 So this demonstration is going to introduce you to the lesson on phase transformations. And what I'm going to do here today is show you the impact of a phase transformation on the mechanical properties of this piano wire. So piano wire is a high carbon steel and as a consequence it's a very stiff material. And to demonstrate that I'm going to bend it and then I'm going to try to straighten it again. And so here I'm going to bend and that took a lot. And now I'm going to try to straighten it and I'm not going to try too hard because I can't do it. And I don't want to cut my hands. But the point is it's a pretty stiff material. It's quite strong and this makes sense, right? Because I'm hitting it with a mallet to produce a sound that's pleasant to the human ear. But as we know if we've done this with a paper clip, what happens with the paper clip of course is that we can do it multiple times and then eventually the paper clip breaks. So this material, this high carbon steel wire is very stiff. It doesn't have the ductility and so as a consequence I can only bend it once. But now what I'm going to do is I'm going to induce a phase transformation. And so this is primarily a phase of what we would call a prolitic phase. So it's a mixture of two different phases, alpha iron and cementite. And what I'm going to do now is I'm going to use our torch and I'm going to heat this piano wire up and then I'm going to quench it. And in doing so I'm going to impart what we call a martensitic phase transformation. So what I want to do with this wire is I want this wire to get nice and hot, glowing red. And so I'm going to put it right here in the flame and we'll get it nice and glowing red. So now that means I'm approaching temperatures of probably 800, 900 degrees Celsius. And now I'm going to take it and immediately stick it in my water and this is called quenching. And now I'll let that cool a little bit while I turn off my torch. And now let's see the impact of quenching this piano wire. So this bend once can't straighten it out. So I've transformed it to the martensitic phase. The martensitic phase is the strongest phase in terms of hardness. But it's also the most brittle phase, obvious. Now what's really cool about this though is that I can reverse the process. And this is a phase transformation that's incurred by a process called tempering. So here what I'm going to do is I'm going to get our torch going again and I'm going to go through the entire process of quenching the sample. But then I'm going to reheat it and let's see if I can bring it back from its brittle behavior. So the first thing, once again, I'm going to heat it, get it glowing red. And now I'm going to quench. And of course we know if I try to bend this right now it's going to snap like a twig. But now I'm going to reheat it. But I have to be careful because there's a certain temperature range that I want to stay in. And so I'm not going to get it glowing red. I'm just going to give it enough heat. And what I'm doing is I'm allowing those molecules to relax and go to an equilibrium position. So, all right. And so now I'll let this cool for a second. And then I will show you that in fact we've gone away from that brittle behavior. And we now have a material that's much closer to what we started with. Ouch, it's still very hot. But you see now I can bend it. So I've done two different types of phase transformation here. I've gone from its as received state to a brittle state. That's a martensitic transformation. And then I've tempered the martensitic form to go back to the original pearlitic form.