GTBL042-10 GTBL042-Callister-v2 August 13, 2007 18:16

Chapter 10 Phase Diagrams

Ascanning electron micrograph showing the microstructure of aplain 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 ferriteand cementite phases. During etching of the surface prior to examination,

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.)

WHY STUDYPhase Diagrams?

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

microstructures, and, consequently, of their thermal

histories. Even though most phase diagrams represent

stable (or equilibrium) states and microstructures, they

are nevertheless useful in understanding the

development 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 (Sections 11.10 and 11.11).

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