At first, let's see a short video clip which shows the evolution of bainite, which is one

of representative non-equilibrium microstructure.

In this micrograph, the starting microstructure is austenite which was cooled from higher

temperatures and this white line indicate the grain boundary of the austenite.

As the temperature of the alloy decreases, you can see the evolution of plate-like micro

structures.

This microstructure evolution shows an important characteristic features of displacive transformation.

As you can see in the video clip, the plate like microstructure grows very quickly and

its growth is confined in one austenite grain.

And you can notice that the growth of plate generates a characteristic roughness on the

surface of the sample.

Those kind of characteristic features of the phase transformation give us very important

clue to understand the detailed mechanism of displacive transformation as we are going

to discuss later.

This slide summarizes the general features of displacive transformation, generating non-equilibrium

micro-structure such as bainite and martensite in iron and steel.

The first important feature is that the phase transformation occurs at lower temperature

where the diffusion is very slow therefore the diffusional motion of individual atom

is difficult to explain the change of the lattice structure in such a short time.

And second one is that the displacive transformation proceeds very quickly even though the transformation

occurs at lower temperature compared to the temperature range for the reconstructive transformation

such as ferrite or pearlite transformation.

Besides, the chemical analysis reveals that the distribution of substitutional alloying

element does not occur in the displacive transformation, and in case of martensite transformation even

the redistribution of interstitial carbon atom does not happen.

Before going further to the mechanism of displacive transformation, let's see at first the characteristic

features of non-equilibrium microstructure which is generated from the displacive transformation.

This is a scanning electron micrograph showing the microstructure of martensite.

Here this white broken line indicates the grain boundary of the prior austenite.

When we look into the structure of the martensite plate formed in one austenite grain, you will

notice certain domain where the growth direction of the martensite plate is nearly the same.

We call that domain as a packet.

And the packet consists of individual plate which is called as a block and further detailed

observation shows that the block contains very fine sub-structure, which is called lath.

So the martensite has a characteristic, hierarchical structure consisting of packet, block and

lath which is closely related to the mechanism of microstructure evolution underwent the

displacive transformation.

Here the typical width of lath is around 0.2 micrometer and the lath boundary is low angle

boundary.

But the interface between block and packet is high angle boundary.

Since the high angle boundary is more formidable obstacle for the dislocation motion, the size

of block or a packet are known to have more significant influence on the mechanical properties

of martensite.

As I mentioned the displacive transformation is accompanied by the occurrence of characteristic

surface roughness.

The surface roughness implies that a certain kind of deformation happens during the transformation

and it is that deformation which eventually convert the FCC lattice structure or parents

austenite into the BCC structure of the product phase.

We call the deformation accompanying the displacive transformation as a shape deformation.

The detailed observation on the surface roughness indicates that the shape deformation has a

character of shear deformation as shown in this figure.

Therefore, considering the difference of atomic volume between FCC and BCC, the shape deformation

should have a character of generalized invariant plane deformation.

The property of invariant plane deformation is that there should be one non-deformed and

non-rotated plane before and after the transformation.

We call the invariant plane as a habit plane of the displaced transformation.

And the habit plane looks like the interface between parent phase and the product phase

in the course of the displacive transformation, as shown in this figure.

Now what we have to think about on the mechanism of displacement transformation is whether

the Shape Deformation here can change the FCC structure into the BCC structure.

In other word, the shape deformation can convert the crystal structure of austenite into that

of bainite or martensite.

In next section, we will think about the answer to this question by introduction of phenomenological

theory of martensite transformation.

Displacive transformation

Ferrous Technology II

Rencontrer les enseignants