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Interstitial and substitutional alloys

At the atomic level, alloys are of two distinct types known as interstitial alloys and substitu­tional alloys. Interstitial alloys are formed when atoms of a small nonmetallic alloying el­ement (such as hydrogen, boron, carbon, or nitrogen) are mixed with the base metal. They become trapped in the spaces, or interstices, in the metal's atomic lattice. Alloys of this type are known for their hardness.

In substitutional alloys, metallic elements with atoms of a size similar to those of the base metal are mixed with it. Atoms of the al­loying element and the base metal can inter­change and replace each other in the base-metal lattice. The most important alloys of this type are the so-called alloy steels. In these, iron atoms are interchanged with those of other transition metal elements, such as chro­mium, manganese, or nickel.

Although a mixture of the two metals cop­per and zinc, brass is an interstitial type alloy. So also are the alloys used to make coins, such as cupronickel (copper and nickel) and cupro-gold (copper and gold).

In practice, many alloys are a mixture of both types—interstitial and substitutional. This is particularly the case with ordinary steels, which often contain very small proportions of silicon, phosphorus, and sulfur (as impurities) and the major alloying elements.

Solid solutions

Alloying can be compared with dissolving a solute (the substance to be dissolved) in a sol­vent (the medium in which the solute is dis­solved). An example is dissolving sugar in


 

The versatility of modern metalsis demonstrated by the table setting in which everything—including the flowers and cloth—is made from steel.

water. An alloy is therefore really a solid solu­tion, and the base metal is said to possess solid solubility. As with liquid solutions, an ex­cess of solute added to the solvent causes pre­cipitation. This means that the excess solute precipitates (separates itself) out from the solu­tion. The part that separates itself out from the solution is known as the precipitate. It is some­times possible to cool alloys very quickly so that expected precipitates do not form. Such alloys are said to be in a metastable (relatively stable) state and have many useful properties.

A modern traderin Oman, whose forebears probably sold earthenware pots, dis­plays a wide variety of alu­minum and stainless steel vessels.

Precipitates formed in alloys may also be compounds of the base metal and the alloying element, rather than just the alloying element. Tungsten carbide is an example. It is formed from the metal tungsten and carbon, as these two elements precipitate out of a tung­sten/carbon alloy. It is an extremely hard alloy, used in the tips of high-speed cutting tools and in mining and petroleum drills.

 

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Commercial electroplat­ingmakes extensive use of complex compounds, from the plating of silverware in small batches (above) to the automated line for brass plating hundreds of small handles (above right).

Most complexeshave characteristic shapes. These are related to the coordina­tion number of the "central" atom (usually a metal). The most common coordination numbers—2,4, and 6—give rise to molecules with the shapes shown. Coordination number

Coordination number

Coordination number


Date: 2015-12-11; view: 915


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