Page 79 - Mechanical Behavior of Materials

80 Chapter 3 A Survey of Engineering Materials

Figure 3.10 Effects of alloying additions and processing (x-axis) on the yield strength
of aluminum alloys. Pure aluminum can be strengthened by cold work, and alloying
increases strength due to solid-solution hardening. The higher strength alloys are heat
treated to produce precipitation hardening. (Adapted from an illustration courtesy of
R. W. Landgraf, Howell, MI.)

3.4 NONFERROUS METALS

Quenching and tempering to produce a martensitic structure is the most effective means of
strengthening steels. In the common nonferrous metals, martensite may not occur; and where it
does occur, the effect is not as large as in steels. Hence, the other methods of strengthening, which
are generally less effective, must be used. The higher strength nonferrous metals often employ
precipitation hardening.
For example, consider the strength levels achievable in aluminum alloys, as illustrated by
Fig. 3.10. Annealed pure aluminum is very weak and can be strengthened only by cold work.
Adding magnesium provides solid-solution strengthening, and the resulting alloy can be cold
worked. Further strengthening is possible by precipitation hardening, which is achieved by various
combinations of alloying elements and aging treatments. However, the highest strength available is
only about 25% of that for the highest strength steel. Aluminum is nevertheless widely used, as in
aerospace applications, where its light weight and corrosion resistance are major advantages that
offset the disadvantage of lower strength than some steels.
We will now discuss the nonferrous metals that are commonly used in structural applications.

3.4.1 Aluminum Alloys

For aluminum alloys produced in wrought form, as by rolling or extruding, the naming system
involves a four-digit number. The first digit specifies the major alloying elements as listed in
Table 3.6. Subsequent digits are then assigned to indicate specific alloys, with some examples being
given in Table 3.7. The UNS numbers for wrought alloys are similar, except that A9 precedes the