Callister - An introduction - 8th edition

440 • Chapter 11 / Applications and Processing of Metal Alloys
Solvent (Al) atom
Solute (Cu)
atom
" Phase particle Phase particle
Figure 11.26 A transmission electron
micrograph showing the microstructure
of a 7150-T651 aluminum alloy (6.2 Zn,
2.3 Cu, 2.3 Mg, 0.12 Zr, the balance Al)
that has been precipitation hardened.
The light matrix phase in the
micrograph is an aluminum solid
solution. The majority of the small
plate-shaped dark precipitate particles
are a transition h phase, the remainder
being the equilibrium h (MgZn 2 )
phase. Note that grain boundaries are
“decorated” by some of these particles.
90,000. (Courtesy of G. H. Narayanan
and A. G. Miller, Boeing Commercial
Airplane Company.)
(a) (b) (c)
Figure 11.25 Schematic depiction of several stages in the formation of the equilibrium
precipitate () phase. (a) A supersaturated solid solution. (b) A transition, , precipitate
phase. (c) The equilibrium phase, within the -matrix phase.
transition phases (denoted as and ), before the formation of the equilibrium
phase (Figure 11.25c). Transition phase particles for a precipitation-hardened 7150
aluminum alloy are shown in the electron micrograph of Figure 11.26.
The strengthening and hardening effects shown in Figure 11.23 result from the
innumerable particles of these transition and metastable phases. As noted in the
figure, maximum strength coincides with the formation of the phase, which may
100 nm
be preserved upon cooling the alloy to room temperature. Overaging results from
continued particle growth and the development of and phases.
The strengthening process is accelerated as the temperature is increased. This
is demonstrated in Figure 11.27a, a plot of tensile 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
11.27b for the same 2014 aluminum alloy at the several temperatures.
Not all alloys that satisfy the aforementioned conditions relative to composition
and phase diagram configuration are amenable to precipitation hardening. In