GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59
2nd Revised Pages11.11 Mechanism of Hardening • 445Duration of precipitation heat treatment (h)
(a)260 °C
(500°F)204 °C
(400°F)149 °C
(300°F)121 °C
(250°F)Yield strength (MPa) Yield strength (ksi)010 –2 10 –1 11 min 1 h 1 day 1 month 1 year10 102 103 10401002003004000102030405060701 min 1 h 1 day 1 month 1 year1030200Ductility (%EL in 2 in. or 50 mm)121 °C
(250°F)149 °C
(300°F)204 °C
(400°F)260 °C
(500°F)Duration of precipitation heat treatment (h)010 –2 10 –1 110102 103 104(b)Figure 11.45 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)
yield 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.]particles for a precipitation-hardened 7150 aluminum alloy are shown in the electron
micrograph of the chapter-opening photograph for this chapter.
The strengthening and hardening effects shown in Figure 11.42 result from the
innumerable particles of these transition and metastable phases. As noted in the
figure, maximum strength coincides with the formation of theθ′′phase, which may
be preserved upon cooling the alloy to room temperature. Overaging results from
continued particle growth and the development ofθ′andθphases.
The strengthening process is accelerated as the temperature is increased. This is
demonstrated in Figure 11.45a, a plot of yield strength versus the logarithm of time
for a 2014 aluminum alloy at several different precipitation temperatures. Ideally,
temperature and time for the precipitation heat treatment should be designed to
produce a hardness or strength in the vicinity of the maximum. Associated with an
increase in strength is a reduction in ductility, which is demonstrated in Figure 11.45b
for the same 2014 aluminum alloy at the several temperatures.