Page 70 - Mechanical Behavior of Materials

Section 3.2 Alloying and Processing of Metals 71

Figure 3.4 Precipitation hardening of aluminum alloyed with 4% Cu. Slow cooling from
a solid solution (a) produces grain boundary precipitates (b). Rapid cooling to obtain
a supersaturated solution (c) can be followed by aging at a moderate temperature to
obtain fine precipitates within grains (d), but overaging gives coarse precipitates (e).

Figure 3.5 Effects of precipitation (aging) time and temperature on the resulting yield
strength in aluminum alloy 6061. (Adapted from [Boyer 85] p. 6.7; used with permission.)

the precipitate to form as fairly uniformly scattered small particles. However, if the intermediate
temperature is too high, or the holding time too long, the particles coalesce into larger ones and
some of the benefit is lost, the particles being too far apart to effectively impede dislocation motion.
Thus, for a given temperature, there is a precipitation (aging) time that gives the maximum effect.
The resulting trends in strength are illustrated for a commercial aluminum alloy, where similar
precipitation hardening occurs, in Fig. 3.5.
If an alloy contains interspersed regions of more than one chemical composition, as for
precipitate particles, as just discussed, a multiple phase situation is said to exist. Other multiple