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    UNIT 6 (PHASE &PHASE

    TRANSFORMATIONS)

    What is a Phase?A phase is a homogenous, physically distinct and mechanically

    separable portion of the material with a given chemical composition and structure.

    Chemical Composition of Phases: It is the chemical composition of each phase in

    the system In a system having more than one phase, each phase will have a unique

    chemicalcomposition which will be different from each other, and will also be

    different fromthe overall composition Not to be confused with overall

    composition

    Limitations to use of Phase Diagrams

    Phase Diagrams are also known as Equilibrium Diagrams Rate of Transformation is missing

    TTT (Time-Temperature- Transformation) diagrams are a complement to Phase Diagrams

    What is a solid solution?

    When foreign atoms are incorporated into a crystal structure, Whether in

    substitutional or interstitial sites, the resulting phase is a solid solution of the

    matrix material (solvent) and the foreign atoms (solute)

    Substitutional Solid Solution: Foreign (solute) atoms occupy normal lattice sites

    occupied by matrix (solvent) atoms,

    E.g. Cu-Ni; Ge-Si

    Interstitial Solid Solutions: Foreign (solute) atoms occupy interstitial sites, e.g.,

    Fe-C

    Phase Transformations:

    Phase transformations: change in the number or character of phases.

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    Most phase transformations begin with the formation of numerous small particles of

    the new phase that increase in size until the transformation is complete

    For Iron transformation to occur, must cool to below 727C

    IRON CABIDE DIAGRAM

    Cooling Curve of Pure Iron

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    TTT(TIME, TEMPERATURE AND TRANSFORMATION) DIAGRAM):

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    Definition and Properties of Structures

    Ferrite () An interstitial solid solution of carbon dissolved in BCC iron

    Iron Iron Carbide Phase Diagram

    -iron.

    Carbon solubility 0.025 wt.% max. at 723 C, 0.008 wt.% min. at 0 C.

    The softest structure that appears on the iron iron carbide diagram.

    Average properties: 40,000 psi TS, 40 % elong. in 2 inch, < RC 0 or < RB 90 hardness.

    Austenite ()

    An interstitial solid solution of carbon dissolved in FCC -iron.

    Carbon solubility 2.00 wt.% max. at 1130 C, 0.80 wt.% min. at 723 C.

    Not stable at room temperature; can be made stable under certain conditions.

    Average properties: 150,000 psi TS, 10 % elong. in 2 inch, RC 40 hardness,

    high toughness.

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    Cementite (Fe3C)

    An interstitial inter metallic compound of iron carbide with an orthorhombic structure.

    Its chemical formula is Fe C and contains 6 67 wt % carbon

    Iron Iron Carbide Phase Diagram

    Definition and Properties of Structures

    Pearlite

    The eutectoid mixture of fine plate-like lamellar mixture of ferrite and cementite.

    Formed from austenite that contains 0.80 wt.% carbon during slow cooling at 723 C.

    Average properties: 120 000 psi TS 20 % elong in 2 inch RC 20 hardness

    Fe3C 6.67 wt.% carbon.

    The hardest and brittle structure that appears on the iron iron carbide diagram.

    Average properties: 5,000 psi TS, high compressive strength.

    Ladeburite

    The eutectic mixture of austenite and cementite.

    Formed from liquid that contains 4.30 wt.% carbon during slow cooling at 1130 C.

    Not stable below 723 C, where austenite of ladeburite transformed into pearlite.

    The structure is then called transformed ladeburite.

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    The IronIron Carbide (FeFe3C) Phase DiagramIn their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The Fe-C phase

    diagram is a fairly complex one, but we will only consider the steel part of the diagram,

    Up to around 7% Carbon.

    Phases in FeFe3C Phase Diagram-ferrite - solid solution of C in BCC Fe

    Stable form of iron at room temperature.

    The maximum solubility of C is 0.022 wt%

    Transforms to FCC -austenite at 912 C

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    -austenite - solid solution of C in FCC Fe

    The maximum solubility of C is 2.14 wt %.

    Transforms to BCC -ferrite at 1395 C

    Is not stable below the eutectoid temperature

    (727 C) unless cooled rapidly

    -ferrite solid solution of C in BCC Fe

    The same structure as -ferrite

    Stable only at high T, above 1394 C

    Melts at 1538 C

    Fe3C (iron carbide or cementite)

    This inter metallic compound is Meta stable, it Remains as a compound

    indefinitely at room T, but decomposes (very slowly, within several years)

    Into -Fe and C (graphite) at 650 - 700 C

    Fe-C liquid solution

    A few comments on FeFe3C system

    C is an interstitial impurity in Fe. It forms a solid solution with , , phases of

    iron Maximum solubility in BCC -ferrite is limited (max.

    0.022 wt% at 727 C) - BCC has relatively small interstitial positions

    Maximum solubility in FCC austenite is 2.14 wt% at 1147 C - FCC has larger

    interstitial positions

    Mechanical properties: Cementite is very hard and brittle -can strengthen steels.

    Mechanical properties also depend on the microstructure, that is, how ferrite and

    cementite are mixed.

    Magnetic properties: -ferrite is magnetic below 768 C, austenite is non-magnetic

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