Welcome back. In this lecture we are actually going to look at some real microstructures of off-eutectic alloys. The first alloy we're going to consider is the aluminum copper alloy, again. In this particular case we have two phases that form behind the advancing liquid. So we have three phases in equilibrium which is what we should have at the eutectic temperature. When the material comes all the way down in temperature for an alloy which is off you tactic. What we see in our microstructure is a composite structure that is made up of the black areas. Our solid solution alpha. So that's aluminum with a certain amount of copper dissolved in it. The background is made up of the alpha phase plus the aluminum two copper, intermetallic compound. Now, we're looking at this in a particular technique in the SCM where the elements, like copper, which are heavier than aluminum, they wind up in dark field being bright. And the aluminum, which has a lower atomic number in comparison, is dark. So, we see that microstructure of the Eutectic in the background between the primary alpha phase that forms. So this is exactly what we calculated in the previous lesson. Now we're just going to look at the microstructure at a slightly higher magnification. So now what we've got is the dark regions are the alpha solid solution. And in between those primary alpha phases we have the material that is the liquid of the composition that transformed into alpha and theta. Now, if you go back to the picture that helped us come up with the understanding of the way these structures develop, one of the things that's very important for us to understand is that in order for the lamellar structure that we've been describing to develop, what we need to have is a cooperative diffusion of process. Where A moves from in front of the beta phase to the alpha phase and component B moves in the other direction. And what happens then is at that solid/liquid interface, what we're able to do is to produce the lamellar eutectic. Now, the key that must be maintained is that in order to have this cooperative diffusion. We essentially have to have a plainer interface or a relatively flat interface where the A and B can move in opposite directions. So we don't want one of the phases, either the alpha phase or the beta phase to advance. Because as a result of one phase going faster than the other phase that's forming. This very pivotal co diffusion process can't occur. Now as it turns out Not all of the microstructures have this regular lamellar eutectic. When we look at a material like aluminum silicon, now recall aluminum is a simple metallic material. Silicon, on the other hand, is not. Silicon can behave like a covalently bonded material. And when we look at the phase diagram ,what we see is the high melting temperature, silicon. The low melting temperature of aluminum. So this is a Eutectic. And as it turns out this is a very popular eutectic especially in engines and things of that nature where what you need is a good hard face that's going to maintain where in the particular automobile engine that you're working with. So if we look at a blown up section of this phase diagram, on the aluminum-rich side we actually see that there is some solubility for silicon and aluminum. But looking at the larger diagram, over the entire composition range, it's clear that there is essentially no solubility for aluminum and silicon. Now when we look at our eutectic microstructure, it's going to appear different than the eutectic microstructure that formed in the aluminum copper case. What we have in the background is the white region. And that white region represents the aluminum that formed as primary aluminum, with a little bit of silicon dissolved in it. If we look between those primary alpha grains, what we're going to see is alternating dark and light. The dark is representing the silicon, and the light is representing the aluminum. So we have this structure analogous to the structure we have with aluminum-copper. But if we look at higher magnification and we look at the details in the aluminum silicon eutectic you find that we don't have a regular structure. We have a silicon and aluminum that have formed but they hadn't formed at the same rate. And they form a structure that's not regular. So some of the microstructures that happened with respect to systems that are eutectic, they may solidify as the regular eutectic or alternatives as the eutectics that's described in this particular micrograph. Thank you.
