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Ch4 - Chapter 04 Equilibrium Phase Diagrams and the...

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9/2/2011 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 square6 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 square6 A phase = any homogeneous mass of material, such as a metal in which the grains all have the same crystal lattice structure square6 In a solid solution, the solvent or base element is metallic, and the dissolved element can be either metallic or nonmetal
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9/2/2011 2 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 square6 In both forms, the alloy structure is generally stronger and harder than either of the component elements Figure 6.1 - Two forms of solid solutions: (a) substitutional solid solution, and (b) interstitial solid solution Intermediate Phases square6 There are usually limits to the solubility of one element in another square6 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 square6 The term intermediate phase is used to describe it because its chemical composition is intermediate between the two pure elements square6 Its crystalline structure is also different from those of the pure metals
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9/2/2011 3 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 square6 In some alloy compositions, the intermediate phase is mixed with the primary solid solution to form a two-phase structure square6 Some two-phase alloys are important because they can be heat treated for much higher strength than solid solutions 4.1 Introduction and 4.2 Phases square6 Phase has a definable structure and composition, any homogeneous mass of material.
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