THE STRUCTURE OF METALS

THE STRUCTURE OF METALS
1. Introduction
Metals are usually crystalline when in the solid form. While very large single crystals can be prepared, the normal metallic object consists of an aggregate of many very small crystals. Metals are therefore polycrystalline. The crystals in these materials are normally referred to as its grains. Because of their very small sizes, an optical microscope, operating at magnifications between about 100 and 1000 times, is usually used to examine the structural features associated with the grains in a metal. Structures requiring this range of magnification for their examination fall into the class known as microstructures. Occasionally, metallic objects, such as castings, may have very large crystals that are discernible to the naked eye or are easily resolved under a low-power microscope. Structure in this category is called macrostructure. On the other hand, there are materials whose grains or sizes are much finer and in the nanoscale range. These microstructures are commonly referred to as nanostructure. There is the basic structure inside the grains themselves: that is, the atomic arrangements inside the crystals. This form of structure is logically called the crystal structure.
A crystal is defined as an orderly array of atoms in space. There are many different types of crystal structures, some of which are quite complicated. Fortunately, most metals crystallize in one of three relatively simple structures: the face-centered cubic, the body-centered cubic, and the close-packed hexagonal.