Structure of MetalsDOE-HDBK-1017/1-93ALLOYSALLOYSMost of the materials used in structural engineering or component fabrication aremetals. Alloying is a common practice because metallic bonds allow joining ofdifferent types of metals. EO 1.10DEFINE the term alloy.EO 1.11DESCRIBE an alloy as to the three possible microstructuresand the two general characteristics as compared to pure metals.EO 1.12IDENTIFY the two desirable properties of type 304 stainlesssteel.An alloy is a mixture of two or more materials, at least one of which is a metal. Alloys canhave a microstructure consisting of solid solutions, where secondary atoms are introduced assubstitutionals or interstitials (discussed further in the next chapter and Module 5, PlantMaterials) in a crystal lattice. An alloy might also be a crystal with a metallic compound at eachlattice point. In addition, alloys may be composed of secondary crystals imbedded in a primarypolycrystalline matrix. This type of alloy is called a composite (although the term "composite"does not necessarily imply that the component materials are metals). Module 2, Properties ofMetals, discusses how different elements change the physical properties of a metal.Alloys are usually stronger than pure metals, although they generally offer reduced electrical andthermal conductivity. Strength is the most important criterion by which many structuralmaterials are judged. Therefore, alloys are used for engineering construction. Steel, probablythe most common structural metal, is a good example of an alloy. It is an alloy of iron andcarbon, with other elements to give it certain desirable properties.As mentioned in the previous chapter, it is sometimes possible for a material to be composedof several solid phases. The strengths of these materials are enhanced by allowing a solidstructure to become a form composed of two interspersed phases. When the material in questionis 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 quenchingdetermines the final solid structure and, therefore, its properties.Rev. 0Page 15MS-01
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