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- Interphase Migration- Classification of phase

Transformation-Decomposition of Austenite

Lecture Series –B-1

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Materials

Science

Model

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Interphase Migration

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Interphase Migration• The great majority of phase transformations in metals and alloys occur by a

process known as nucleation and growth of the new phase β, nucleate and subsequently grow into surrounding matrix of metastable parent α phase.

• At any time during the transformation the system can be divided into parent and product phases.

• In other words, an interface is created during the nucleation stage and then migrates into the surrounding parent phase during the growth stage.

• There are basically two different types of interface: glissile and non-glissile.

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• Glissile interface migrates by dislocation glide that results in the shearing of the parent lattice into the product.

• The migration of glissile interface produces a macroscopic shape change in the crystal.

• The motion of glissile interfaces is relatively insensitive to temperature and is therefore known as athermal migration.

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• Non-Glissile interface: Most interfaces are non-glissile and migrate by the random jumps of individual atom/atoms across the interface.

• The migration of non-glissile interface cannot produce a change in shape in the parent crystal.

• The extra energy that the atom needs to break free of one phase and attach itself to the other is supplied by thermal activation.

• The migration of non-glissile interface is therefore, diffusion controlled and extremely sensitive to temperature.

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Classification of Phase

Transformations

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Classification of Phase Transformations• The nucleation and growth transformations can be classified into two

groups:

1. the phases that grow by the movement of glissile interface .

2. the phases that grow by the movement of non-glissile interfaces.

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• Transformations produced by the migration of a glissile interface are referred to as military transformations i.e., the coordinated motion of atoms crossing the interface just like soldiers moving in ranks on the parade ground.

• During a military transformation the nearest neighbours of any atom are essentially unchanged.

• Therefore, the parent and product phases must have the same composition and no diffusion is involved in the transformation.

• Martensite and twin formation in steels and other alloy systems occur by the motion of glissile interfaces.

• Since there is no change in composition, the new phase will be able to grow as fast as the atoms can move across the interface. Such transformations are interface controlled.

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• In contrast the uncoordinated transfer of atoms across a non-glissile interface results in what is known as a civilian transformation.

• During civilian transformations the parent and product may or may not have the same composition.

• When the parent and product phases have different compositions, growth of the new phase will require long-range diffusion and is dependant upon diffusion rate is called diffusion controlled.

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• Classification of nucleation and growth transformations according to interface migration process is summarized in Table 1.

• Table 1: Classification of Phase Transformations (Adapted from J.W. Christian, 'Phase transformations in metals and alloys -An introduction', in Phase Transformations, Vol. 1, p. 1, Institute of Metallurgists, 1979.)

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Type Military CivilianEffect of

temperature change

Athermal Thermally activated

Interface type Glissile(coherent or

semicoherent)

Non-Glissile(coherent, semicoherenl, incoherent, solid/liquid or solid/vapour)

Composition of parent and

product phases

Same composition

Same composition Different compositions

Nature of diffusion processes

No diffusion Short-range diffusion (across

interface)

Long-range diffusion(through lattice)

Interface, diffusion or

mixed control?

Interface control Interface control Mainly interface control

Mainly diffusion control

Mixed control

Examples Martensite Twinning

Symmetric tilt boundary

Massive Ordering Polymorphic

Recrystallization Grain growth Condensation

Precipitation/ Dissolution

Bainite Condensation Evaporation

Precipitation/ Dissolution… Solidification and melting

Precipitation/ Dissolution Eutectoid

Cellular precipitation

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• While many transformations can be classified into the above system, but there are transformations, where difficulty arises.

• For example, 1.Widmanstätten ferrite, 2.Upper bainite 3.Lower bainite and 4.acicular ferrite

• The above listed transformations take place by thermally activated growth(non-gillissile), but it also produces a shape change similar to produced by the motion of a gillissile interface.

• These transformations can conveniently be classed as Displacive transformations as explained in Table 2.

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IPS: Invariant-Plane Strain-If the operation of strain, leaves one plane of the parent crystal completely un-rotated and undistorted, this is call IPS.

/no (Old Ref:)

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Austenite with two different kinds of atoms. It can transform into new/different crystal structure by two methods.1. Displacive Transformation2. Reconstructive Transformation

Displacive transformation involves a Homogeneous deformation of the crystal structure into a new shape. important characteristic: we get a macroscopic shape change which is in the form of IPS with a large shear component and there is atomic correspondence b/w the product and parent Phase

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Military (Displacive) Transformation: Atoms (large and small) moves in a discipline manner.

Civilian (Reconstructive) Transformation: Atoms (large and small) does not move in a discipline manner

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Para-Military Transformation:• Small atoms diffused but

large atoms are displaced during transformation. • So the change in crystal

structure is achieved by displacive mechanism but small atom(like Carbon) are able to partition b/w the parent and the product phase during transformation.• (e.g. bainite, Widmanstatten

and Acicular Ferrite .

Large atom moves in disciplined manner but small atoms go

and occupy wherever they like to

occupy.

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• Fig. 1.6 Summary of the variety of phases generated by the decomposition of austenite.• Ref: Steels

Bhadeshia

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• Fig: Temperature composition regions in which the various morphologies are dominant in specimens with ASTM grain size Nos. 0-1. • GBA = grain boundary allot

riomorphs, • W = Widmanstatten

sideplates and/or intragranular plates, • M = Massive ferrite,

Introduction:

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Introduction:

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Introduction:

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log{time}

Tem

pera

ture

/ °C

Ms

ferritepearlite

bainite

martensite

Ae3

upper bainite

lower bainite

Widmanstatten ferriteBs

Ws

Introduction:

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Decomposition of Austenite

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• Allotriomorphic Ferrite• Idiomorphic Ferrite• Massive Ferrite.• Pearlite

Reconstructive decomposition of Austenite

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Allotriomorphic Ferrite (AF) OR

Grain boundary Allotriomorphs, GBA

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Definition and Characteristics:• Ferrite forms at the austenitic Grain boundary,

because austenitic grain boundary having easier diffusion path. It tends to grow (easy grow) more rapidly along the γ grain boundary, not within the grain. OR

• The allotriomorphic ferrite grains nucleate at the highest temperatures, i.e. just below Ae3 or typically above (>) 600 where Fe-atom are mobile.

• Because, Austenitic grain boundary is the easiest nucleation site in steel.

• The shape of AF does not reflect its internal crystalline symmetry. (we don’t get nice beautiful shape with straight faceted, because it is dominated/controlled by γ -grain boundary)

• Note: Faceting means, develop some planes with particular crystallography.

• There is no shape deformation other than volume change.

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• Fig: Optical micrographs showing heterogeneous nucleation of allotriomorphic ferrite at prior austenite grain boundaries and they subsequently grew along these boundaries and growth of allotriomorphic ferrite at prior austenite grain boundaries in Fe-0.22C-2.05Si-3.07Mn-0.7Mo (wt. %) steel

• Austenitised at 1100 oCfor 10 min and transformed at (a) 750 oC @ 20 hr (b) 735 °C@ 20 hr.

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Important point for AF:• Thickening rate (normal to the

boundary) is much slower than the lengthening rate.• This Layer is not a single crystal of

ferrite, we can nucleate many grain of the ferrite along the grain boundary. But are limited to the thickening.• Somebody treated this problem

as one-dimensional growth of ferrite,

because a plan moving normal and diffusion is happening in one direction.

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Idiomorphic Ferrite (IF) ORIntragranular idiomorphs, (II) OR

Intragranular Ferrite (IF)

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Definition and Characteristics:• The term “idiomorphic” implies that the phase

concerned has faces belonging to its crystalline form.• These are equi-axed crystals which nucleate inside

the austenite grains, usually on non-metalic inclusions or other heterogeneous nucleation sites present in the steel. • An idiomorph forms without contact with the

austenite grain surfaces and nucleated intra-granuarly.• Therefore It has a shape which reflect the

symmetry of the ferrite and the austenite in which it grows. It is the superimposed symmetry of α and γ (α + γ)• So, we can see nice facets/surfaces.• Note: Faceting means, develop some planes with

particular crystallography.

• The mechanism of transformation is same as AF, the shape is different because these ferrite grows inside the austenite (intra-granularly nucleated).

• Because, inclusion are the next/second easiest nucleation sites in steel.

• There is no shape deformation other than volume change.

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• Can see nice flat interfaces with a particularly crystallographic indices

Martensite + Idiomorphic Ferrite

Fig: Formation of Idiomorphic ferrite in alloy Fe-0.39C-2.05Si-4.08Ni(wt%) steel, Austenitised at 1300oC @ 30min and transformed at 680oC @ 3 hr.

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Massive Ferrite (MF) Diffusionless Civilian Transformation

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• Definition and Characteristics:• Massive ferrite grows by a reconstructive transformation mecha nism i.e.,

formed by short-range movement/diffusion of atoms and across the boundaries classed as diffusionless civilian transformation.• The Product phase has the same composition as the parent austenite.• The ability to cross parent austenite grain boundaries seems particularly

pronounced during massive transformation; • the final ferrite grain size can be larger than the initial grain size of the

austenite. • These factors combined to give a single-phase microstructure of larger grains

of ferrite which have an approximately equiaxed morphology due to impingement between neighbouring grains. as shown in the micrograph of Fig.

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• In γ→α transformation of massive ferrite can form if the γ is quenched sufficiently rapidly to avoid transformation near equlibrium, slow enough to avoid the formation of martensite. • Massive transformations should not be confused with martensite.

Although the martensitic transformation also produces a change of crystal structure without a change in composition, the transformation mechanism is quite different. E.g. (See Slide No 14)

• Massive ferrite has its own C curve on TTT or CCT diagram as shown in figure.

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• Fig. . A possible CCT diagram for systems showing a massive transformation.• Slow cooling (1) produces

equiaxed α.• Widmanstiitten

morphologies result from faster cooling (2).• Moderately rapid quenching

(3) produces the massive trans formation, • while the highest quench rate

(4) leads to a martensitic transformation.

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• The effect of cooling rate on the temperature at which transformation starts in pure iron is shown in Fig. 5.79.

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• The microstructure of massive ferrite is shown in Fig. 5.80.

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• Fig.: Formation of massive ferrite in alloy Fe-0.05C-2.05Si-4.08Ni (wt. %) steel,

austenitised at 1300 0C @ 10 min and transformed at 600 0C @ 60 s.

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Summary for Massive Ferrite:

• Short range diffusion. • No change in chemical composition• Interface controlled.• Diffuionless-Civilion Transformation

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Pearlite

- Larger T: colonies are smaller

- Smaller T: colonies are larger

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• Pearlite is a common microstructure in wide variety of steels and received intensive research attention because of it substantial strength contribution to the steel.•Morphologically it is a lamellar mixture of ferrite and

carbide.•When austenite containing about 0.8wt% C is cooled below

the Ae1 tem perature it becomes supersaturated with respect to ferrite and cementite and a eutectoid transformation results, i.e. γ → α + Fe3C• The resultant microstructure comprises lamellae, or sheets,

of cementite embedded in ferrite as shown in Fig. 5,55. This is known as pearlite,

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End of Reconstructive Transformation

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Invariant-Plane Strain: If the operation of a strain, leaves one plane of the parent crystal completely unrotated and undistorted; this is known as an invariant-plane strain (IPS).

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