The last decomposition process we want to talk about is the formation of martensite from austenite. As we have discussed previously, the transformation of austenite to martensite is diffusionless. It occurs without the aid of diffusion of individual atoms. It occurs as a result of cooperative homogeneous motion of atoms that result in a change of structure. And, these movements are small, usually less than the interatomic distances between the planes. And, the atoms maintain their relative relationships between one another. And the most commonly encountered transformation of these type, is martensitic transformations of iron and carbon and because of the importance of this material, this will be the focus. Although there are other materials systems in which martensitic transformations also occur. Again, we go back to our eutectoid portion of the diagram. And now what we're going to do is to quench down to a low temperature, and we're going to take our austenite, and what we're going to do is we're going to form our martensite. Now the martensite phase, again, is a single phase material. So we're looking at what happens when we take material and we take the austenite and we transform it to the martensite. And again, we see that it's a thermal, so the lines are horizontal, and the process is diffusionless, and it's a single phase microstructure. And there is the same composition that the austenite has that goes to the composition that is the martensite at the end. So these two compositions are the same. But there's one thing that we'll talk about later on, and that is the dramatic difference in strength, of those two phases associated with the formation of this martensitic structure. So we quench it rapidly, and we go below the martensite finish. And what we're going to wind up with is a high strength material that is made up of these regions of martensite. In the sequence of photo micrographs, we're going to be looking at the athermal nature of the martensitic phase change. Now if we look at the microstructure, at one temperature what we find is that at particular temperature we have some martensite plates or platelets that have formed at the temperature of 25C. And the bar that I have indicated here is the fact that what we're going to do Is take all of the micrographs, look at all of the micrographs at exactly the same magnification. And these materials have been polished, so that they can be reinserted into temperature baths at lower temperatures to see how the progress of the martensite occurs. So the material is dropped to a temperature of this time -60C. The red arrows indicate the presence of the original martensite platelets. And now, what we've done is the microstructure of austenite is beginning to fill in and more of the austenite is being consumed by the development of the martensite platelets that are forming at the lower temperature. And now we go to an even lower temperature, this time now below -100C. And we see that the austenite is beginning to be completely filled, although there's still plenty of space for the martensite to form. And again, you can see the red arrows indicating where those original martensite platelets were that formed at 25C. So we're seeing the athermal nature of this transformation. And we're following how temperature affects the amount of material that has been transformed. Decreasing the temperature results in an increase in the fraction transformed and that's what these diagrams are indicating, as we go from left to right. Now let's take a look at how we might be able to come up with a structural argument that describes the way the process of martensite forms. This is a relatively simple model and because we start out with the austenite as an FCC structure, let's go back and draw two unit cubes. And now what we're going to do is we're going to define our X, Y and Z, and we're going to put spots on these and those are my corner positions where I can have iron atoms sitting. Here are the face positions that I can have the iron atoms sitting. And so we now have the appropriate six-atom sites that sit inside of the unit cell. The four that are on the corners are one lattice point, and the six that are on the faces give you the other three lattice points for a total of four lattice points. Now, rather than defining the FCC cell, we could come up with an alternative description of this crystal structure. And that's going to require the addition of atoms or sites that are in the adjacent unit cell. So what I'm going to do then is I'm going to start putting those sites in. So what I have done in this particular figure is to make a structure which connects the locations of the surface and the corners, and the bottom and top, and the center position. And so what you see is rather than an FCC description of the unit cell, what you have now is the body center tetragonal alternative form of the distribution of atoms that you have in the austenite. In addition to having the blue atoms that represent the iron positions. The red atoms represent where the carbon atom positions are located. And what we can do is we can now start looking at the unit cells. And we can look at the unit cell parameters, our original parameter where we have the edge of the unit cell, represents a gamma. And then when we look at the distance between the positions at the corner and the faces, we see that those parameters are now A gamma divided by the square root of 2. So that represents our unit cell which is tetragonal because along that base we have a gamma over the square root of 2 for those two dimensions, and then a gamma perpendicular to that plane. So, that's our BCT alternative to the FCC austenite distribution. So, now, what we're going to do is to take that distribution of potential carbon sites and atom sites, and we're going to take those positions, and we're going to cause the structure to compress. And it's going to change magnitude along the C-axis and it's going to change along the two orthogonal directions to it. So this is what's happening as a consequence of going from the austenite to the ferrite. There is a change in the dimension of the unit cell, and that will depend upon how much carbon we actually have managed to put into the particular alloy. So as the carbon content of the alloy increases we're going to have more carbon stuffed into this body centered tetragonal unit cell. And so what we're seeing is relatively small displacements associated with the positions of the atoms in the original cell to the positions of the atoms in the martensitic structure. Here is our picture again, and what we see is the transformation of the, from that body centertetragonal austenite to the body centered cubic iron. So we go from the equilibrium phase at high temperature to the ferrite phase after all of the carbon has left the lattice. And so what we're doing is diffusing the carbon out of the body center to tetragonal lattice. We are producing the combination of ferrite and cementite. So with time at temperature, we relieve some of the stress associated with the carbon being trapped in the BCT structure, and the precipitation of the second phase, namely Fe3C. Thank you.
