MARTENSITIC_TRANSFORMATION

2. Martensitic transformation
Habit plane
This shape to reduce the strain energy
Habit planes are irrational therefore approximated
A homogeneous deformation of the lattice of the austenite occurs to change it to
the lattice of the martensite
A diffusion less transformation occurs due to either too quick transformation
(steel) or due to too slow transformation (iron nickel alloy at 5 K)
A homogeneous deformation of the lattice of
the austenite occurs to change it to the lattice
of the martensite
No compositional change during transformation therefore Aust. And martensite
compositions are same which can be confirmed by EDS analysis

3. NW vs KS OR
KS relation
KS OR is 5.26 more rotation in NW so that (111) α matches with the (011) γ
In both cases, (011)α || (111) γ

4. Importance of glissile disl. In martensite
transformation
As glissile dislocations moves, it diffuses one part of the crystal into other
hence shape change occur
As glissile dislocations are necessary requirements for the martensitic
transformation
Do not require diffusion to
move
Require diffusion to move
Sessile Jogs
Sets cuts each other
The other part pin both
dislocations to move
This phen. Is imp. In work
hardening
As glissile dislocations can not contain more than one set of the
Dislocations

6. The only set of dislocations line must lie along the invariant line
Thefore interfacial energy per unit area is very small
Shear deformation
γ α’

7. Martensitic transformation
Martensitic transformation is both
volume change as well as shear
deformation
Invariant plane
Shear strain is high
Volume change is low
Strain energy for martensite transformation
C – thickness of the plate
Therefore to minimise the energy
thickness is very small
s; shear strain

8. Reason for thin plate shape in martensite
and mechanical twins
Although overall shear strain
is uniform but since invariant
plane is unchanged and to
minimise the strain energy,
at the corner it takes the
shape of thin plate
• Due to a similar reason, the mechanical twin forms plate like structure,
1. since invariant plane is unchanged at corner displacement is minimum
for same shear strain, 2. To minimises the strain energy
• while the annealing twins forms the slab like structure because they
don’t have to accommodate the deformation

9. γ-α or α’ Deformation (Bain strain)
Same Lattice but refined the structure as BCT
FCC

10. Compression
Expansion
But we can not get invariant plane from this loading
Therefore a slight rigid body rotation is
required to match oa and oa’ to get
invariant plane

11. Slipped and twinned martensite

12. Twins and martensite
HKDH Bhadesia

13. Twin involves the shearing
of close pack plane
Twin involves the shearing
of close pack plane
Crystal structure of the
twin is same as the crystal
str. Of the matrix/ Only
orientation changes

14. Mod-01 order-disorder transformation
Ms depends upon the carbon
content, alloying elemnt and
grain size
Smaller the gap b/w T0 and
TMs, smaller the shear is
requires