Alloys are usually stronger than pure metals, although they generally offer reduced electrical and thermal conductivity. Strength is the most important criterion by which many structural materials are judged. Therefore, alloys are used for engineering construction. Steel, probably the most common structural metal, is a good example of an alloy. It is an alloy of iron and carbon, with other elements to give it certain desirable properties.

As mentioned in the previous chapter, it is sometimes possible for a material to be composed of several solid phases. The strengths of these materials are enhanced by allowing a solid structure to become a form composed of two interspersed phases. When the material in question is an alloy, it is possible to quench (discussed in more detail in Module 2, Properties of Metals) the metal from a molten state to form the interspersed phases. The type and rate of quenching determines the final solid structure and, therefore, its properties.

Type 304 Stainless Steel

Type 304 stainless steel (containing 18%-20% chromium and 8%-10.5% nickel) is used in the tritium production reactor tanks, process water piping, and original process heat exchangers. This alloy resists most types of corrosion.

Composition of Common Engineering Materials

The wide variety of structures, systems, and components found in DOE nuclear facilities are made from many different types of materials. Many of the materials are alloys with a base metal of iron, nickel, or zirconium. The selection of a material for a specific application is based on many factors including the temperature and pressure that the material will be exposed to, the materials resistance to specific types of corrosion, the materials toughness and hardness, and other material properties.

One material that has wide application in the systems of DOE facilities is stainless steel. There are nearly 40 standard types of stainless steel and many other specialized types under various trade names. Through the modification of the kinds and quantities of alloying elements, the steel can be adapted to specific applications. Stainless steels are classified as austenitic or ferritic based on their lattice structure. Austenitic stainless steels, including 304 and 316, have a facecentered cubic structure of iron atoms with the carbon in interstitial solid solution. Ferritic stainless steels, including type 405, have a body-centered cubic iron lattice and contain no nickel. Ferritic steels are easier to weld and fabricate and are less susceptible to stress corrosion cracking than austenitic stainless steels. They have only moderate resistance to other types of chemical attack.

Other metals that have specific applications in some DOE nuclear facilities are inconel and zircaloy. The composition of these metals and various types of stainless steel are listed in Table 2 below.

Summary

The important information in this chapter is summarized below.

Alloys Summary

An alloy is a mixture of two or more materials, at least one of which is a metal.

Alloy microstructures

Solid solutions, where secondary atoms introduced as substitutionals or interstitials in a crystal lattice.

Crystal with metallic bonds

Composites, where secondary crystals are imbedded in a primary polycrystalline matrix.

Alloys are usually stronger than pure metals although alloys generally have reduced electrical and thermal conductivities than pure metals.

The two desirable properties of type 304 stainless steel are corrosion resistance and high toughness.