SO8 Diffless Transform 5

8/9/2019 SO8 Diffless Transform 5

http://slidepdf.com/reader/full/so8-diffless-transform-5 1/20

Diffusionless

Transformations - 5SOLID STATE



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



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



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.



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



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



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>



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



Lath martensite in very low-C HSLA

steels Can see prior austenite g.b.

Microstructure with high dislocation density



Retained austenite between Laths of

martensite



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



New austenite nucleating at the lath

boundaries of aged martensite



New austenite grows

between laths

Some retained Some transformed to

dislocated martensite

Some transformed totwinned martensite

(micro-twins evident)



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



Transformations in steels

morphology of ferrite, pearlite, bainite and

martensite

mechanisms of transformationcrystallography

SO8 Diffless Transform 5

Documents

Transcript of SO8 Diffless Transform 5