Alloys, Light and Ferrous. Definitions

Although alloys have been part of industrial practice since antiquity (bronze, an alloy of copper and tin, has been in use for thousands of years), the systematic development of alloys dates to the middle of the nineteenth century. Due to improvements in techniques for chemical analysis and the rise of systematic testing of material properties, the basic theoretical principles of alloys were developed in the late 1800s.

Broadly speaking, the development and use of alloys in the twentieth century was essentially an extension of the discoveries made in the nineteenth century. Refinements in the purity of source materials, the systematic testing of different alloy combinations and methods of heat treatment, and improvements in manufacturing techniques led to significant improvements in material properties. However, no new fundamental principles that led to radical breakthroughs were discovered during the twentieth century.

During the twentieth century, steel was the dominant engineering material in the industrialized world due to its low cost and versatility. Much of that versatility is due to a class of steels known as alloy steels. By adding other metals to the basic mix of iron and carbon found in steel, the properties of alloy steels can be varied over a wide range. Alloy steels offer the potential of increased strength, hardness, and corrosion resistance as compared to plain carbon steel. The main limitation on the use of alloy steels was that they typically cost more than plain carbon steels, though that price differential declined over the course of the twentieth century.

Although steel was the dominant engineering material in the twentieth century, a number of other alloys developed during the twentieth century found widespread use in particular applications. Higher cost limited the use of specialty alloys to particular applications where their material properties were essential for engineering reasons. This entry covers the use of alloys in mechanical applications. Alloys used in electrical applications are discussed in the entry Alloys, Magnetic.

Definitions. An alloy is a mixture of two or more metallic elements or metallic and nonmetallic elements fused together or dissolving into one another when molten. The mixture is physical, and does not involve the formation of molecular bonds. Strictly speaking, steel is an alloy, since it is a mixture of iron and carbon, but is not normally referred to in that way. Rather, when one speaks of alloy steel, one is referring to steel (iron plus carbon) with other elements added to it.

The formal definition of alloy steel is a steel where the maximum range of alloying elements content exceeds one or more of the following limits: 1.6 percent manganese, 0.6 percent silicon, or 0.6 percent copper. In addition, alloy steels are recognized as containing specific (minimum or otherwise) quantities of aluminum, boron, chromium (up to 3.99 percent), cobalt, nickel, titanium, tungsten, vanadium, zirconium, or any other alloying element that is added in order to obtain a desired alloying effect.

Somewhat confusingly, a number of alloys that are commonly referred to as alloy steels actually contain no carbon at all. For example, maraging steel is a carbon-free alloy of iron and nickel, additionally alloyed with cobalt, molybdenum, titanium and some other elements.

Another commonly used industry term is ‘‘special’’ (or in the U.S. ‘‘specialty’’) steel. Most, though not all, special steels are alloy steels, and the two terms are often used interchangeably. Other industry terms refer to the properties of the steel rather than a specific material composition. For example, ‘‘high strength’’ steel refers to any steel that can withstand loads of over 1241 MPa, while ‘‘tool-and-die’’ steel refers to any steel hard enough to be used for cutting tools, stamping dies, or similar applications.

The names of nonsteel alloys are usually defined by the names of their primary constituent metals. For example, nickel-chromium alloy consists of a mix of approximately 80 percent nickel and 20 percent chromium. Titanium alloys are primarily titanium mixed with aluminum, vanadium, molybdenum, manganese, iron or chromium. However, some alloys are referred to by trade names that have become part of the standard engineering vocabulary. A good example is Invar, an alloy of 64 percent iron and 36 percent nickel. The name is a contraction of the word ‘‘invariable,’’ reflecting Invar’s very low rate of thermal expansion.

Alloys are useful for industrial purposes because they often possess properties that pure metals do not. For example, titanium alloys have yield strengths up to five times as high as pure titanium, yet are still very low in density. Even when alloys have the same properties as pure materials, alloy materials—particularly alloy steels—are often cheaper than a pure material for a given purpose.

The differences in properties between a pure material and its alloys are due to changes in atomic microstructure brought about by the mixture of two or more types of atoms. The addition of even small amounts of an alloying element can have a major impact on the arrangement of atoms in a material and their degree of orderly arrangement. In particular, alloying elements affect the way dislocations are formed within microstructures. These changes in microstructure lead to large-scale changes in the properties of the material, and often change the way a material responds to heat treatment.

It is important to note that the addition of alloying elements can have both positive and negative effects on the properties of a material from an engineering point of view. In the manufacture of alloys, it is often just as important to avoid or remove certain chemical elements as it is to add them. Careful control of the chemical composition of raw materials and various processing techniques are used to minimize the presence of undesirable elements.