PHASE TRANSFORMATIONS in METALS and ALLOYS Phase Transformation in Metals Development of...

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Transcript of PHASE TRANSFORMATIONS in METALS and ALLOYS Phase Transformation in Metals Development of...

  • PHASE

    TRANSFORMATIONS

    in METALS and ALLOYS

    IE-114 Materials Science and General Chemistry

    Lecture-12-13

  • Why do we study phase

    transformations?

    The desirable mechanical properties of a material can be obtained

    as a result of phase transformations using the right heat treatment

    process.

    In order to design a heat treatment for some alloy with desired RT

    properties, time and temperature dependencies of some phase

    transformations can be represented on modified phase diagrams.

  • Phase Transformation in Metals

     Development of microstructure in both single- and two-phase alloys involves phase transformations-which involves the alteration in the number and character of the phases. Phase transformations take time and this allows the definition of transformation rate or kinetics.

    Phase transformations alter the microstructure and there can be three different classes of phase transformations:

    a) Diffusion dependent transformations with no change in number of phases and composition ( solidification of a pure metal, allotropic transformations, etc.)

    b) Diffusion dependent transformations with change in number of phases and composition (eutectoid reaction)

    c) Diffusionless transformations (martensitic transformation in steel alloys)

  • Steel is heat treated in order to:

     Increase/decrease strength (Hardening, normalizing, annealing)

     Reduce internal stresses (stress relief annealing)

     Adjust grain size (normalizing)

     Remove the effects of cold working (normalizing)

     Improve machinability (annealing)

     Avoid microsegregation (homogenizing)

    All of the heat treatment operations conducted on steels based on the

    heating of the material to some temperature to form fully austenite and

    cooling the material to low temperatures at different rates (formation

    of different phases depending on the cooling rate applied)

    HEAT TREATMENT: A combination of heating and cooling

    operations, timed and applied to a metal or alloy in the solid state in

    a way that will produce desired properties (physical and sometimes

    chemical properties)

  • 1) Continuous cooling ( very slow, moderate or fast cooling)

    2) Interrupted cooling (very fast cooling to a temperature(undercooling) and wait at that temperature long enough for transformation of austenite to take place, then cooling to

    low temperatures (isothermal transformation), e.g.room temperature)

    Heat treatment of steels

    1.Step: Austenitization (heat treatment to obtain ~100% austenite phase)

    Cooling;

    2.Step:Cooling to low temperatures at different rates

    We have to decide the lowest possible temp. in the -region

    because of grain coarsening (25oC higher than the -trans. temp.)

     For each 1 inch thickness the austenitization time is 45 mins.

  • Very slow continuous cooling of eutectoid steel

    (under equilibrium conditions)

    When the carbon content of steel and the temperature are known fraction and

    composition of phases can be predicted using Fe-Fe3C diagram.

    However, the total transformation time is not known. Transformation time is

    missing in Fe-Fe3C diagram

    Upon very slow cooling, transformation of austenite to pearlite occurs by diffusion

    of carbon atoms(time is required for carbon diffusion). So, this type of

    transformation is called DIFFUSIONAL(Time Dependent) TRANSFORMATION.

  • Fe-Fe3C diagrams are equilibrium phase diagrams and they don’t give

    information about non-equilibrium cooling conditions

    Cooling rate is so fast that carbon atoms cannot find enough

    time to go and locate their equilibrium positions

    DIFFUSIONLESS (e.g.martensitic) TRANSFORMATIONS

    Under non-equilibrium conditions TTT-diagrams are used to investigate the

    transformation fraction, temperatures and time.

    TTT(Time-Temperature-Transformation)-diagrams

    IT-diagrams (IsothermalTransformation diagrams)

    CT-diagrams (Continuous Cooling Transformation diagrams)

    (used in interrupted cooling conditions) (used in continuous cooling conditions)

  • Fraction transformed

    Phase Transformation involves:

    1) Nucleation (the formation of very small particles of the new phase at

    some imperfection sites, e.g.,grain boundaries

    2) Growth (nuclei increase in size)

    Transformation rate:

    Fraction of transformation is measured at constant temperatures as a function of time by

    either microscopic examination or measurement of some physical property such as

    electrical conductivity.Then, the fraction of transformed material versus the logarithm of

    time graphs are drawn.

    AVRAMI EQUATION

    y: fraction of transformation

    t : time

    k,n : time independent constants

    Rate of transformation,r : Reciprocal of time required for 50% transformation

    Rate,r : 1/t50%

  • • Reaction rate increases with T.

    TRANSFORMATION RATE ~ UNDERCOOLING( T)

    As undercooling is increased finer microstructures are formed

    increasing T

  • ISOTHERMAL TRANSFORMATION(IT) DIAGRAMS

    • Fe-C system, Co = 0.77wt%C (Eutectoid steel)

    • Transformation at T = 675oC.

    logt

  • IT-diagram of an eutectoid steel

  • Proeutectoid ferrite

    Proeutectoid cementite

    Coarse Pearlite

    Fine Pearlite

    Upper Bainite

    Lower Bainite

    Martensite

    Depending on the alloy composition, amount of undercooling and

    isothermal transformation time, one can obtain one of the following

    phases or combination of them;

    IT diagram of a hypoeutectoid steel

  • EUTECTOID STEEL (0.77 wt.%C)

    IT -DIAGRAMS

    The position of IT diagrams

    Two factors will change the position of

    the curves;

    1) Chemical composition

    2) Austenitic grain size

    With few exceptions, an increase in

    carbon or alloy content or in grain size

    of the austenite always retards

    transformation (transformation lines shift

    to longer times)

  • • Eutectoid steel, Co = 0.77wt%C

    • What happens if an eutectoid steel is austenitized at T > 727oC then it is

    rapidly cooled to 625oC and hold isothermally for about 100 seconds;

    PEARLITE TRANSFORMATION in eutectoid steel

  • FINE and COARSE PEARLITE (Eutectoid steel)

    - Smaller T:

    colonies are larger

    - Larger T:

    colonies are smaller

    • Ttransf just below TE --Larger T: diffusion is faster

    --Pearlite is coarser.

    • Ttransf well below TE --Smaller T: diffusion is slower

    --Pearlite is finer.

    Interlamellar distance

    is very close

  • Comparison of mechanical properties of

    fine and coarse pearlite

  • BAINITE PHASE (Phase mixture of and Fe 3 C)

     Austenite transforms to -lathes (strips) and rods of Fe3C isothermally between

    the nose region and Ms temperature

     Bainite is a phase mixture of and Fe3C

    Transformation is a diffusion controlled process

     ***For plain carbon steels bainite is only formed by isothermal

    transformation***

    Schematic IT diagram for eutectoid steel

  • Upper bainite

    Lower

    bainite

    Upper(feathery) and Lower(Needlelike) Bainite:

    HARDNESS Lower bainite > upper bainite > fine pearlite > medium pearlite > coarse pearlite

    UPPER BAINITE

    LOWER BAINITE

    Upper bainite

  •  Transformation of (FCC) to Martensite (BCT, body centered tetragonal)

     Transformation is rapid!(shear transformation)

     % transformation depends on temperature only.

    M a rte

    n tite

    n e e d le

    s A

    u s te

    n ite

    60 m EUTECTOID STEEL (Quenching from Austenitization temp.)

    MARTENSITE

  • MARTENSITE

    Avrami-type Every steel has a specific TTT-

    diagram and on which its Ms and Mf are indicated

     Both Ms and Mf decrease as carbon content increases

  • x x x

    x

    x

    x potential C atom sites

    Fe atom sites

    (involves single atom jumps)

    Expansion c

    a

    EXPANSION occurs because atoms of martensite are less densely

    packed than that of austenite. This expansion during the formation of

    martensite produces high localized stress which result in the plastic

    deformation of the matrix.

    The degree of expansion depends on carbon content

    • HARDNESS INCREASES DUE TO HIGHLY DISTORTED LATTICE

    MARTENSITE

  • Altough martensite is very hard unfortunately it is very BRITTLE for industrial use. In

    order to toughen the steel and make it more ductile, a heat treatment called

    tempering is applied.

    Tempering is applied by reheating a martensitic steel to a temperature below

    eutectoid temperature and cooling it at any rate to increase the ductility and

    toughness.

    TEMPERING

    1) Conventional Quenching and Tempering

  • As-q