324 Section 4 Nonferrous Metallurgy length in one direction is different than the others, leading to four rectangular faces. A few metals have more complex unit cells. The face-centered cubic elements generally have higher electrical and thermal conductivity than elements with other crystal structures. Silver (Ag), copper (Cu), and gold (Au), which are all face-centered cubic, have the highest conductivity of all metals. Metals such as beryllium (Be), cobalt (Co), magnesium (Mg), and zinc (Zn) form hexagonal close-packed unit cells, which dramatically changes dislocation motion, and thus ductility. In some unit cells, such as with tin (Sn), the body-centered cube is stretched in one direction to form a body-centered tetragonal structure. Some metals, notably bismuth (Bi), uranium (U), and polonium (Po), form different unit cells. The ability of these metal crystals to deform is less than for other crystals, so the metal is less ductile. 15.1.2 Sl ip Planes and Dislocation Tangles In the as-cast condition, all metals form regions with atoms neatly aligned in unit cells, called grains. When metal deforms, the atoms in a crystal grain slide past one another, forming dislocations. The dislocations appear on the nanometer scale as lines, and repeat displacements occur along the same planes, as discussed in Chapter 5. As slip continues, dislocations along different planes intersect one another, and dislocation tangles build up. The force required to move dislocations increases. On the macroscale, we see the metal has become stronger. In very dense tangles, almost no dislocations can move, so the ductility drops to zero. When yet more force is applied, the metal atoms separate, forming macroscale cracks in the metal. You created these cracks in copper yourself in the Self-Discovery activity “Effects of Cold Work on Copper,” found at the end of Chapter 5. Metals with face-centered cubic (fcc) atom structures have more slip planes than body-centered cubic (bcc) metals. Thus, fcc metals usually have greater ductility. The ductility of gold and silver, both fcc, is higher than that of bcc iron or molybdenum. Metals with an hcp structure have yet fewer slip planes, and tend to be less ductile. This is one reason why wheel rims made of magnesium (hcp) chip more easily than those made of aluminum (fcc). Removing Tan gles—Recrystallization You know that heating steel above 1300°F (700°C) allows the “tangled” crystals to be replaced by fresh crystal grains. The same recrystallization happens with nonferrous metals. The recrystallization temperature depends on the metal—zinc is liquid when molybdenum is still filled with dislocation tangles. Metals can recrystallize at about 55% of the melting point temperature as measured on the kelvin temperature scale. Figure 15-5 illustrates how each scale reports temperatures. On the kelvin, or absolute, scale, 0 is the lowest possible temperature—the equivalent of –459°F or –273°C. For example, tin (Sn) can recrystallize at room temperature, because room temperature is at 58% of its melting point. Copyright Goodheart-Willcox Co., Inc.