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1. - Interphase Migration
- Classification of phase
Transformation
-Decomposition of Austenite
1
Lecture Series –B-1

2. 2
Materials
Science
Model

3. Interphase Migration
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4. 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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5. • 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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6. • 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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7. Classification of Phase
Transformations
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8. 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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9. • 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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10. • 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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11. • 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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12. 12
Type Military Civilian
Effect 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

13. • 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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14. 14
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:)

15. 15
Austenite with two different kinds of atoms.
It can transform into new/different crystal
structure by two methods.
1. Displacive Transformation
2. 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

17. 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 .
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Large atom moves in
disciplined manner
but small atoms go
and occupy
wherever they like to
occupy.

18. • Fig. 1.6
Summary of the
variety of
phases
generated by
the
decomposition
of austenite.
• Ref: Steels
Bhadeshia
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19. • 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,
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Introduction:

20. 20
Introduction:

21. 21
Introduction:

22. 22
Introduction:

23. Decomposition of Austenite
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24. • Allotriomorphic Ferrite
• Idiomorphic Ferrite
• Massive Ferrite.
• Pearlite
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Reconstructive decomposition of Austenite

25. Allotriomorphic Ferrite (AF)
OR
Grain boundary Allotriomorphs, GBA
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26. 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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27. 27
• 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.

28. 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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29. Idiomorphic Ferrite (IF) OR
Intragranular idiomorphs, (II) OR
Intragranular Ferrite (IF)
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30. 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.
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• 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.

32. Massive Ferrite (MF)
Diffusionless Civilian Transformation
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33. • Definition and Characteristics:
• Massive ferrite grows by a reconstructive transformation mechanism 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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34. • 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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35. • 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
transformation,
• while the highest quench
rate (4) leads to a
martensitic transformation.
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36. • 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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37. • The microstructure of massive ferrite is shown in Fig. 5.80.
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38. • 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. 38

39. Summary for Massive Ferrite:
• Short range diffusion.
• No change in chemical composition
• Interface controlled.
• Diffuionless-Civilion Transformation
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40. 40
Pearlite
- Larger T:
colonies are
smaller
- Smaller T:
colonies are
larger

41. • 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 temperature 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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43. End of Reconstructive Transformation
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44. 44
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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