IE370Lecture4 - Solid Solutions Chapter 04 Equilibrium...

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9/13/2008 1 Chapter 04: Equilibrium Phase Diagrams and the Iron-Carbon System Alloys An alloy = a mixture or compound of two or more elements, at least one of which is metallic s Two main categories: 1. Solid solutions 2. Intermediate phases Solid Solutions An alloy in which one element is dissolved in another to form a single-phase structure s A phase = any homogeneous mass of material, such as a metal in which the grains all have the same crystal lattice structure s In a solid solution, the solvent or base element is metallic, and the dissolved element can be either metallic or nonmetal Two Forms of Solid Solutions 1. Substitutional solid solution - atoms of solvent element are replaced in its unit cell by dissolved element 2. Interstitial solid solution - atoms of dissolving element fit into vacant spaces between base metal atoms in the lattice structure s In both forms, the alloy structure is generally stronger and harder than either of the component elements
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9/13/2008 2 Figure 6.1 - Two forms of solid solutions: (a) substitutional solid solution, and (b) interstitial solid solution Intermediate Phases s There are usually limits to the solubility of one element in another s When the amount of the dissolving element in the alloy exceeds the solid solubility limit of the base metal, a second phase forms in the alloy s The term intermediate phase is used to describe it because its chemical composition is intermediate between the two pure elements s Its crystalline structure is also different from those of the pure metals Types of Intermediate Phases 1. Metallic compounds – consist of a metal and nonmetal, such as Fe 3 C 2. Intermetallic compounds - two metals that form a compound, such as Mg 2 Pb s In some alloy compositions, the intermediate phase is mixed with the primary solid solution to form a two-phase structure s Some two-phase alloys are important because they can be heat treated for much
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IE370Lecture4 - Solid Solutions Chapter 04 Equilibrium...

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