Transformation Of Martensite

Advanced Metallic MaterialsPROF. Alexey N. Solonin

By: Asad Alamgir Shaikh

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.

• 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

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.

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:

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Drawback (inconsistency) of Bain Model:

• Although Bain Model has been accepted but, 1. This model neither involve- shear transformation-

an important feature of martensitic transformation2. 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.

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

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• 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 needlesAustenite

60

m

xx xxx

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

TE

0%100%50%

A

A

M + AM + A

M + A

0%50%90%

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the amount of reaction is found to be virtually independent of time.

1%

50%

95%

Volume fraction of Martensite Vα’ is a function of Temperature not time. no matter how long hold you at that temperature

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).

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What is To ?

Driving force for the nucleation of Martensite at the Ms temperature:

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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.

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• Fully coherent interface is impossible for the γ → α′ transformation because, the lattice deformation BR is an invariant–line strain.

WhereB = Bain StrainR = 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

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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.

Migration of atoms by dislocation glide that results in the shearing of the parent lattice into the product.

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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.

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.

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.

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

Lath Martensite

Fe-31%Ni-0.28%C (Ms=192K)

Lenticular Martensite (Optical micrograph)

Fe-29%Ni-0.26%C (Ms=203K)

Thin plate Martensite

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.

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.