In this lecture, we're going to talk a little bit about defects. Defects are extremely important because sometimes we try to avoid them when we're growing these large silicon crystals, we're trying to avoid impurities, dislocations, and other bad things. But in other cases, we try to introduce defects to strengthen materials. It's important for us to have an idea or understanding of defects. What we got to do is initially talk about the dimensionality because that's how we classify our defects. We have these things called point defects, which are vacancies, interstitials, and substitutional. Vacancies are a missing atom from a lattice site. If I look here, there's lattice site, boom, boom, boom, boom, boom. I got 1, 2, 3, 4, 5, 6, 7, 8 going across. If I do the same count, 1, 2, 3, 4, 5, 6, 7. On that specific plane, I'm missing an atom and that defect is called a vacancy, a missing atom. Then we can have what we refer to as interstitials. Interstitials reside between the atoms. They have to be much smaller than the host atom. For example, carbon in iron to make steel. Then we have substitutional. Substitutional often are going to be on the same size of the matrix. In that particular case, they have to reside on lattice sites. You can see here, there's my lattice site and I have those residing on it. In each case, these types of defects can alter properties. Vacancy concentration can influence many properties, creep, diffusion. In the case of interstitial, the amount of carbon I have in a steel will dictate its mechanical properties. Then if I have diamond, if I have impurities like nitrogen in there, it will give me, or I think it's nitrogen, will give me the champagne diamond. If I want a clean diamond, I don't want any impurities in there and I don't want any defects like stacking faults and dislocation. Those are going to be our zero-point defects. Now, we have a defect a lot called a line defect, and that's going to be a dislocation. Dislocation in this example for now, you could think of it as an extra half plane of atoms. Here's a plane, here is a plane, here is a plane, full plane, here is a full plane. But here I have a half plane of atom. You can this guy is going to be in a different stress state above the dislocation and below the dislocation. Dislocations arrived from plastically deforming metal, so if I start bending, beating on the metal, I will introduce these types of defects. Dislocations will control my mechanical properties. Then there are other type defects 2D defects called grain boundary. Grain boundary is where we have a change in orientation between single crystal regions. Because these guys can have a different normal vector, h, k, and l so they would have a different orientation. Now, grain boundaries can strengthen, but also grain boundaries can be areas where they are going to be susceptible to chemical attack. Because you can start to think about these grain boundary as array of dislocations and they're going to have a higher energy state. Also, you got to think about when I do plastic deformation, I'm putting energy into the system so the chemical potential goes up and it becomes more reactive. As I deform it, I can make it susceptible to corrosion. If I overwork it or put too much energy, I can introduce cracks. Now, we look at defect density. Let's say solidification. Just randomly, let's just take some molten metal, we're going to heat it up and then we just pour it into a mold. What's going to happen? Inside the liquid, I'm going to nucleate these little embryos. As it continues to cool these little embryos will grow and then some of them will be near each other and they will grow into a larger crystal. The crystals have random orientation and then they're going to completely solidify. These crystals, now since they are randomly oriented, when they touch, they are going to introduce grain boundary. Each one of these 2D areas is a grain with a specific crystallographic orientation. Now, we saw where we nucleate solid and we start to cool. These nucleus embryos grow, then they touch ones, they solidify. They grow larger to eventually it completely solidifies. The adjacent grains touch each other and we have a solid. Now, we're just talking about the density of defects, this is in a metal that we've cooled. We'll say there's 10^17 vacancies per centimeter cubed, 10^8th dislocations per centimeter cubed. Then when we plastically deform it, we're going to do work on the system, we can get the dislocation density is high as 10^13th dislocations per centimeter cubed. We'll have a variety of 2D defects, that's the grain boundaries. Zero degree defects as the vacancies and 1D defects, the dislocations.
