c09 - 2nd REVISE PAGES 1496T_c09_252-310 12/21/05 8:30 Page...

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252 Chapter 9 Phase Diagrams A scanning electron micrograph showing the microstructure of a plain carbon steel that contains 0.44 wt% C. The large dark areas are proeutectoid ferrite. Regions having the alternating light and dark lamellar structure are pearlite; the dark and light layers in the pearlite correspond, respectively, to ferrite and cementite phases. During etching of the surface prior to examina- tion, the ferrite phase was preferentially dissolved; thus, the pearlite appears in topographical relief with cementite layers being elevated above the ferrite layers. 3000 ± . (Micrograph courtesy of Republic Steel Corporation.) One reason that a knowledge and understanding of phase diagrams is important to the engineer relates to the design and control of heat-treating procedures; some properties of materials are functions of their mi- crostructures, and, consequently, of their thermal his- tories. Even though most phase diagrams represent stable (or equilibrium) states and microstructures, they are nevertheless useful in understanding the develop- ment and preservation of nonequilibrium structures and their attendant properties; it is often the case that these properties are more desirable than those associated with the equilibrium state. This is aptly illustrated by the phenomenon of precipitation hardening (Section 11.9). WHY STUDY Phase Diagrams ? 1496T_c09_252-310 12/21/05 8:30 Page 252 2nd REVISE PAGES
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9.1 INTRODUCTION The understanding of phase diagrams for alloy systems is extremely important be- cause there is a strong correlation between microstructure and mechanical properties, and the development of microstructure of an alloy is related to the characteristics of its phase diagram. In addition, phase diagrams provide valuable information about melting, casting, crystallization, and other phenomena. This chapter presents and discusses the following topics: (1) terminology asso- ciated with phase diagrams and phase transformations; (2) pressure–temperature phase diagrams for pure materials;(3) the interpretation of phase diagrams;(4) some of the common and relatively simple binary phase diagrams, including that for the iron–carbon system; and (5) the development of equilibrium microstructures, upon cooling, for several situations. Definitions and Basic Concepts It is necessary to establish a foundation of definitions and basic concepts relating to alloys, phases, and equilibrium before delving into the interpretation and uti- lization of phase diagrams. The term component is frequently used in this discus- sion; components are pure metals and/or compounds of which an alloy is composed. For example, in a copper–zinc brass, the components are Cu and Zn. Solute and solvent, which are also common terms, were defined in Section 4.3. Another term used in this context is system, which has two meanings. First, “system” may refer to a specific body of material under consideration (e.g., a ladle of molten steel). Or it may relate to the series of possible alloys consisting of the same components, but without regard to alloy composition (e.g., the iron–carbon system).
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This note was uploaded on 10/03/2009 for the course E E344 taught by Professor Libera during the Spring '09 term at Stevens.

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c09 - 2nd REVISE PAGES 1496T_c09_252-310 12/21/05 8:30 Page...

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