Callister - An introduction - 8th edition

WHY STUDY Phase 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
(Section 11.9).
In the processing/structure/properties/performance
scheme, reasons for studying phase diagrams are
as follows:
• Concepts discussed in this chapter provide a foundation
that is necessary for us to understand phase
transformations that occur in steel alloys, as well as
the consequences of these transformations—that is,
microstructural and property alterations (as presented
in Chapter 10).
Learning Objectives
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
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.
9.1 INTRODUCTION
The understanding of phase diagrams for alloy systems is extremely important
because 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 associated
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.
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