Page 326 - Materials Science and Engineering An Introduction

WHY STUDY Phase Diagrams?

One reason that a knowledge and understanding of are nevertheless useful in understanding the develop-
phase diagrams is important to the engineer relates to ment and preservation of nonequilibrium structures
the design and control of heat-treating procedures; and their attendant properties; it is often the case
some properties of materials are functions of their that these properties are more desirable than those
microstructures and, consequently, of their thermal associated with the equilibrium state. This is aptly
histories. Even though most phase diagrams represent illustrated by the phenomenon of precipitation
stable (or equilibrium) states and microstructures, they hardening (Section 11.9).

Learning Objectives
After studying this chapter, you should be able to do the following:
1. (a) Schematically sketch simple isomorphous congruent phase transformations;
and eutectic phase diagrams. and
(b) On these diagrams, label the various phase (b) write reactions for all these transformations
regions. for either heating or cooling.
(c) Label liquidus, solidus, and solvus lines. 4. Given the composition of an iron–carbon alloy
2. Given a binary phase diagram, the composition containing between 0.022 and 2.14 wt% C, be
of an alloy, and its temperature; and assuming able to
that the alloy is at equilibrium, determine the (a) specify whether the alloy is hypoeutectoid or
following: hypereutectoid,
(a) what phase(s) is (are) present, (b) name the proeutectoid phase,
(b) the composition(s) of the phase(s), and (c) compute the mass fractions of proeutectoid
(c) the mass fraction(s) of the phase(s). phase and pearlite, and
3. For some given binary phase diagram, do the (d) make a schematic diagram of the micro-
following: structure at a temperature just below the
(a) locate the temperatures and compositions of eutectoid.
all eutectic, eutectoid, peritectic, and

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, cast-
ing, 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.

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 term component is 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. Solute and solvent,
298 •