An alloy is a substance composed of two or more elements, at least one of which is a metal. Since metals are seldom 100 percent pure, the term alloy is usually reserved for metallic mixtures that are formulated as such rather than those that are mixtures as a consequence of impurities. When formulating an alloy, the chief aim is often to retain or reinforce the desirable properties of an element while eliminating undesirable properties. Copper, for example, is an excellent conductor of electricity, but it is too soft for many applications. Additions of small quantities of other elements to copper produce alloys that are good conductors and that are hard enough to withstand mechanical machining processes. In addition to mechanical strength, properties such as conductivity, temperature, and corrosion resistance can be modified by alloying.
Making alloys
In the simplest alloy-making technique, the components of an alloy are melted together in the appropriate proportions. However, this is possible only when there is an overlap between the temperature ranges for which the constituent elements are liquid. In other cases, one component might boil at a temperature lower than the melting point of another component, and much of the low-boiling component would be lost by evaporation if all components were heated together.
There is a great difference between the melting and boiling points of the components of brass, for example, which are copper and zinc. To overcome the loss of zinc by evaporation, brass is made by first melting copper—melting point 1983°F (1084°C)—in a heat-resistant pot, called a crucible. Zinc—melting point 786°F (419°C), boiling point 1665°F (907°C)—is then added. The alloy forms without the zinc evaporating to a great extent. If the metals were simply heated together, much of the zinc would boil away before the copper even started to melt.
Another technique for blending metals that have widely differing melting points uses master alloys. Consider the example of an alloy that consists of 5 percent metal A, which, when pure, melts at 2000°F (1093°C), and 95 percent of metal B, which melts at 842°F (450°C). The alloy may be made by first melting metal A and adding an equal amount of low-melting metal B. The resulting blend—the master alloy—melts at a lower temperature than does metal A. Therefore, the master alloy can be diluted with metal B to the required final composition at lower temperatures than would be necessary if using pure A. The use of a master alloy therefore reduces the amount of heat energy required for blending.
Structure and hardness
Many properties of alloys are best understood by examining the changes that occur in metallic structures when different types of atoms mix together in one substance. More precisely, changes in properties such as hardness and corrosion resistance can be understood by considering the defects that occur in the structures of pure metals and how such defects are "healed" by the inclusion of atoms of other elements.
The atoms in solid metals form crystalline lattices, in which a simple geometric arrangement of atoms repeats countless times in a regular manner. Such structures are mechanically soft, since their atoms are arranged in flat planes that slip over one another if sufficient external stress is applied. When the stress is removed, the atoms fall into the closest lattice positions without returning to their original positions. Hence, the shape of the metal changes permanently—a phenomenon called plastic flow.
In reality, the ease with which atomic planes slide over each other is increased by the presence of defects, called dislocations, in the crystal structure. The movement of dislocations in a metal can be compared to the movement of rucks in a carpet. A ruck can be pushed from one end of a carpet to the other without great effort, and the whole carpet moves as a result. To move the carpet through the same distance by pulling one edge would require much more effort. Similarly, when stress is applied to a metal that has dislocations, the dislocations shift through the metal and allow adjacent crystalline regions to move relative to one another so as to relieve the stress.
When stress is applied to a metal that contains impurities, any dislocations in the structure move until they strike impurity atoms, which stop their progress. Hence, the presence of different types of atoms, whether introduced intentionally to make an alloy or present as impurities, increases the hardness of a metal. Also, since dislocations at the surface of a metal are prone to chemical attack, the elimination of dislocations by the inclusion of appropriate impurities increases the resistance to chemical attack and corrosion.
