Martensitic Transformation in steel

1. Transformation Of Martensite
Advanced Metallic Materials
PROF. Alexey N. Solonin
By: Asad Alamgir Shaikh

2. MARTENSITIC TRANSFORMATIONS
• Martensitic transformations are (usually)
first order, diffusionless, shear
(displacive) solid state structural changes.
• Their kinetics and morphology are
dictated by the strain energy arising from
shear displacement.
• The atoms move in an organized manner
relative to their neighbours and therefore
they are known as a military
transformations in contrast to diffusional
civilian transformations.

3. • Why study martensitic transformations?!
• They occur in many different metal, ceramic & polymer systems, and are
generally important to understand.
• Steels represent the classical example (and a rate case of a mechanically
hard martensite). Also, there are remarkable devices that exploit the shape
memory effect (a consequence of martensitic transformation) such as stents
that open up once at body temperature.The martensites in this case are
generally soft, mechanically speaking.
Medical Stents

4. Atomic model - the Bain Model
• The first suggestion of a possible transformation
mechanism was made by Bain in 1934.
• For the case of fcc Fe transforming to bct(Fe-C
martensite), there is a basic model known as the
Bain model.
• The essential point of the Bain model is that it
accounts for the structural transformation with a
minimum of atomic motion.
• Start with two fcc unit cells: contract by 20% in
the z direction, and expand by 12% along the x
and y directions.

5. The displacement can be described as a
combination of homogeneous lattice
deformation, known also as “Bain
Distortion”, and shuffles.
In a homogeneous lattice deformation
one Bravais Lattice is converted to
another by the coordinated shift of
atoms.
A shuffle is a coordinated shift of atoms
within a unit cell, which may change the
crystal lattice but does not produce
homogeneous lattice distortive strain.
Driving force for the nucleation of
martensite:

7. Drawback (inconsistency) of Bain
Model:
• Although Bain Model has been accepted but,
1. This model neither involve- shear
transformation- an important feature of
martensitic transformation
2. It does not explain orientation relationship.
3. It does not explain a well established Habit Plan.
4. No undistorted plane is available in Bain
Distortion Model.
5. It is not possible to explain IPS associated with
martensitic transformation.
7

8. Martensitic Transformation
• Martensite: is the transformation of austenite quenched to room temperature
• Austenite  Martensite does not involve diffusion  no activation: athermal
transformation.
• Each atom displaces small (sub-atomic) distance to transform FCC -Fe (austenite) to
martensite, a Body Centered Tetragonal (BCT) unit cell .
• Martensite is metastable - persists indefinitely at room T: transforms to equilibrium
phases on at elevated temperature
• Since martensite is a metastable phase, it does not appear in phase Fe-C phase
diagram.
• The amount of martensite formed is a function of the temperature to
which the sample is quenched and not of time.
• The shear changes the shape of the transforming region:
→ results in considerable amount of shear energy
→ plate-like shape of Martensite

9. 9
• Martensite:
-- (FCC) to Martensite (BCT)
Adapted from Fig. 10.21, Callister &
Rethwisch 8e. (Fig. 10.21 courtesy
United States Steel Corporation.)
Adapted from Fig. 10.20,
Callister & Rethwisch 8e.
Martensite: A Nonequilibrium Transformation
Product
Martensite needles
Austenite
60m
x
x x
x
x
x
potential
C atom sites
Fe atom
sites
Adapted from
Fig. 10.22,
Callister &
Rethwisch 8e.
• Isothermal Transf. Diagram
•  to martensite (M) transformation..
-- is rapid! (diffusionless)
-- % transf. depends only on T to
which rapidly cooled
10 103
105
time (s)10-1
400
600
800
T(ºC)
Austenite (stable)
200
P
B
TEA
A
M + A
M + A
M + A
0%
50%
90%

10. 10
the amount of reaction is found to be virtually
independent of time.
1%
50
%
95
%
Volume
fraction of
Martensite
Vα’ is a
function of
Temperatur
e not time.
no matter
how long
hold you at
that
temperature

11. Characteristics of Difusionless
Transformations
Fig. (a), (b) Growth of martensite with increasing cooling below Ms. (c)-(e)
Different martensite morphologies in iron alloys: (c) low C (lath), (d) medium C
(plate), (e) Fe-Ni (plate).

12. 12
What is To ?
Driving force for the
nucleation of Martensite
at the Ms temperature:

13. Structure of the Interface b/w γ & α’:
• What is Interface: is simple a set of dislocation
that allow to connect two crystals.
• The formation of martensite cannot depend on the
thermal activation. There must be a high level of
continuity(link) across the interface, which must
(may) be coherent and semi–coherent.
13

14. • Fully coherent interface is impossible for the γ → α′ transformation
because, the lattice deformation BR is an invariant–line strain.
14
Where
B = Bain Strain
R = Rigid Body
Rotation
• Invariant-Line:
there is no
distortion and
rotation b/w γ & α′
along that line.
• Means Austenite
and martensite
match perfectly
along that line.
• Therefore, the
interface b/w γ &
α′ must be semi
coherent in a
special way.
Fig: Fully
Coherent
Fig: A semi-coherent
interface

15. semi coherency in a special way that
there is one set of dislocation:
1. A semi– coherent interface must be such that the
interfacial dislocations can glide as the
interface moves (climb is not permitted)
2. There is an additional condition for a semi–
coherent interface to be glissile.
15
Migration of atoms by
dislocation glide that results
in the shearing of the parent
lattice into the product.

16. 16
Q: When do we get slipped Martensite and Twinned Martensite?
When steel
deform at
normal strain
rate-
When steel
blasted
produces
lots of
mechanical
twin.-
• Martensite forms extremely rapidly
will be twinned.
• Martensite which has dislocated
interface and produces slip steps it
tends to be slipped.
• If both of these processes happen
perfectly no one can find
dislocation in the Martensite.

17. Self-accommodation by variants
• A typical feature of martensitic transformations is that each
colony of martensite laths/plates consists of a stack in which
different variants alternate. This allows large shears to be
accommodated with minimal macroscopic shear.

18. Carbon in ferrite
• One consequence of the occupation of
the octahedral site in ferrite is that the
carbon atom has only two nearest
neighbors.
• Each carbon atom therefore distorts the
iron lattice in its vicinity.
• The distortion is a tetragonal distortion.
• If all the carbon atoms occupy the same
type of site then the entire lattice
becomes tetragonal, as in the martensitic
structure.
• Switching of the carbon atom between
adjacent sites leads to strong internal
friction peaks at characteristic
temperatures and frequencies.

19. Morphology and crystallography of a’ (bcc or
bct) Martensite in ferrous alloys

20. Lath
Martensite

21. Fe-31%Ni-0.28%C
(Ms=192K)
Lenticular Martensite (Optical
micrograph)
Fe-29%Ni-0.26%C
(Ms=203K)

22. Thin plate
Martensite

23. Mechanisms
• The mechanisms of military transformations are not entirely
clear. The small length scales mean that the reactions
propagate at high rates transformation. The high rates are
possible because of the absence of long range atomic
movement (via diffusion).
• Possible mechanisms for martensitic transformations include
(a) dislocation based
(b) shear based
• Martensitic transformations strongly constrained by
crystallography of the parent and product phases.
• This is analogous to slip (dislocation glide) and twinning,
especially the latter.

24. Summary
• Martensitic transformations are characterized by
a diffusionless change in crystal structure.
• The lack of change in composition means that
larger driving forces and undercoolings are
required in order for this type of transformation
to occur.
• The temperature below which a diffusionless
transformation is possible is known as “T0”.
• Technological applications abound - quenched
and tempered steels, Nitinol shape memory
alloys etc.