phase trans#2

8/4/2019 Phase Trans#2

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IE 114-Material Science andGeneral Chemistry

1

Phase Transformations

Chapter 11

Callister, 2001

5th Edition


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Why do we study phase transformations?The tensile strength of an Fe-C alloy of eutectoid composition can be

varied between 700-2000 MPa depending on the heat treatmentprocess adopted. This shows that the desirable mechanicalproperties of a material can be obtained as a result of phasetransformations using the right heat treatment process. In order todesign a heat treatment for some alloy with desired RT properties,

time and temperature dependencies of some phasetransformations can be represented on modified phase diagrams.

Based on this, we will learn:

a) Phase transformations in metals

b) Microstructure and property dependence in Fe-C alloy system

c) Precipitation HardeningCrystallization, Melting, and GlassTransition


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Phase Transformation in MetalsDevelopment of microstructure in both single- and two-phase alloys

involves phase transformations-which involves the alteration inthe number and character of the phases. Phase transformationstake time and this allows the definition of transformation rate orkinetics.

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

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

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

c) Diffusionless transformations (martensitic transformation in steelalloys)


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Kinetics of Solid State Reactions:Transformations (formation of a newphase with a different composition andstructure) involving diffusion dependon time (Chapter 6). Time is alsonecessary for the energy increase

associated with the phase boundariesbetween parent and productphases.Moreover, nucleation, growthof the nuclei, formation of grains andgrain boundaries and establishment ofequilibrium take time. As a result wecan say the transformation rate

(progress of the transformation) is afunction of time.In kinetic investigations, the fraction of

reaction completed is measured as afunction of time at constant T.Tranformation progress can bemeasured by microscopic examination

or measuring a physical property (e.g.,conductivity). The obtained data isplotted as fraction of the transformationversus logarithm of time.

k and n are time independent constants.


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Temperature controls the kinetics of the transformations. For therecrystallization of Cu:

For a specific temperature range, rate increases according to :


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Multiphase transformations: Phase transformations may beaccomplished by varying temperature, composition and externalpressure. Most of the phase transformations require some finite timeto go to completion and the rate of transformation is important in the

relationship between heat treatment and microstructuredevelopment.

The rate of transformation to achieve the equilibrium state is very slowand equilibrium conditions are maintained if the heating/cooling isreally slow. This is usually unpractical. In general, transformationsare shifted to lower or higher temperatures for cooling and heatingrespectively. These phenomena are termed supercooling andsuperheating respectively. The more rapid the cooling or heating,the greater the supercooling or superheating. For example, Fe-Ceutectoid reaction is typically displaced 10-20C lower than theequilibrium transformation temperature.

For many alloys, the preferred state is metastable state(intermediate between initial and eqm . States )


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7

Microstructural and Property Changes in Fe-C Alloys

1) Isothermal Transformation Diagrams:

Pearlite is a microstructural product of the transformation of

Temperature is important in this transformation:

Each curve is obtained by rapidly cooling the austenite to the temperatureindicated.


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The dependance of transformation to temperature and time can be analyzedbest using the diagram below:

isothermal transformation diagramfor Fe-C alloy of eutectoid composition

Data for the construction of isothermaltransformation diagram is obtained froma series of plots of the percentagetransformation versus logarithm of timeinvestigated over a range of temperatures.

727CAt T just below 727C, very long times(on the order of 105 s) are required for 50%

transformation and therefore transformationrate is slow. The transformation rateincreases as T decreases, for example,at 540C 3 s is required for 50% completion.

This type of diagram is valid for constant T.


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This observation is in clear contradiction with the equation of

This is because in T range of 540C-727C, the transformation rate is mainlycontrolled by the rate of pearlite nucleation and nucleation rate decreases withT increase. Q in this equation is the activation energy for nucleation and it increaseswith T increase. It has been found that at lower T, the austenite decompositionis diffusion controlled and the rate behavior can be calculated using Q for diffusion

which is independent of T.

Isothermal phase diagrams are also called time-temperature-transformation (T-T-T) plots.


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An actual isothermal heat treatment curve on the isothermaltransformation diagram:

rapid cooling

isothermal treatment

the thickness ofthe ferrite tocementite layersis about 8 to 1.


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The layer thickness depends on temperature at which the isothermaltransformation occurs. For example at T just below the eutectoid,relatively thick layers of both ferrite and cementite phases are

produced. This structure is called coarse pearlite. At lower T,diffusion rates are slower, which causes formation of thinner layersat the vicinity of 5400C. This structure is called fine pearlite.


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For Fe-C alloys of other compositions, a proeutectoid phase of ferrite orcementite will coexist with pearlite and therefore the isothermaltransformation diagram has additional curves:


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Bainite: is another microstructure formed as a result of transformationof austenite. Bainite consists of ferrite and cementite and diffusionprocesses take place as a result. This structure looks like needles or

plates. There is no proeutectoid phase in bainite.

nose5400C

2150C


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Pearlitic and bainitic transformations are competitive andtransformation from to another requires reheating. The kinetics ofbainite formation follows the Equation relating the rate to

temperature.

If steel alloy with pearlitic or bainitic structure is heated to and left at atemperature below the eutectoid temperature (such as 7000C) for 18to 24 hours, another microstructure, called spheroidite, forms.

cementite

ferrite


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Another microstructure is formed when austenite is rapidly cooled orquenched to a relatively low temperature (ambient T) calledmartensite. Martensite is a single phase nonequilibrium structure

and produced as a result of diffusionless transformation of austenite.The quenching rate should be very high to prevent carbon diffusion.

FCC austenite undergoes a polymorphic transformation to a bodycentered tetragonal (BCT) martensite.

carbon

Steels can maintain their martensitic structure indefinetely at RT. Since martensitictransformation does not involve diffusion, it is almost instantaneous. Therefore its

transformation is independent of time.

Fe


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Since martensite is in a nonequilibrium phase, it does not appear on the phasediagram of Fe-Fe3C.


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The austenite to martensite transformation is shown in the isothermaltransformation diagram:

Temperatures of these transformationschange with the composition of alloy andtransformation to martensite only depends

on T not time. This type of transformationis called athermal transformation.


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Steels in which C is the prime alloying element are called plain carbonsteels, whereas alloy steels containg other elements, such as, Cr,Ni, Mo, W, etc.

In the presence of other elements the isothermal transformaiondiagrams may be different:

1. shifting to longer times thenose of austenite to pearlite

transformation(a proeutectoid phase nose ifit exists)

2. formation of separate bainitenose

Remarks about the diagram:


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Mechanical Behavior of Fe-C Alloys:

Pearlite: Cementite is much harder but more brittle than ferrite.Therefore increasing the fraction of Fe3C will make the resulting

material harder and stronger. Since Fe3C is brittle, increasing itscontent decreases ductility of the material.


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The layer thickness is also important for the mechanical behavior of thematerial. Fine pearlite is harder and stronger than coarse pearlite.Coarse pearlite is more ductile than fine pearlite because of greaterrestriction to plastic deformation of the fine pearlite.


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IE 114-Material Science andGeneral Chemistry 21

There is a large adherence between the two phases across a boundary of andFe3C. The strong and rigid cementite layers restricts the deformation of soft

ferrite layers and as the phase boundary area increases per unit volume of

the material, the degree of reinforcement is higher. In addition phase

boundaries act like barriers for dislocation motions. This is why fine

pearlite has a greater strength and hardness.

Spheroidite: has lower strength and hardness than pearlitic microstructures.

This is becuase of the smaller phase boundary area in spheroiditic

microstructures. Of all the steel alloys, those that are softest and weakest

have a spheroidite microstructure. The spheroidized steels have higher

ductility than coarse pearlite.


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Bainite: Bainitic steels have a finer structure and therefore they are stronger

and harder than pearlitic ones. They have a good combination of strength

and ductility

Martensite:The hardest and strongest and the most brittle form of the steel alloy.It has a negligibly small ductility. Its hardness is controlled by C content up to0.6 wt% rather than its microstructure. These properties are the results of effectivenessof the interstitial C atoms in hindering dislocation motion and existence of few slipsystems for BCT structure.


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Tempered Martensite: Martensite is hard but also very brittle so that itcan not be used in most of the applications. Any internal stress thathas been introduced during quenching has a weakining effect. The

ductility and toughness of the material can be enhanced by heattreatment called as tempering. This also helps to release anyinternal stress.

Tempering is performed by heating martensite to a T below eutectoid temperature(2500C-6500C) and keeping at that T for specified period of time. The formationof tempered martensite is by diffusional processes.

very small cementite particles dispersedhomogeneously.

ferrite matrix


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Tempered martensite may be nearly as hard and strong as martensite,but with substantially enhanced ductility and toughness. Thehardness and strength may be due to large area of phase boundaryper unit volume of the material. The phase boundary acts like abarrier for dislocaitons. The continuous ferrite phase in temperedmartensite adds ductility and toughness to the material.

The size of the cementite particles is important factor determining themechanical behavior.

As the cementite particle size increases, material becomes softer andweaker. The temperature of tempering determines the cementiteparticle size. Since martensite-tempered martensite transformationinvolves diffusion, T increase will accelerate the diffusion and rate ofcementite particle growth and rate of softening as a result.

Heat treatment of martensite has two variables:

Temperature and time.


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This type of data is usually providedby the steel manufacturer. For this data,martensite is quenched in oil and temperingtime is 1 hour.

This data is obtained for water quenchedeutectoid composition. As tempering timeincreases the hardness decreases.Overtempered martensite is relatively softand ductile.


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Tempering of some steels may result in decrease in toughness and thisis called temper embrittlement. These are the alloys with highconcentrations of alloying elements, P, As, Ni, Cr, Sb and Sn. Thepresence of alloying elements increases the T at which theductile-to-brittle transition occurs.

This has been observed when steel is tempered at a temperatureabove about 5750C followed by slow cooling to RT or whentempering is carried out b/w 375-5750C.

Crack propagation in this type of materials is intergranular, that is thefracture path follows the grain boundaries of the precursoraustenite phase.

We can avoid the temper embrittlement by

1) compositional control

2) tempering above 5750C or below 3750C followed by quenching toRT.


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Summary of the Phase Transformations for Fe-C Alloy System


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Precipitation Hardening: is the enhancement of the strength andhardness by forming extremely small uniformly dispersed particles ofa second phase within the original phase matrix. This can be doneby phase tranformations at appropriate temperatures.

Small particles in the new phase are called precipitates.

For example: Al-Cu, Cu-Be, Cu-Sn, Mg-Al and some ferrous alloys areprecitation hardenable.

Heat Treatments-Precipitation hardenable alloys usually contain two or

more alloying elements. The simplest system is binary system: A-Bsystem.


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There are conditions for the precipitation hardening to be applied:

a) considerably high solubility of the elements in each other

b) solubility limit should rapidly decrease in concentration of themajor element as T decreases.

c) The composition of a preipitation hardenable alloy must be lessthan the maximum solubility.

There are two different types of heat treatment:

1) Solution heat treating: all solute atoms are dissolved to form a singlephase solid solution.

Consider the alloy with C0 composition, which is heated to T0 -andwaiting all phase to dissolve completely. At this point there is only with C0 composition. Then it is quenched to T1, which is usually RT so

that any diffusion and formation of phase is prevented. The resultingmaterial is phase solid solution supersaturated with B atoms at T1.


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Mechanism of Hardening: Precipitation hardening is commonly used inAl alloys with high strength. For example Al-Cu alloys.

phase is a substitutional

solid solution of Cu in Al phase is intermetallic

compound, CuAl2

During the initial hardening stage Cu atoms cluster together in small discs (fewatoms thick and about 25 atoms in diameter) at countless positions within phase. Thediscs are very small that they are not considered as particles. With time and diffusion of Cu

atoms, the discs become particles and increase in size. The particles undergoes two transition

phases ( and ) before the formation of equilibrium phase.


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The strengthening process is improved by T increase.


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Crystallization, Melting, and Glass Transition in Polymers:

Crystallization: is the production of an ordered solid phase upon coolingfrom a liquid melt having no crystalline structure. The degree ofcrystallization controls the mechanical properties of the polymers.The crystallization of polymers follows the same steps as: nucleationand growth processes.

Time dependence of crystallization can be described by

(Avrami equation)

Since 100 % crystallinityof the polymer is not possible,the vertical axis is scaled asnormalized fraction crystallized.

Fraction 1.0 is the highest level

of crystallization.


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Melting: is the opposite of crystallization and occurs at meltingtemperature. Melting of a polymer takes place over a range oftemperature and behavior of the polymer during melting depends onthe history of the material, such as the temperature of crystallization,

rate of heating.

Glass transition: This occurs in amorphous and semicrystallinepolymers. As a result of glass transition, motions of molecular chainsare restricted at lower temperatures. As T is decreased, a gradualtransformation is observed from a liquid to a rubbery material and

finally a rigid solid. The temperature of transition from rubbery torigid solid phase is called glass transition temperature (Tg).Stiffness, heat capacity and thermal expansion coefficient are themajor properties changed during this transition.

Melting and glass transition temperatures are important since they

define the upper and lower temperature limits of applications.


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The temperature of melting and glass transition can be determined from aplot of specific volume (reciprocal of the density) versus temperature.


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IE 114 Material Science and 38

Factors influencing melting temperature:

Molecular chemistry and structure, chain stiffness (double bonds and aromaticgroups lowers the flexibility), size and type of the side groups, molecularweight, degree of branching.

Factors affecting glass transition temperature:Molecular characteristics controlling the chain flexibility or stiffness control Tg

to some degree. As chain flexibility is diminished glass transitiontemperature increases.

Molecular weight also affects Tg:

phase trans#2

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