Callister - An introduction - 8th edition

11.9 Precipitation Hardening • 439
Strength or hardness
Zones
"
'
Overaging
Figure 11.23 Schematic
diagram showing strength
and hardness as a
function of the logarithm
of aging time at constant
temperature during the
precipitation heat
treatment.
Logarithm of aging time
Mechanism of Hardening
Precipitation hardening is commonly employed with high-strength aluminum alloys.
Although a large number of these alloys have different proportions and combinations
of alloying elements, the mechanism of hardening has perhaps been studied
most extensively for the aluminum–copper alloys. Figure 11.24 presents the
aluminum-rich portion of the aluminum–copper phase diagram. The phase is a substitutional
solid solution of copper in aluminum, whereas the intermetallic compound
CuAl 2 is designated the phase. For an aluminum–copper alloy of, say,
composition 96 wt% Al–4 wt% Cu, in the development of this equilibrium phase
during the precipitation heat treatment, several transition phases are first formed
in a specific sequence. The mechanical properties are influenced by the character
of the particles of these transition phases. During the initial hardening stage (at
short times, Figure 11.23), copper atoms cluster together in very small and thin discs
that are only one or two atoms thick and approximately 25 atoms in diameter; these
form at countless positions within the phase. The clusters, sometimes called zones,
are so small that they are really not regarded as distinct precipitate particles. However,
with time and the subsequent diffusion of copper atoms, zones become particles
as they increase in size. These precipitate particles then pass through two
Figure 11.24 The
aluminum-rich side of
the aluminum–copper
phase diagram.
(Adapted from J. L.
Murray, International
Metals Review, 30, 5,
1985. Reprinted by
permission of ASM
International.)
Temperature (°C)
0 5 10 20 30
700
600
500
400
+ L
Composition (at% Cu)
+
L
+ L
(CuAl 2 )
1200
1000
800
Temperature (°F)
300
0 10 20 30 40 50
600
(Al)
Composition (wt% Cu)