Mechanical Knowledge


The use of metals has always been a key factor in the development of the social systems of man. Of the roughly 100 basic elements of which all matter is composed, about half are classified as metals. The distinction between a metal and a nonmetal is not always clear cut. The most basic definition centers around the type of bonding existing between the atoms of the element, and around the characteristics of certain of the electrons associated with these atoms. In a more practical way, however, a metal can be defined as an element that has a particular package of properties.

Metals are crystalline when in the solid-state and, with few exceptions ( e. g. , mercury), are solid at ambient temperatures. They are good conductors of heat and electricity and are opaque to light. They usually have a comparatively high density. Many metals are that is, their shape can be changed permanently by the application of a force ductile— without breaking. The forces required to cause this deformation and those required finally to break or fracture a metal are comparatively high, although, the fracture forces is not nearly as high as would be expected from simple considerations of the forces required to tear apart the atoms of the metal.    One of the more significant of these characteristics from our point of view is that of crystallinity. A crystalline solid is one in which the constituent atoms are located in a regular three-dimensional array as if they were located at the corners of the squares of a three-dimensional chessboard O. The spacing of the atoms in the array is of the same order as the size of the atoms, the actual spacing being a characteristic of the particular metal. The directions of the axes of the array define the orientation of the crystal in space. The metals commonly used in engineering practice are composed of a large number of such crystals, called grains. In the most general case, the crystals of the various grains are randomly oriented in space. The grains are everywhere in intimate contact with one another and joined together on an atomic scale. The region at which they join is known as a grain boundary.

An absolutely pure metal (i. e. , one composed of only one type of atom) has never been produced. Engineers would not be particularly interested in such a metal even if it were to be produced, because it would be soft and weak. The metals used commercially inevitably tain small amounts of one or more foreign elements, either metallic or nonmetallic. These foreign elements may be detrimental, they may be beneficial, or they may have no influence at all on a particular property. If disadvantageous, foreign elements tend to be known as impurities. If advantageous, they tend to be known as alloying elements. Alloying elements are commonly added deliberately even in substantial amounts in engineering materials. The result is known as an alloy.

The distinction between the descriptors "metal" and "alloy" is not clear cut. The term metal" may be used to encompass both a commercially pure metal and its alloys. Perhaps it can be said that the more deliberately an alloying addition has been made and the larger the amount of the addition, the more likely it is that the product will specifically be called an alloy. In any event, the chemical composition of a metal or an alloy must be known and controlled within certain limits if consistent performance is to be achieved in service. Thus chemical composition has to be taken into account when developing an understanding Of the factors which determine the properties of metals and their alloys.

Of the 50 or so metallic elements, only a few are produced and used in large quantities in engineering practice. The most important by far is iron, on which are based the ubiquitous steels and cast irons (basically alloys of iron and carbon). They account for about by weight of all metals produced. Next in importance for structural uses (that is, for Structures that are expected to carry loads) are aluminum, copper, nickel, and titanium. Aluminum accounts for about O. 8% by weight of all metals produced, and copper about O., leaving only 0. 5% for all other metals. As might be expected, the remainder is all used in rather special applications. For example, nickel alloys are used principally in corrosion-and heat-re_ resistant applications, while titanium is used extensively in the aerospace industry because its alloys have good combinations of high strength and low density. Both nickel and titanium are used in high-cost, high-quality applications, and. indeed, it is their high cost that tends to restrict their application.

We cannot discuss these more esoteric properties here. Suffice it to say that a whole complex Of properties in addition to structural strength is required Of an alloy before it will be accepted into, and survive in, engineering practice. It may, for example, have to be strong and yet have reasonable corrosion resistance;

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