The Structure of Metal

The crystalline structure of metals can be described as closest possible packing of spherical atoms. One way to visualize this macroscopically is to put a quantity of steel balls on a tray and watch how they stack when the tray is tilted. Metallic structures include the body-centered cubic and the closest packed structures; face-centered cubic and hexagonal closest packed. When two or more metals bond together, it is called an alloy.

Some types of metal can have two different crystalline structures, which occur at differing temperatures. This is called allotropy. Scandium, titanium, chromium, manganese, iron, and cobalt all exhibit this phenomenon. (Tro, page 1031) For example, iron demonstrates body-centered cubic structure below 909°C and face-centered cubic structure above 909°C. (see example here) The characteristics for the metal change when the structure changes. For example, the body-centered structure for iron is ferromagnetic, whereas the face-centered structure is not ferromagnetic. The face-centered iron structure can also occur at room temperature as a thin inclusion within another crystal.

Alloys can be simple solid solutions of different elements or they can have specific stochiometric ratios of the components. Substitutional alloys occur when a metal atom substitutes for another metal atom in the structure. In order to be stable, the substituting atom must be close in size, within 15%, to the other atoms in the crystal structure. In substitutional alloys, the crystal structure may stay the same or change. Brass contains copper and zinc. Copper has a face-centered cubic structure, whereas zinc has a hexagonal closest packed structure. Once the brass alloy has more than 35% zinc it starts to develop a body-centered cubic structure. (see example here)

Interstitial alloys occur when much smaller atoms fill the gaps in between the larger metal atoms. Most often the interstitial atoms will be non-metallic. At low concentrations this type of alloy is better described as a solution. At higher concentrations, it is better described stoichiometrically. Hydrogen, boron, nitrogen, and carbon can often fit into the holes in a closest packed structure. In closest-packed crystalline structures (face-centered cubic and hexagonal) the holes can be octahedral or tetrahedral. Octahedral holes occur where there are three closest packed atoms in one layer, with three closest-packed atoms atop them in the next layer (kind of looks like a star of David). Tilted onto one point, one can clearly see the symmetry of four spheres making a square on the central plane. The Pythagorean theorem can be used to calculate the hole. Simplified the radius of the hole is 41.4% the size of radius of the metal atom. In closest-packed structures, the octahedral holes are in a 1:1 ratio to the number of atoms. Tetrahedral holes occur when a group of three closest packed atoms have another atom on top of their holes (like a three sided pyramid). A tetrahedral hole’s radius is 23% the size of the radius metal atom. Tetrahedral holes occur at a 2:1 ratio to the metal atoms in a closest-packed structure.

An easy-to-read way of depicting alloys is with a binary phase diagram. The x-axis represents the composition of the alloy (in mole percent) and the y-axis represents the temperature. It is assumed that that pressure is held constant. The phases indicated on the diagram are liquid and solid, specifying which type of crystal structures are observed. Often the diagrams will include a two-phase region which lies between two other structure phases. The two phases are mixed together at these compositions. Whichever phase is is closest to the composition of the alloy is the most abundant phase. This is called the lever rule.

Alloys can be useful for many applications. Many alloys are more durable or harder than pure metals. A eutectic alloy solidifies at a lower temperature than any other composition of the two elements and at an exact point of temperature instead of a range. Alloys can be refractory with high melting temperatures, some are especially resistant to corrosion, and others have desirable magnetic, thermal, or electrical properties. The structures of the metal and alloy crystals give them their diverse properties that are useful for applications from industrial production to medicine.

 

References

Tro, Nivaldo J. Chemistry: a molecular approach. New Jersey: Pearson Education, Inc., 2011. Print

Van Vlack, Lawrence H., et al. “Alloy.” McGraw-Hill Encyclopedia of Science & Technology. 10th ed. Vol. 1. New York: McGraw-Hill, 2007. 482-487. Gale Virtual Reference Library. Web. 29 May 2013.

Schmid, Michael. “Crystallography of Iron Films.” Institut für Angewandte Physik. 2009. Web. 29 May 2013. <http://www.iap.tuwien.ac.at/www/surface/stm_gallery/fe_crystallography>.

 

Callcut, Vin. “Introduction to Brasses (Part II).” Copper Development Association. 2013. Web. 29 May 2013. <http://www.copper.org/publications/newsletters/innovations/2000/01/brasses02.html>.

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