Page 385 - Materials Science and Engineering An Introduction

WHY STUDY Phase Transformations?

The development of a set of desirable mechanical treatment for some alloy that will yield the desired
characteristics for a material often results from a phase room-temperature mechanical properties. For example,
transformation that is wrought by a heat treatment. the tensile strength of an iron–carbon alloy of eutec-
The time and temperature dependencies of some toid composition (0.76 wt% C) can be varied between
phase transformations are conveniently represented approximately 700 MPa (100,000 psi) and 2000 MPa
on modified phase diagrams. It is important to know (300,000 psi) depending on the heat treatment
how to use these diagrams in order to design a heat employed.

Learning Objectives
After studying this chapter, you should be able to do the following:
1. Make a schematic fraction transformation- 3. Cite the general mechanical characteristics for
versus-logarithm of time plot for a typical solid– each of the following microconstituents: fine
solid transformation; cite the equation pearlite, coarse pearlite, spheroidite, bainite,
that describes this behavior. martensite, and tempered martensite; briefly
2. Briefly describe the microstructure for each of explain these behaviors in terms of microstruc-
the following microconstituents that are found ture (or crystal structure).
in steel alloys: fine pearlite, coarse pearlite, 4. Given the isothermal transformation (or
spheroidite, bainite, martensite, and tempered continuous-cooling transformation) diagram for
martensite. some iron–carbon alloy, design a heat treatment
that will produce a specified microstructure.

10.1 INTRODUCTION
One reason metallic materials are so versatile is that their mechanical properties
(strength, hardness, ductility, etc.) are subject to control and management over relatively
large ranges. Three strengthening mechanisms were discussed in Chapter 7—namely
grain size refinement, solid-solution strengthening, and strain hardening. Additional
techniques are available in which the mechanical behavior of a metal alloy is influenced
by its microstructure.
The development of microstructure in both single- and two-phase alloys typically
involves some type of phase transformation—an alteration in the number and/or char-
acter of the phases. The first portion of this chapter is devoted to a brief discussion of
some of the basic principles relating to transformations involving solid phases. Because
most phase transformations do not occur instantaneously, consideration is given to the
transformation rate dependence of reaction progress on time, or the transformation rate. This is followed
by a discussion of the development of two-phase microstructures for iron–carbon alloys.
Modified phase diagrams are introduced that permit determination of the microstruc-
ture that results from a specific heat treatment. Finally, other microconstituents in addi-
tion to pearlite are presented and, for each, the mechanical properties are discussed.

Phase Transformations

10.2 BASIC CONCEPTS

phase transformation A variety of phase transformations are important in the processing of materials, and
usually they involve some alteration of the microstructure. For purposes of this dis-
cussion, these transformations are divided into three classifications. In one group are

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