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Diffusionless
Transformations - 5SOLID STATE
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Diffusionless Transformation
Martensite forms in plate or lath form It is a supersaturated solid solution of
ferrite
The first plates form at Ms temperatureand the transformation is completed at
Mf temperature
Usually 100% martensite is neverobtained. Some austenite remains - Retained Austenite.
10-15% retained austenite is common in
quenched steels
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C in Solid Solution
There are two possible positions for
interstitial C: Tetrahedral sites (4 nearest neighbors)
Octahedral sites (6 nearest neighbors)
In FCC , the largest interstitial atomthat can be accommodated is d4 = 0.225 D and d6 = 0.414 D
DFe = 0.252 nm (atomic diameter of Fe)
Then d4 = 0.056 nm and d6=0.104 nm
DC = 0.154 nm (atomic diameter of C).
Hence C occupies octahedral sites. But
the lattice is distorted
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In BCC , d4 = 0.291 D and d6 = 0.155 D
Note the interstitial size in BCC is less than in
FCC
Note d6 < d4. Yet C occupies octahedral sites in .
Because of less distortion energy
In , Octahedral sites have 2 nearest neighbor atoms
Tetrahedral sites have all 4 nearest neighbors.
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For containing C in solution severely distorted in the <100> C atoms take ordered positions causing distortion
in one of the <100> directions leading to a BCT
crystal
c/a = 1 + 0.045 (wt% C)
In FCC
C is randomly in octahedral sites.
Hence the crystal remains cubic but lattice
parameter increases
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Martensite Crystallography
Two orientation relationships found (i) Kurdjumov-Sachs
(ii) Nishiyama/Greininger and Troiano
{111} // {110}’ and <11
-2> // <1-10> ’
The closest packed planes are parallel in both
The correspondence between FCC and
BCT lattices was first pointed out by Bain
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A BCT unit cell could be outlined
between two FCC unit cells. To obtain
the lattice parameters of martensite,
give a 17% contraction along [001] and
12% expansion along <100>
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This is not the mechanism of
transformation nor does it predict the
orientation relationships
Scratch observations indicate that the
martensite plates are tilted about the
junction plane with austenite (habitplane) - homogeneous shear has taken
place
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Lath martensite in very low-C HSLA
steels Can see prior austenite g.b.
Microstructure with high dislocation density
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Retained austenite between Laths of
martensite
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Tempering of Martensite
25-100°C: Carbon segregation to dislocations &boundaries
100-200 °C: Transition carbide precipitation
200-350 °C: Transformation of retained austenite
350-550 °C: Segregation of impurity/alloying atoms toboundaries
400-600 °C: Recovery of dislocation substructure Reduced c/a ratio
Conversion of BCT to BCC-ferrite
500-700°C:
Formation of alloy carbides (secondary hardening)
Formation of austenite & its subsequent transformation (secondaryhardening)
600-700 °C: RXN and GG, coarsening of carbides
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New austenite nucleating at the lath
boundaries of aged martensite
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New austenite grows
between laths
Some retained Some transformed to
dislocated martensite
Some transformed totwinned martensite
(micro-twins evident)
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Recap
Homogeneous nucleation energeticsand rates
Heterogeneous nucleation phenomenon
Growth rates of precipitates. Zenerapproximation
Cellular transformation kinetics
Precipitation hardening in Al-Cu system
Spinodal decomposition
Particle coarsening
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Transformations in steels
morphology of ferrite, pearlite, bainite and
martensite
mechanisms of transformationcrystallography
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