And welcome back once again to the materials teaching laboratories here in Kemper Hall on the University of California Davis Campus. Today we're gonna talk about ductile to brittle transition and find out how small changes in ambient temperature can make a big difference in terms of mechanical performance. [MUSIC] And so I'm standing here today in front of a Charpy tester. This is a long time traditional mechanical testing machine that basically does in one very quick swoop. And you'll see an example of that, In our segment today, in that one swing of a large pendulum hammer arm the breaking of a material by impact. And essentially what it's doing in that split second is doing the same thing we did in the Instron machines in our earlier segment, talking about the big four mechanical properties. But in this case, we're doing it in a very rapid sequence. We're taking the material to a very quick elastic deformation, then well on to substantial plastic deformation and then final failure. And as we'll emphasize in our next segment, we've notched that sample, put a small defect in the surface, in order to control where it fractures and to ensure that it does fracture in an efficient manner. So it reminds us, actually of the discovery of what we call the ductile-to-brittle transition. Back in the World War II era, there were some ships called the Liberty Ships, which were a British design, but manufactured largely in the United States. Utilitarian, not terribly attractive Navy vessel that was meant to provide resources during the war to our allies in Europe, hence the name liberty ship. But in the process, when these ships were sent into very cold North Atlantic waters, some failed. Some literally split in half. They fractured catastrophically. At first it was thought that perhaps there were bad weld nuts to explain the failure. But when the materials themselves were sent back to Cambridge University in England for analysis, is when metalers then discovered this ductile to brittle transition concept. And it's a case in which a material that performed just fine in a room temperature, ambient, atmosphere of the laboratory, in a very cold environment became brittle, like a piece of glass. So today we're looking at an impact energy measuring device, the Charpy tester. And in order to appreciate why that is so, we need to look at three crystal structures in this brief introduction to material science. But we certainly don't expect you to learn all the details of crystallography and the nature of the structures on the atomic scale of these important metallurgical alloys. But keep in mind three main structures which are representative of most of the commercial metal alloys that we deal with in everyday engineering. The face centered cubic crystal structure, the body centered cubic crystal structure and the hexagonal close packed crystal structure. Most common metals fall into one of those three categories. And what we see in a very careful analysis of the crystallography of these materials is that these highly densely packed atomic arrangements, the face center cubic structure has, what we call, 12 slip systems. A slip system is nothing more than a combination of the high atomic density planes and high atomic density directions, in which dislocations move. And remember that segment in which we talked about how dislocations are at the basis of plastic deformation. Well, for face-centered cubic alloys that dislocation motion is an easy one because, again, of the large number of slip systems. By contrast, in the hexagonal close-packed structure, equally highly dense arrangement of atoms because of the geometry of the hexagonal arrangement, a limited number of slip systems, that material is relatively brittle. So those HCP or hexagonal close packed crystal structure alloys tend to be brittle in nature over a wide temperature range. Face center cubic materials tend to be ductile over a wide temperature range. But the body center cubic crystal structure alloys which include so many of the common ferris alloys, the common structural steels that we make so many things from in everyday engineering practice, those are somewhat intermediate. They also, as with the FCC, have 12 flip systems, so they have the ability to be very ductile. And so again the Liberty ship materials that were tested at room temperature in the laboratories in England initially were just fine. However, when they were retested at Cambridge University at very low temperatures representative of the cold, north Atlantic sea temperatures, they turned out to be very brittle. And for the simple reason that the body-centered cubic crystal structure is one, in which the dislocation motion is slower. And so as a result, as the material is cooled down to a lower temperature, that cold North Atlantic Sea ambient temperature then, that material behaves in a very brittle way. The dislocations can't move, plastic deformation is choked off and as a result the material was brittle, almost like a piece of glass. Well in appreciating those differences then, again, we can monitor that now routinely in laboratories, by simply measuring the performance of various metal alloys at various temperatures. And again, it's critically important to monitor these body centered cubic alloys, such as the common structural steels, at a range of temperatures from well above room temperature to well below room temperature. The plot of those then will very often show a transition, very low impact energy corresponding to brittle fracture at low temperatures and then high impact energies corresponding to ductile fracture at relatively higher temperatures.
