Page 134 - Materials Science and Engineering An Introduction

WHY STUDY Imperfections in Solids?

The properties of some materials are profoundly example, brass (70% copper/30% zinc) is much harder
influenced by the presence of imperfections. and stronger than pure copper (Section 7.9).
Consequently, it is important to have a knowledge Also, integrated-circuit microelectronic devices
about the types of imperfections that exist and the found in our computers, calculators, and home
roles they play in affecting the behavior of materials. appliances function because of highly controlled
For example, the mechanical properties of pure metals concentrations of specific impurities that are
experience significant alterations when the metals are incorporated into small, localized regions of
alloyed (i.e., when impurity atoms are added)—for semiconducting materials (Sections 18.11 and 18.15).

Learning Objectives
After studying this chapter, you should be able to do the following:
1. Describe both vacancy and self-interstitial the weight percent and atom percent for each
crystalline defects. element.
2. Calculate the equilibrium number of vacancies in 5. For each of edge, screw, and mixed dislocations:
a material at some specified temperature, given (a) describe and make a drawing of the
the relevant constants. dislocation,
3. Name the two types of solid solutions and (b) note the location of the dislocation line, and
provide a brief written definition and/or (c) indicate the direction along which the
schematic sketch of each. dislocation line extends.
4. Given the masses and atomic weights of two 6. Describe the atomic structure within the vicinity of
or more elements in a metal alloy, calculate (a) a grain boundary and (b) a twin boundary.

4.1 INTRODUCTION
Thus far it has been tacitly assumed that perfect order exists throughout crystalline ma-
terials on an atomic scale. However, such an idealized solid does not exist; all contain
imperfection large numbers of various defects or imperfections. As a matter of fact, many of the prop-
erties of materials are profoundly sensitive to deviations from crystalline perfection;
the influence is not always adverse, and often specific characteristics are deliberately
fashioned by the introduction of controlled amounts or numbers of particular defects,
as detailed in succeeding chapters.
A crystalline defect refers to a lattice irregularity having one or more of its dimen-
sions on the order of an atomic diameter. Classification of crystalline imperfections is
frequently made according to the geometry or dimensionality of the defect. Several
point defect different imperfections are discussed in this chapter, including point defects (those as-
sociated with one or two atomic positions); linear (or one-dimensional) defects; and in-
terfacial defects, or boundaries, which are two-dimensional. Impurities in solids are also
discussed, because impurity atoms may exist as point defects. Finally, techniques for the
microscopic examination of defects and the structure of materials are briefly described.

Point Defects

4.2 VACANCIES AND SELF-INTERSTITIALS
vacancy The simplest of the point defects is a vacancy, or vacant lattice site, one normally oc-
cupied but from which an atom is missing (Figure 4.1). All crystalline solids contain
vacancies, and, in fact, it is not possible to create such a material that is free of these

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