Page 328 - Materials Science and Engineering An Introduction

300 • Chapter 9 / Phase Diagrams

9.3 PHASES

phase Also critical to the understanding of phase diagrams is the concept of a phase. A phase may
be defined as a homogeneous portion of a system that has uniform physical and chemical
characteristics. Every pure material is considered to be a phase; so also is every solid, liquid,
and gaseous solution. For example, the sugar–water syrup solution just discussed is one
phase, and solid sugar is another. Each has different physical properties (one is a liquid,
the other is a solid); furthermore, each is different chemically (i.e., has a different chemical
composition); one is virtually pure sugar, the other is a solution of H 2 O and C 12 H 22 O 11 . If
Tutorial Video: more than one phase is present in a given system, each will have its own distinct properties,
Phases and
Solubility Limits and a boundary separating the phases will exist, across which there will be a discontinuous
and abrupt change in physical and/or chemical characteristics. When two phases are present
What is a Phase?
in a system, it is not necessary that there be a difference in both physical and chemical
properties; a disparity in one or the other set of properties is sufficient. When water and ice
are present in a container, two separate phases exist; they are physically dissimilar (one is
a solid, the other is a liquid) but identical in chemical makeup. Also, when a substance can
exist in two or more polymorphic forms (e.g., having both FCC and BCC structures), each
of these structures is a separate phase because their respective physical characteristics differ.
Sometimes, a single-phase system is termed homogeneous. Systems composed of two
or more phases are termed mixtures or heterogeneous systems. Most metallic alloys and,
for that matter, ceramic, polymeric, and composite systems are heterogeneous. Typically,
the phases interact in such a way that the property combination of the multiphase system
is different from, and more desirable than, either of the individual phases.

9.4 MICROSTRUCTURE
The physical properties and, in particular, the mechanical behavior of a material often
depend on the microstructure. Microstructure is subject to direct microscopic observa-
tion using optical or electron microscopes; this is touched on in Sections 4.9 and 4.10.
In metal alloys, microstructure is characterized by the number of phases present, their
proportions, and the manner in which they are distributed or arranged. The microstruc-
ture of an alloy depends on such variables as the alloying elements present, their con-
centrations, and the heat treatment of the alloy (i.e., the temperature, the heating time
at temperature, and the rate of cooling to room temperature).
The procedure of specimen preparation for microscopic examination is briefly out-
lined in Section 4.10. After appropriate polishing and etching, the different phases may be
distinguished by their appearance. For example, for a two-phase alloy, one phase may ap-
pear light and the other phase dark. When only a single phase or solid solution is present,
the texture is uniform, except for grain boundaries that may be revealed (Figure 4.15b).

9.5 PHASE EQUILIBRIA
equilibrium Equilibrium is another essential concept; it is best described in terms of a thermody-
namic quantity called the free energy. In brief, free energy is a function of the internal
free energy
energy of a system and also the randomness or disorder of the atoms or molecules (or
entropy). A system is at equilibrium if its free energy is at a minimum under some speci-
fied combination of temperature, pressure, and composition. In a macroscopic sense,
this means that the characteristics of the system do not change with time, but persist
indefinitely—that is, the system is stable. A change in temperature, pressure, and/or
composition for a system in equilibrium results in an increase in the free energy and
in a possible spontaneous change to another state by which the free energy is lowered.
phase equilibrium The term phase equilibrium, often used in the context of this discussion, refers
to equilibrium as it applies to systems in which more than one phase may exist. Phase