In this lecture, we're going to talk about different processing conditions for cold working versus hot working. Cold working processing typically at room temperature. Hot working processing at elevated temperatures. Okay, as we mentioned before, when we do work on the system, we increase the internal energy. And that is because the amount of work that's done which increases the internal energy and the fact that we increase the defect concentration. Now, when we look at that in terms of a annealing, so now we take the cold work material and we're going to heat it up. We're going to lower the internal energy of the system by decreasing the number of defects, reducing the amount of plastic deformation, and subsequently forming new grains. So here we have a cold work material. We're going to heat it up. And after 10 minutes at 700°C you can see we've actually nucleated and grown new grains. Now hits the reduction in the overall free energy of the system is going to be the driving force for defect annihilation in the new grain formation and grain growth. Okay, so how does annealing affect the properties? We know that by a annealing the cold work material we are going to modify, essentially introduce a completely new grain or micro structure. And the way we're going to do this is three mechanisms. The first is called recovery, then recrystallization, and then grain growth, right? If I look at how it affects the properties during recovery. Okay, west monitor, let's say monitor the tensile strength. During recovery I don't see a lot of change in the strength. But I do see a substantial change in, say point defects, And electrical properties. So if I look at like the resistivity will improve because I'm annealing these point defects. And these point defects account for scattering centers for the electrons, sorry. And then, if I look at during recovery though, I get a reduction in point defects. And if you notice I get my dislocations to align themselves in a lower energetic configuration, okay? And that's when you get this. You can see this sort of this low energy configuration. And if you recall a grain boundary is an array of dislocations. Now when I go through recrystallization, I nucleate these small or a new set of grains that are small, okay? But during recrystallization I see a substantial change in the strength. I lower my strength and I increase my ductility. And then I will go into a phase of grain growth. Grain growth is where the larger grains grow at the expense of the smaller grains, okay? And they're typically my properties plateau, okay? So we start off with cold work material and then if I heat it, I give it some heat. I'm going to go through the three stages recovery, recrystallization, and grain growth. During recovery I see a change in point defects, I don't see a whole lot of change in my mechanical properties. But I do see changes in electrical properties. And then my most substantial change in mechanical properties occur during recrystallization, where [COUGH] I take that sub grain structure of the dislocation aligning themselves in low energy configuration. They nucleate new grains. And then I have grain growth where the larger grains grow at the expense of the smaller grains. So if typically if we say a one hour heat treatment at some temperature, let's say greater than 0.6 times the melting temperature in Kelvin, I should see a substantial decrease in the yield strength and tensile strength and an increase in the ductility.
