COURSE The Structure of Metals Lesson: The Crystal Structure of Metals
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The Crystal Structure of Metals When metals solidify from a molten state, the atoms arrange themselves into various orderly configurations, called crystals; this atomic arrangement is called crystal structure or 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 a single crystal can have many unit cells. There are three basic atomic arrangements in metals.
Body-centered cubic (bcc Body-centered cubic (bcc): examples: alpha iron, chromium, molybdenum, tantalum, tungsten, and vanadium The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells.
Face-centered cubic (fcc Face-centered cubic (fcc): examples: gamma iron, aluminum, copper, nickel, lead, silver, gold, and platinum 2R The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells.
Hexagonal close-packed (hcp) Hexagonal close-packed (hcp): examples: beryllium, cadmium, cobalt, magnesium, alpha titanium, zinc, and zirconium O. The hexagonal close-packed (hcp) crystal structure: (a) unit cell; and (b) single crystal with many unit cells
Deformation and Strength of Single Crystals When a single crystal is subjected to an external force, it first undergoes elastic deformation; that is, it returns to its original shape when the force is removed A simple analogy to this type of behaviour is a helical spring that stretches when loaded and returns to its original shape when the load is removed. If the force on the crystal structure is increased sufficiently, the crystal undergoes plastic deformation or permanent deformation; that is, it does not return to its original shape when the force is removed.
Deformation and Strength of Single Crystals There are two basic mechanisms by which plastic deformation takes place in crystal structures. One is the slipping of one plane of atoms over an adjacent plane (called the slip plane) under a shear stress. Note that this behaviour is much like the sliding of playing cards against each other. Shear stress is defined as the ratio of the applied shearing force to the cross-sectional area being sheared. Just as it takes a certain magnitude of force to slide playing cards against each other, a single crystal requires a certain amount of shear stress (called critical shear stress) to undergo permanent deformation. Thus, there must be a shear stress of sufficient magnitude within a crystal for plastic deformation to occur; otherwise the deformation remains elastic.
Deformation and Strength of Single Crystals The shear stress required to cause slip in single crystals is directly proportional to the ratio b/a, where a is the spacing of the atomic planes and b is inversely proportional to the atomic density in the atomic plane. As b/a decreases, the shear stress required to cause slip decreases. Thus, slip in a single crystal takes place along planes of maximum atomic density; in other words, slip takes place in closely packed planes and in closely packed directions. Atomic planes Shear stress Slip plane
Deformation and Strength of Single Crystals Because the b/a ratio varies for different directions within the crystal, a single crystal exhibits different properties when tested in different directions, this property is called anisotropy. A simple example of anisotropy is the behaviour of woven cloth, which stretches differently when pulled in different directions. Another example is the behaviour of plywood, which is much stronger in the planar direction than along its thickness direction. Note, for example, how plywood splits easily when a thick nail is driven through its thickness. Atomic X planes Shear stress Slip plane
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