Page 32 - Materials Science and Engineering An Introduction

4 • Chapter 1 / Introduction

Figure 1.2 Three thin disk specimens of
aluminum oxide that have been placed over a
printed page in order to demonstrate their
differences in light-transmittance characteristics.
The disk on the left is transparent (i.e., virtually
all light that is reflected from the page passes
through it), whereas the one in the center is
translucent (meaning that some of this reflected
light is transmitted through the disk). The disk
on the right is opaque—that is, none of the light
passes through it. These differences in optical
properties are a consequence of differences in Specimen preparation, P. A. Lessing
structure of these materials, which have resulted
from the way the materials were processed.

reflected light passes through it), whereas the disks in the center and on the right are, respec-
tively, translucent and opaque. All of these specimens are of the same material, aluminum
oxide, but the leftmost one is what we call a single crystal—that is, has a high degree of
perfection—which gives rise to its transparency. The center one is composed of numerous
and very small single crystals that are all connected; the boundaries between these small
crystals scatter a portion of the light reflected from the printed page, which makes this ma-
terial optically translucent. Finally, the specimen on the right is composed not only of many
small, interconnected crystals, but also of a large number of very small pores or void spaces.
These pores also effectively scatter the reflected light and render this material opaque.
Thus, the structures of these three specimens are different in terms of crystal
boundaries and pores, which affect the optical transmittance properties. Furthermore,
each material was produced using a different processing technique. If optical transmit-
tance is an important parameter relative to the ultimate in-service application, the per-
formance of each material will be different.

1.3 WHY STUDY MATERIALS SCIENCE
AND ENGINEERING?
Why do we study materials? Many an applied scientist or engineer, whether mechani-
cal, civil, chemical, or electrical, is at one time or another exposed to a design problem
involving materials, such as a transmission gear, the superstructure for a building, an
oil refinery component, or an integrated circuit chip. Of course, materials scientists
and engineers are specialists who are totally involved in the investigation and design
of materials.
Many times, a materials problem is one of selecting the right material from the
thousands available. The final decision is normally based on several criteria. First, the
in-service conditions must be characterized, for these dictate the properties required of
the material. On only rare occasions does a material possess the maximum or ideal com-
bination of properties. Thus, it may be necessary to trade one characteristic for another.
The classic example involves strength and ductility; normally, a material having a high
strength has only a limited ductility. In such cases, a reasonable compromise between
two or more properties may be necessary.
A second selection consideration is any deterioration of material properties that
may occur during service operation. For example, significant reductions in mechanical
strength may result from exposure to elevated temperatures or corrosive environments.