GTBL042-10 GTBL042-Callister-v2 August 13, 2007 18:16
Learning Objectives
After careful study of this chapter you should be able to do the following:- (a)Schematically sketch simple isomorphous and
eutectic phase diagrams.
(b)On these diagrams label the various phase
regions.
(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
following:
(a)locate the temperatures and compositions of
all eutectic, eutectoid, peritectic, and
congruent phase transformations; and
(b)write reactions for all these transformations
for either heating or cooling.
4.Given the composition of an iron–carbon alloy
containing between 0.022 wt% C and 2.14 wt%
C, be able to
(a)specify whether the alloy is hypoeutectoid or
hypereutectoid,
(b)name the proeutectoid phase,
(c)compute the mass fractions of proeutectoid
phase and pearlite, and
(d)make a schematic diagram of the
microstructure at a temperature just below
the eutectoid.10.1 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 as-
sociated 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 utiliza-
component tion of phase diagrams. The termcomponentis frequently used in this discussion;
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.Soluteandsolvent,
which are also common terms, were defined in Section 5.4. Another term used in this
system context issystem,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).
The concept of a solid solution was introduced in Section 5.4. By way of review,
a solid solution consists of atoms of at least two different types; the solute atoms