Callister - An introduction - 8th edition

11.9 Precipitation Hardening • 441
Figure 11.27 The
precipitation
hardening
characteristics of a
2014 aluminum alloy
(0.9 wt% Si, 4.4 wt%
Cu, 0.8 wt% Mn, 0.5
wt% Mg) at four
different aging
temperatures:
(a) tensile strength,
and (b) ductility
(%EL). [Adapted
from Metals
Handbook:
Properties and
Selection: Nonferrous
Alloys and Pure
Metals, Vol. 2, 9th
edition, H. Baker
(Managing Editor),
American Society for
Metals, 1979, p. 41.]
Tensile strength (MPa)
Ductility (% EL in 2 in. or 50 mm)
500
400
300
260°C
(500°F)
Duration of precipitation heat treatment (h)
(a)
121°C
(250°F)
149°C
(300°F)
204°C
(400°F)
200
30
0 10 –2 10 –1 1 10 10 2 10 3 10 4
30
20
10
1 min 1 h 1 day 1 month 1 year
1 min 1 h 1 day 1 month 1 year
204°C
(400°F)
260°C
(500°F)
149°C
(300°F)
121°C
(250°F)
70
60
50
40
Tensile strength (ksi)
0
0 10 –2 10 –1 1 10 10 2 10 3 10 4
Duration of precipitation heat treatment (h)
(b)
natural aging,
artificial aging
addition, lattice strains must be established at the precipitate–matrix interface. For
aluminum–copper alloys, there is a distortion of the crystal lattice structure around
and within the vicinity of particles of these transition phases (Figure 11.25b). During
plastic deformation, dislocation motions are effectively impeded as a result of
these distortions, and, consequently, the alloy becomes harder and stronger. As the
phase forms, the resultant overaging (softening and weakening) is explained by a
reduction in the resistance to slip that is offered by these precipitate particles.
Alloys that experience appreciable precipitation hardening at room temperature
and after relatively short time periods must be quenched to and stored under
refrigerated conditions. Several aluminum alloys that are used for rivets exhibit this
behavior. They are driven while still soft, then allowed to age harden at the normal
ambient temperature. This is termed natural aging; artificial aging is carried out at
elevated temperatures.
Miscellaneous Considerations
The combined effects of strain hardening and precipitation hardening may be employed
in high-strength alloys. The order of these hardening procedures is important
in the production of alloys having the optimum combination of mechanical