Page 67 - Mechanical Behavior of Materials

66 Chapter 3 A Survey of Engineering Materials

In this chapter, each major class of materials is considered in more detail. Groups of related
materials within each major class are identified, the effects of processing variables are summarized,
and the systems used for naming various materials are described. Metals and alloys are the dominant
engineering materials in current use in many applications, so more space is devoted to these than to
the others. However, polymers, ceramics and glasses, and composites are also of major importance.
Recent improvements in nonmetallic and composite materials have resulted in a trend toward these
replacing metals in some applications.
An essential part of the process of engineering design is the selection of suitable materials
from which to make engineering components. This requires at least a general knowledge of
the composition, structure, and characteristics of materials, as summarized in this chapter. For a
particular engineering component, the choice among candidate materials may sometimes be aided
by systematic analysis, for example, to minimize mass or cost. Such analysis is introduced near
the end of this chapter. Materials selection is also aided by specific prediction of strength, life,
or amount of deformation, as described in later chapters related to yielding, fracture, fatigue,
and creep.

3.2 ALLOYING AND PROCESSING OF METALS

Approximately 80% of the one-hundred-plus elements in the periodic table can be classed as
metals. A number of these possess combinations of availability and properties that lead to their
use as engineering metals where mechanical strength is needed. The most widely used engineering
metal is iron, which is the main constituent of the iron-based alloys termed steels. Some other
structural metals that are widely used are aluminum, copper, titanium, magnesium, nickel, and
cobalt. Additional common metals, such as zinc, lead, tin, and silver, are used where the stresses
are quite low, as in various low-strength cast parts and solder joints. The refractory metals, notably
molybdenum, niobium, tantalum, tungsten, and zirconium, have melting temperatures somewhat
or even substantially above that of iron (1538 C). Relatively small quantities of these are used as
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engineering metals for specialized applications, particularly where high strength is needed at a very
high temperature. Some properties and uses for selected engineering metals are given in Table 3.1.
A metal alloy is usually a melted-together combination of two or more chemical elements,
where the bulk of the material consists of one or more metals. A wide variety of metallic and
nonmetallic chemical elements are used in alloying the principal engineering metals. Some of
the more common ones are boron, carbon, magnesium, silicon, vanadium, chromium, manganese,
nickel, copper, zinc, molybdenum, and tin. The amounts and combinations of alloying elements used
with various metals have major effects on their strength, ductility, temperature resistance, corrosion
resistance, and other properties.
For a given alloy composition, the properties are further affected by the particular processing
used. Processing includes heat treatment, deformation, and casting. In heat treatment, a metal or
alloy is subjected to a particular schedule of heating, holding at temperature, and cooling that causes
desirable physical or chemical changes. Deformation is the process of forcing a piece of material
to change its thickness or shape. Some of the means of doing so are forging, rolling, extruding,
and drawing, as illustrated in Fig. 3.1. Casting is simply the pouring of melted metal into a mold
so that it conforms to the shape of the mold when it solidifies. Heat treatment and deformation or
casting may be used in combination, and particular alloying elements are often added because they