La ciencia del clima terrestre y sus variaciones

The metallic bond, described in Chapter 2, holds metal atoms close to each other by an electron cloud of valence electrons. The electrostatic attraction between the negatively charged electrons and the positive nuclei increases inversely with the square of the distance between them, so that the strongest bond is the shortest bond. Consequently, metal atoms try to fit around each other as tightly as possible.

Metal Crystals

A close packing arrangement of atoms in three dimensions is possible in two different patterns, both found in metals, although why a certain metal prefers one pattern over the other is not completely understood. To complicate the picture further, some metals crystallize in a pattern that is not quite close packed.

The hexagonal close-packed (hcp) crystal structure, shown in Figure 3.1, is favored by a number of high-strength, light-weight metals (see Table 3.1) as well as zinc. Some hcp metals switch to a body- centered cubic (bcc) structure at high temperatures where thermal expansion forces the atoms farther apart.

Face-centered cubic (fcc) crystal structures are preferred by the coinage metals, including aluminum, as well as some others (Table 3.1). This crystal arrangement allows easy, sharply-defined deformation so its advantage in coinage is obvious. The fcc structure is also close packed, although its name doesn't indicate it, and is made up of close-packed planes of atoms just stacked differently than hcp.

Table 3.1 Crystal Structures of Common Metals at 20°C. FCC Ag, Al, Au; Co (hcp > 427°C); Cu, Ni; Pb; Pt. HCP Be (bcc > 1256°C); Cd; Mg; Ti (bcc > 883°C);

Zn; Zr (bcc > 872°C).

hcp fcc bcc

Figure 3.1 Schematic sketches of the three common metal crystal structures, showing atom locations.

Question: A certain homogeneous brass (alloy of Cu and Zn) consists of 75 wt.% Cu, 25 wt.% Zn.

What crystal structure does it have and how will the atoms be arranged in the pattern?

Answer: The structure will be fcc because Cu is fcc and is the major component. Zinc atoms will

substitute for some copper atoms in the structure–approximately one zinc atom per cube.

Body-centered cubic metals include iron (and steel) at moderate temperatures as well as several less- common metals. Iron, however, changes to the fcc structure when heated to 912°C, then switches back to bcc at 1394°C, and sticks with that structure up to its melting point. The low-temperature bcc structure is commonly called ferrite, the fcc iron structure is austenite, and the high-temperature bcc structure is delta ferrite. Although the bcc crystal pattern is a fairly efficient way to pack atoms tightly together, it is not close packed, as fcc and hcp are.

Figure 3.1 shows the three common metal crystal structures. For clarity, only the atom positions are shown; in reality, the atoms are large enough to touch each other. For example, in bcc structures the center atom touches the eight corner atoms. In fcc the face-centered atoms touch the four nearby corner atoms, and in hcp the center atoms in the top and bottom bases touch the six surrounding atoms in the base.

Question: The crystal pattern for bcc is shown in Figure 3.1. How many atoms are there in each

cube? (The crystal is made up of these cubes all packed together so that each corner atom is actually a part of eight cubes.)

Answer: 8 corner atoms ÷ 8 cubes + 1 center atom/cube = 2 atoms/cube. Crystal Defects

As metals begin to solidify from the melt, nuclei form at various places in the melt and begin to grow into crystals. Within the crystals the repeating pattern of atoms continues almost perfectly in three dimensions, but as two crystals grow up against each other the pattern in one is not aligned with the pattern in another. The result in the solid metal is an agglomeration of crystals, also called "grains," separated by grain boundaries as illustrated in Figure 3.2.

Impurity atoms in the liquid metal will not be exactly the right size for the crystal, may want to bond differently, or for whatever reason may prefer to remain in the liquid, so that many will end up in the last liquid to freeze–the grain boundaries. Consequently, a grain boundary tends to be filled with impurity atoms, and, being a misfit area between tightly-packed crystals, tends to be an easy pathway for atoms to move around.

Question: Which do you think diffuses faster: a vacancy or a substitutional impurity atom? Why? Answer: Since a substitutional impurity diffuses by simply changing places with a vacancy, you

might think that they diffuse at the same rate. However, a vacancy can change places with any of the other atoms on regular crystal sites also, making it much more mobile than an impurity atom.

Interstitial positions and vacancies are not the only paths for atomic diffusion, however. Metals contain dislocations, and small, interstitial atoms especially prefer to run along these dislocation "pipes." Grain boundaries also offer paths for rapid diffusion of all sorts of atoms because of the extra space between atoms and weaker bonding. These short-circuiting paths are only slightly affected by temperature, functioning almost as well at room temperature as at high temperatures.

The fastest diffusion for solids, though, is definitely surface diffusion. Atoms of all sizes are free to move without any squeezing between neighboring atoms. Atom movement over an external surface or internal crack surface is not only rapid, but seems to be almost independent of temperature.