaction Phase Phase diagram Phase equilibrium Primary phase Proeutectoid cementite Proeutectoid ferrite Solidus line Solubility limit Solvus line System Terminal solid solution Tie line
