3. Martensitic transformation
Rapidly quenched γ
from temp. above A1
to temp. below Ms
Very slow diffusion
rate at temp. below Ms
Rapid cooling prevents decomposition of
γ by diffusional process.
γ → α + Fe C transformation cannot
3
occur by any diffusion transformation
3
4. Martensitic transformation
γ will minimize its free energy by
Transforming to a metastable phase γ → α’
with a lower energy by a shear process
The energy decreases with displacement
of atoms and causes change in
microstructure.
This mode of transformation kinetics is
referred to as athermal, meaning without
thermal activation.
4
5. Martensitic transformation
Martensite
A general term for
microstructure formed
by diffusionless
phase transformation
γ → α’
Parent γ and product α’ phases have a
specific crystallographic relationship
No change in composition
5
6. Martensitic transformation
Atomic movements during transformation
are cooperative in a regimented fashion
with distance less than one interatomic
spacing
Cause shape change
in the transformed
region
Surface tilt if the product
α’ phase intersects a free
surface of the parent γ phase
6
7. Martensitic transformation
Shear mechanism
Shear is on habit planes, on which
the transformation was initiated.
Ideally, martensite crystal has
planar interfaces with the parent austenite
Preferred crystal planes on interfaces
Midrib is considered to be the starting plane
for the formation of a martensite plate.
7
8. Martensitic transformation
A martensite unit
Flatand Lies in a habit plane
parallel to a crystallographic plane in γ
Once nucleated inside a γ grain at M ,
s
α’ propagates ~ 1 km/s
In a direction lying in the plane of α’ plate
α’ plate shape is similar to a lens.
Grow until reaching grain boundary,
twin boundary, or another martensite
Can form again inside retained γ but 8 smaller
9. Martensitic transformation
Thermodynamic driving force
Temperature Ms reflects the
amount of thermodynamic
driving force required to initiate
shear transformation.
Smaller martensite is still required
further cooling rate for next α’
to form.
Each α’ platelet will introduce its
own strain.
9
10. Martensitic transformation
Thermodynamic driving force
Next α’ platelet will require higher
energy (∆T) to overcome extra
stress from previous α’ platelets
and to make the next martensitic
transformation to occur.
The martensitic transformation
will end at Mf.
Density of α’ plates is not
a function of the γ grain size.
10
11. Martensitic transformation
Surface tilting
Martensite crystal is displaced by
the shear partly above and partly
below the surface of the austenite.
Shear transformation tilts the originally
horizontal surface of the parent phase into
a new orientation.
An important feature of shear-type or
martensitic transformation
11
12. Martensitic transformation
Surface tilting
Large change in shape would
cause large strain.
Minimized by deformation
Twinning
Slip
with no crystal structure change
12
13. Martensitic transformation
Martensite crystalline structure
In steel
Parent phase usually has fcc
Product phase can be bcc, bct, or hcp
Iron-based martensite
Fe-Ni and Fe-Ni-Cr usually have bcc
Fe- >15% Mn alloys can form martensite
with hcp crystal structure.
13
14. Martensitic transformation
Martensite crystalline structure
fcc → bcc transformation
Kurdjumov-Sachs orientation relationship
for high-carbon steels
{111}γ || {101}α’ and <110>γ || <111>α’
with {225}γ habit planes
Kurdjumov-Sachs orientation relationship
for low-carbon steels
{111}γ || {101}α’ and <110>γ || <111>α’
with {557}γ habit planes 14
16. Martensitic transformation
During cooling to Ms DC = 1.54 Å
2 types of interstitial
site in fcc
Tetrahedral site
DFe = 2.52 Å
surrounded by 4 atoms
d fcc = 0.225D
w/o matrix distortion, max. space 4
0.568 Å
Octahedral site
surrounded by 6 atoms
d fcc = 0.414D
w/o matrix distortion, max. space 6
∴ cause distortion of γ lattice 16 1.044 Å
17. Martensitic transformation
Temp. below Ms
Interstitialatoms cause more distortion
to the bcc lattice.
Distortion causes the martensitic Fe-C
structure to form a bct structure.
c/a ratio varies with %Carbon
c/a = 1.005 + 0.045 (wt.% C)
Distortion in z direction causes
contraction in x and y directions.
17
18. Martensitic transformation
Exp. Ms decreases
significantly with
increasing %C in
Fe-C alloys and
carbon steels.
Carbon in solid solution increases strength or
shear resistance of austenite
Higher carbon alloys require larger
undercooling or driving force to begin
the shear for martensite formation. 18
19. Martensitic transformation
Martensite plates
Form in the shape of laths that are grouped
into larger sheaves, or packets
Generally called lath martensite, massive
martensite or packet martensite
ASM Handbook
19
20. Martensitic transformation
Martensite plates
Form as individual, lenticular, or plate-
shape units
Generally called plate martensite
Forme as thin sheet
Called sheet martensite
20
ASM Handbook
22. Martensitic transformation
After martensitic transformation
Large interaction energy btw carbon and strain fields
Carbon tends to lower its chemical potential by
diffusing to sites close to dislocations and to form
carbon-rich clusters.
In low-carbon low-alloy steels
Plenty of time during
quenching for carbon to
segregate or precipitate
as ε-carbide (Fe2.4C) or Fe3C
22
23. Carbide
ε-carbide (Fe2.4C)
Similar to Fe3C
Having the same form of precipitate
Cementite (Fe3C)
Lath-likeprecipitate
Coarsens into a spheroidal forms at high T
23
24. Martempering
Interrupted quench from the A1
Delay cooling just above
martensitic transformation for
a length of time to equalize T
throughout the piece
Reduce thermal gradient
btw surface & center
Minimize distortion, cracking,
and residual stress.
24
25. Geometric observation
When martensite is formed
Observe shape deformation
(macroscopic deformation)
Well-defined displacement
of scratches due to a martensite plate
Straight lines/vectors are transformed
into other straight lines/vectors.
Planes are transformed into other planes
Maintain interfacial coherency btw
martensite & parent at the habit plane
25
26. Geometric observation
When martensite is formed
Martensite plate leaves the
scratches uninterrupted as
they cross the interface.
Ifthe shape deformation causes any significant
rotation of the habit plane
Ifthere is any distortion existing in the habit
plane due to shape deformation
26
27. Geometric observation
If any significant rotation of the habit plane
due to shape deformation
Plastic deformation of the matrix material
next to the martensite plate would reveal
itself additional plastic deformation of the
scratches in the parent phase.
27
28. Geometric observation
If any distortion existing in the
habit plane due to shape deform.
Scratches across the interface
would appear discontinuous.
28
29. Geometric observation
Additional plastic deformation is not
observed.
Scratches across the interface appear to
be continuous.
∴ Unrotated and undistorted
habit plane due to shape
deformation
29
31. Bain correspondence
Bain (1924) purposed a simple mechanism for
the deformation of austenite lattice
Incomplete model for entire martensitic
transformation
Martensitic transformationis taken place by
a cooperative movement of atoms
Soatomic neighbors are maintained before
and after the transformation.
31
32. Bain correspondence
α lattice with bcc can be generated from an fcc
γ lattice by
Compression about 20% along
one principle axis and
a simultaneous uniform
expansion about 12% along
the other two axes perpendicular to the
first principle axis
This homogeneous distortion making one
lattice change to another is termed
a lattice deformation.
32
33. Bain correspondence
Bain distortion
Lattice deformation of fcc to bcc/bct
transformation
Bain correspondence
Method of determining
a correspondence
between lattice
points in the →
initial and ao → c
final lattices 33
0.7ao → a
34. Bain correspondence
Bain distortion of fcc to bcc/bct
[001] becomes [001]
f b
[110] becomes [100]b
f
Consider corresponding vectors are
x1b = x1f − x2f
x1b = x1f + x2f
x3 = x3f
b
→
x1 1 −1 0 x1
x = 1 1 0 x ao → c
2 2
x3 b 0 0 1 x3 f
Bain correspondence matrix 34
0.7ao → a
35. Bain correspondence
Does not provide actual crystal orientation
relationships between initial and final lattices.
The most feasible deformation
Involves the smallest relative atomic
displacements
Results in the smallest strain energy.
Unrotated habit plane
Satisfy
Undistorted habit plane
Not yet satisfy
35
