Welcome back. I'd like to describe in this lesson the development of the microstructure that is associated with the eutectic systems. We have described the eutectic as being a reaction and a variant reaction in which liquid transforms to two solids upon cooling. Now, what we're also going to do in this case is to describe what we mean by a Eutectic microstructure. If we looked at a very common eutectic system, namely, the aluminum copper system. And we're on the aluminum rich side of the aluminum copper phase diagram. When you take a look at the diagram, what you see is the eutectic reaction at 548 degrees C. I've indicated the eutectic composition that's at 33%. The eutectic is made up of the liquid as well as the two phases, alpha which is a solid solution of aluminum with copper in it and the intermetallic particle theta, which is Al2Cu. When you look at structures that are produced from a composition of 33%, you develop a structure that's referred to as a lamellar structure. So if you think of each one of these phases, the light phase is the aluminum to copper phase, and the dark phase is the alpha solid solution aluminum copper. So when you look at the microstructure, coming out of the board is a platelet of alternating dark and light, dark and light. And these are the alternating lamelli of the alpha and the beta phases. So when we cool compositions through the eutectic, this a typical section that we would see in the microscope. And as it turns out, there are numerous microstructures of eutectic systems that appear this way and so consequently it's important for us to describe in a bit of detail how this microstructure evolves. Based on these experimental observations, we want to be able to describe how this alternating microstructure of alpha beta comes about. If we look at a section through this alternating structure, what we're seeing is a liquid and behind the liquid are the two phase structure of alpha and beta and alpha and beta, as indicated by gray and blue sections. Now at the equilibrium temperature, we simply recognized what the compositions of all the phases are. We know that the alpha phase is a phase which is rich in A. And the beta phase is rich in B. If we go to our section of the eutectic diagram, we see the composition of the alpha phase indicated by the gray arrow. The composition of the beta phase, as indicated by the blue arrow and the composition of the eutectic as given by the black arrow. So these give us the three phases that are in equilibrium along that solid liquid interface. Now in order for the interface to advance, what has to happen is, there has to be a slight drop in the temperature, and there must be a change in composition locally in the liquid in front of the alpha phase and in front of the beta phase. And this works in the following way. So here is our interface of alpha beta alpha and beta. And what you see in this visual is that we are going to start moving A in front of the beta phase toward the alpha to the left and the alpha to the right. And we're doing this along the liquid solid interface. Similarly, if we take a look at the component B, what we see is in front of the alpha phase, what we're going to do is move the component B in front of the beta phase and B in front of the beta phase to the right. So this process occurs at a slightly lower temperature than the eutectic, and it is occurring along the solid liquid interface. It turns out that we'll describe this exact mechanism when we get into the module that describes the kinetic processes. But I wanted to introduce the idea of exactly how these phases begin to appear in this cooperative method by diffusion of A in one direction, and diffusion of B in the other direction. So here is our solid liquid interface, and the motion of A and B in front of their required components wind up increasing the local composition so that the alpha and the beta phase can advance. In a previous lesson, I presented a eutectic diagram where a number of the compositions along the phase boundaries had been indicated. What I'd like to do here is to use that same data, and what I'm looking at is, in particular the eutectic alloy composition. If I look at a temperature above the eutectic, I have essentially 100% liquid. When I reach the eutectic temperature, I begin to form a very small amount of alpha and beta. And I'm assuming that the amount of solid alpha and beta I have is very, very, very small and the compositions can be given by the compositions on the eutectic tie line. As soon as I drop below T6, what happens is the liquid phase disappears and what I have left are the two phases alpha and beta. And from their compositions and the composition of the alloy, which is the eutectic composition, I'm able to calculate what the fraction of solid alpha and the fraction of solid beta are. And at the next temperature, at T8, I'm able to do the same thing. So what I can do is I can actually plot the data in the form of a phase fraction chart. What I see is above T6 I have 100% liquid and as soon as I reach T6 I am in equilibrium with three phases, just a little bit of solid alpha and beta and essentially 100% liquid. As soon as the temperature begins to drop the amount of liquid I have goes to zero and I'm beginning to develop my alpha phase and my beta phase, according to the compositions that are given along the phase boundaries at those two respective temperatures. Now I can also show how the microstructure changes as a consequence of cooling above the eutectic temperature down to below the eutectic temperature. So when I'm above the eutectic temperature I see I have 100% liquid and I'm observing that in the microscope. As soon as I drop below the eutectic what I begin to form is that alternating alpha and beta mixture that comes about as a result of eutectic composition. We'll be continuing this description and looking at eutectic systems that are off the eutectic composition in the next lesson. Thank you.
