P2 Metal Structure.pdfVIP

Part 2 Metal Structure Dr.Wanhua Yu ustbywh@ Outline When metals solidify from a molten state, the atoms arrange themselves into various orderly configurations, called crystals[1]. This arrangement of the atoms in the crystal is called, crystalline structure. The smallest group of atoms showing the characteristic lattice structure of a particular metal is known as a unit cell. It is the building block of a crystal, and single crystal can have many unit cells. Here are three basic atomic arrangements and some of the metals that use each: 1. body-centered cubic (bcc)--alpha iron, chromium, molybdenum, tantalum, tungsten, and vanadium; 2. face-centered cubic (fcc)--gamma iron, aluminum, copper, nickel, lead, silver, gold, and platinum; 3. hexagonal close-packed (hcp)--beryllium, cadmium, cobalt, magnesium, zinc, and zirconium. Body-Centered Cubic Crystal Structure The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley Sons, 1976. Face-Centered Cubic Crystal Structure Figure 1.3 The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley Sons, 1976. Hexagonal Close-Packed Crystal Structure These structures are represented by the illustrations given above. Each sphere in these illustrations represents an atom. The order of magnitude of the distance between the atoms in these crystal structures is 0.1 nm (10-8 in.). The models are known as hard-ball or hard-sphere models; they can be likened to tennis balls arranged in various configurations in a box. The way in which these atoms are arranged determines the properties of a particular metal. We can modify these arrangements by adding atoms of some other metal or metals kn