Structure of metals

Strength together with plasticity is the combination of properties that makes metals so valuable in industry. In addition to strength and plasticity, metals have many favourable characteristics, such as resistance to corrosion, electrical and heat conductivity, etc.

The characteristics of metals are due to two structural factors: first, the atoms of which the metallic state is composed; and second, the way in which these atoms are arranged.

When a metal becomes solid, it crystallizes. The crystallization or solidification is accompanied by a complete change in the atomic arrangement of the metal. The atoms of liquid metal become arranged into a definite pattern, forming small solid bodies of regular geometric shape such as cubes, etc., when crystallization occurs.

If liquid is cooled slowly in a crucible, nuclei form at the temperature of freezing, and these nuclei continue to grow until the liquid has changed to solid.

Knowing that metals are composed of many crystals or grains, and that each grain in turn is composed of a great many atoms all arranged in some pattern, how can we understand the plastic flow that must take place when metals are deformed during a bending operation or during the drawing out of a piece of a metal? This deformation may be understood as shearing; that is, when a metal is subjected to stresses exceeding its elastic limit, the crystals of the metal elongate by an action of slipping or shearing which takes place within the crystals and between the crystals. If deformation of the metal continuous, the crystals become remarkably elongated. This plastic flow of the metal, resulting in permanent deformation of the crystals is accompanied by marked changes in the physical properties of the metal. The tensile strength, yield point and particularly hardness are increased, but not the scratch hardness or ductility of cutting, as in machine operations in a lathe. The stiffness remains about the same, though in some cases it may be increased as much as 3 per cent. With the increase in hardness and strength, the plasticity or formability is reduced. Ultimately, if deformation of the crystals is continued, the metal becomes brittle. This process changing the physical properties of a metal is called work-hardening.

If the temperature of work-hardened metal is raised above normal, the deformation begins to disappear and the metal returns to the normal condition of structure and properties.

Дата добавления: 2014-11-25