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chap10 - Chapter A 10 Phase Diagrams scanning electron...

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C h a p t e r 10 / Phase Diagrams A scanning electron micro- graph which shows the micro- structure 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 examination, the fer- rite phase was preferentially dissolved; thus, the pearlite appears in topographical re- lief with cementite layers be- ing elevated above the ferrite layers. 3000 . (Micrograph courtesy of Republic Steel Corporation.) Why Study Phase Diagrams? One reason why a knowledge and understanding of phase diagrams is important to the engineer relates to the design and control of heat treating proce- dures; some properties of materials are functions of their microstructures, and, consequently, of their thermal histories. Even though most phase dia- grams represent stable (or equilibrium) states and microstructures, they are, nevertheless useful in un- derstanding the development and preservation of nonequilibrium structures and their attendant prop- erties; it is often the case that these properties are more desirable than those associated with the equi- librium state. This is aptly illustrated by the phe- nomenon of precipitation hardening (Sections 11.10 and 11.11). 281
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L e a r n i n g O b j e c t i v e s After studying this chapter you should be able to do the following: 1. (a) Schematically sketch simple isomorphous and eutectic phase diagrams. (b) On these diagrams label the various phase re- gions. (c) Label liquidus, solidus, and solvus lines. 2. Given a binary phase diagram, the composition of an alloy, its temperature, and assuming that the alloy is at equilibrium, determine: (a) what phase(s) is (are) present; (b) the composition(s) of the phase(s); and (c) the mass fraction(s) of the phase(s). 3. For some given binary phase diagram, do the fol- lowing: (a) locate the temperatures and compositions of all eutectic, eutectoid, peritectic, and congru- ent phase transformations; and 10.1 I NTRODUCTION The understanding of phase diagrams for alloy systems is extremely important because there is a strong correlation between microstructure and mechanical proper- ties, and the development of microstructure of an alloy is related to the characteris- tics 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 associ- ated with phase diagrams and phase transformations; (2) the interpretation of phase diagrams; (3) some of the common and relatively simple binary phase diagrams, including that for the iron–carbon system; and (4) the development of equilibrium microstructures, upon cooling, for several situations.
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