Phase transformation (Material Science)

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Phase TransformationMYO ZIN AUNG28J16121Ship Design Lab. (NAOE)

Phase Transformation - ContentsChange of Crystal Structure (Micro)Shape MemoryTemperature Dependency of Linear Expansion Coefficient (Macro)2

Earthquake Resistant Building Technologies2

Why STUDY Phase Transformation?Tensile strength of iron-carbon alloy of eutectoid composition can be varied between 700 MPa and 2000 MPa depending on heat treatment employed. This shows that the desirable mechanical properties of a material can be obtained as a result of phase transformations using heat treatment processes.The time and temperature dependencies of phase transformations are represented on phase diagrams.It is important to know how to use these phase diagrams in order to design a heat treatment for alloy to obtain the desired room-temperature mechanical properties.3

Phase Diagram for Water4

3 PhasesSolidLiquidVapor

Crystal Structure

5

Face Centered Cubic Crystal Structure (FCC)Body-centered cubic crystal structure (BCC)Hexagonal close-packed crystal structure (HCP)

Atomic Packing Factor6

StructureAPFBCC0.68FCC0.74HCP0.74

3 ClassificationsDiffusion-dependent transformation (Simple)No change in number or composition of the phases presentSolidification of a pure metalAllotropic TransformationsRecrystallization and Grain GrowthDiffusion-dependent transformationSome alternation in phase compositionsOften alternation in the number of phases presentFinal microstructure ordinarily consists of 2 phasesEutectoid reactionDiffusionless transformationMetastable phase is producedMartensitic transformation in some steel alloys7

Polymorphism or Allotropy8

Iron exists in both BCC and FCC form depending on the temperature. Metals exist in more than one crystalline formChange of these forms is called Allotropic Transformation

Phase Diagram of Pure Iron9

3 Solid Phases Fe (BCC) Fe (FCC) Fe (BCC)

Cooling Curve of Pure Iron10

Take times between Phases

White to Gray Tin

11

Body-centered tetragonal Crystal structure similar to diamond The rate at which this change takes place is extremelyslow; however, the lower the temperature (below13.2 C) the faster the rateIncrease in volume (27%), a decrease in density (from 7.30 g/cm3 to 5.77 g/cm3). This volume expansion results in the disintegration of the white tin metal into a coarse powder of the grey allotrope

This white-to-gray-tin transition producedsome rather dramatic results in 1850 in Russia.Thewinter that year was particularly cold, and recordlow temperatures persisted for extended periodsof time. The uniforms of some Russian soldiers hadtin buttons, many of which crumbled because ofthese extreme cold conditions, as did also many ofthe tin church organ pipes. This problem came tobe known as the tin disease.Earthquake Resistant Building Technologies11

How transform?Most phase transformations do not occur instantaneouslyThey begin by the formation of numerous small particles of the new phase(s), which increase in size until the transformation has reached completion2 stages of Phase TransformationNucleationNucleation involves the appearance of very small particles, or nuclei of the new phase which are capable of growing.GrowthDuring the growth stage these nuclei increase in size, which results in the disappearance of some (or all) of the parent phase.12

Nucleation & Growth13

tFor sufficientUndercooling

Iron-Carbon System (Steel)Fe-Fe3C (Iron-Iron Carbide) Phase Diagram

14

Phases of Iron-Carbon Alloys15

Steel is stronger than pure iron because of the carbon atoms in the void space of unit cell.

16-ferriteAustenite (-iron)

Fe-Fe3C (Iron-Iron Carbide) Phase Diagram

176.7 wt% C means 100% Fe3C

Not interested in more than 6.7 wt% C

Mechanically, cementite is very hard and brittle; the strength of some steels is greatly enhanced by its presence.Steel

Eutectoid composition - 0.76 wt% C

Eutectoid temperature 727 CCast IronIronCementite0.008%2.14%6.7%

The ironcarbon alloys that contain between 0.008 and 2.14 wt% C are classified as steels.

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Eutectoid Alloys (0.76 wt% C)

18Pearlite: a micro-constituent consisting of alternating layers of ferrite and cementite.

Nucleation & growth of pearlite19

Hypoeutectoid Alloys (< 0.76 wt% C)

20

Hypereutectoid Alloys (> 0.76 wt% C)

21

Ferrite/Cementite Transformation22

23

Properties of Different Phases of Steel TypeTensileStrength (psi)Hardness(Rockwell)Elongation(2 in.)Ferrite40,000C 0 or B 9040 %softest structure on the diagramsmall amount of carbon dissolved in (BCC) ironFerromagnetic & Fairly ductilePearlite120,000C 20 or B 95-10020 %-Ferrite + CementiteAustenite150,000~ C 4010 %normally not stable at room temperature. But, under certain conditions it is possible to obtain austenite at room temperatureCarbon dissolved in (F.C.C.) ironNon-magnetic & ductileCementite~ 5,000Hardest structure in the diagram and BrittleClassified as ceramic in pure formOrthorhombic Crystal Structure

24

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Microstructures and Mechanical Properties for IronCarbon Alloys

25

How to do Phase Transformations?By varying Temperature, Composition, and the external PressureTemperature Changes by means of Heat Treatments are most conveniently utilizedCrossing a Phase Boundary on the CompositionTemperature phase diagram as an alloy of given composition is heated or cooledMost phase transformations require some finite time to go to completion (to get the equilibrium state) need to wait to finishThe speed or rate is often important in the relationship between the heat treatment and the development of microstructureOne limitation of phase diagrams is their inability to indicate the time period required for the attainment of equilibrium26

Equilibrium vs MetastableThe rate of approach to equilibrium for solid systems is so slow.Equilibrium conditions are maintained only if heating or cooling is carried out at extremely slow and unpractical rates.For other-than-equilibrium cooling, transformations are shifted to lower temperatures than indicated by the phase diagram. (Supercooling)for heating, the shift is to higher temperatures (Superheating)For many technologically important alloys, the preferred state or microstructure is a metastable one (e.g. Martensite)Intermediate between the initial and equilibrium statesIt thus becomes imperative to investigate the influence of time on phase transformations.27

Austenite to Pearlite

28AustenitePearlite

Eutectoid Steel (0.76 wt% C)Eutectoid Temp = 727 C

Isothermal transformation diagram ( TTT Diagram )

29

With superimposed isothermal heat treatment curve (ABCD)

30Shortest time interval for Transformation

31Coarse & Fine Pearlite

Coarse PearliteFine Pearlite

Bainite

32

The microstructure of bainite consists offerrite and cementite phases, and thus diffusional processes are involved in its formation

SpheroiditeIf a steel alloy having either pearlitic or bainitic microstructures is heated to, and left at, a temperature below the eutectoid for a sufficiently long period of timefor example, at about 700C (1300F) for between 18 and 24 hyet another microstructure will form called spheroiditeInstead of the alternating ferrite and cementite lamellae (pearlite) or the microstructure observed for bainite, the Fe3C phase appears as spherelike particles embedded in a continuous aphase matrix.The kinetics of spheroidite formation is not included on isothermal transformation diagrams.33

Spheroidite microstructure

34

MartensiteMartensite is formed when austenite alloys are rapidly cooled (or quenched) to a relatively low temperature (in the vicinity of the ambient).Martensite is a nonequilibrium single-phase structure that results from a diffusionless transformation of austenite.It may be thought of as a transformation product that is competitive with pearlite and bainite.The martensitic transformation occurs when the quenching rate is rapid enough to prevent carbon diffusion.Any diffusion whatsoever results in the formation of ferrite and cementite phases.35

Unit Cell of Martensite

36

Body-centered tetragonal (BCT) Structure

37

FerriteCementite

Ferrite matrix and elongated particles of Fe3CPearlite

Bainite

Diffusion Dependent

Austenite (FCC)Martensite (BCT)Diffusionless TransformationNo enough time to form Pearlite or BainiteVery Hard and BrittleAusteniteVery Rapid Cooling(Quenching)Moderate CoolingSlow CoolingCoolingSuper-saturated solid solution of carbon in ferrite

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Martensite

38

The needleshape grains are the Martensite phase, and the white regions are austenite that failed to transform during the rapid quench

Cooling Rate

39Continuous-cooling transformation diagram for a eutectoid ironcarbon alloy and superimposed cooling curves, demonstrating the dependence of the final microstructure on the transformations that occur during cooling

Tempered MartensiteIn the as-quenched state, martensite, is very hard, but so brittle So it cannot be used for most applicationsAny internal stresses that may have been introduced during quenching have a weakening effect.The ductility and toughness of martensite may be enhanced and these internal stresses relieved by a heat treatment known as tempering.By heating to a temperature below the eutectoid for a