Page 139 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

70o -
Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment

'-‘quid X-solid soiurion
600 _ XA-quenched, solid solution retained
K + liquid
K AB-age hardened, precipitation starts
f " (submicroscopic)
500 - 9 X AC-overaging, precipitate
.E agglomerates
a A
Q- ‘
§
200 - f *- " ’
K _l' (b
8 "'
<16
A B C
100 95 90 Aluminum (Al) Time->
O 5 10 Copper (Cu)
Composition (% by weight)

(8) (D)
FIGURE 4.2l (a) Phase diagram for the aluminum-copper alloy system. (b) Various
microstructures obtained during the age-hardening process.

precipitates form because the solid solubility of one element (one component of the
alloy) in the other is exceeded.
Three stages are involved in precipitation hardening; they can best be described
by reference to the phase diagram for the aluminum-copper system (Fig. 4.21a). For
an alloy with the composition 95.5% Al-4.5% Cu, a single-phase (kappa phase) sub-
stitutional solid solution of copper (solute) in aluminum (solvent) exists between 500°
and 5 70°C. This kappa phase is aluminum rich, has an fcc structure, and is ductile.
Below the lower temperature (that is, below the lower solubility curve) there are two
phases: kappa (K) and theta (6), which is a hard intermetallic compound of CuAl2.
This alloy can be heat treated, and its properties are modified by two different meth-
ods: solution treatment and precipitation hardening.

4.9.l Solution Treatment

In solution treatment, the alloy is heated to within the solid-solution kappa phase-
say, 54O°C-and then cooled rapidly-for instance, by quenching it in water. The
structure obtained soon after quenching (A in Fig. 4.21b) consists only of the single
phase kappa; this alloy has moderate strength and considerable ductility.

4.9.2 Precipitation Hardening
The structure obtained in A in Fig. 4.21b can be made stronger by precipitation
hardening. In this process, the alloy is reheated to an intermediate temperature and
then held there for a period of time, during which precipitation takes place. The
copper atoms diffuse to nucleation sites and combine with aluminum atoms; this
process produces the theta phase, which forms as submicroscopic precipitates
(shown in B by the small dots within the grains of the kappa phase). This structure
is stronger than that in A, although it is less ductile. The increase in strength is due
to increased resistance to dislocation movement in the region of the precipitates.

(°C)
Temperature