Callister - An introduction - 8th edition

11.9 Precipitation Hardening • 437
Temperature
Figure 11.21 Hypothetical
phase diagram for a
L
precipitation-hardenable alloy
of composition C 0 .
+ L
+ L
T
0 M
C
T 2
+
T 1
C C 0
A
N
B
Composition (wt% B)
precipitation
hardening
solution heat
treatment
that are induced by appropriate heat treatments. The process is called precipitation
hardening because the small particles of the new phase are termed precipitates. Age
hardening is also used to designate this procedure because the strength develops
with time, or as the alloy ages. Examples of alloys that are hardened by precipitation
treatments include aluminum–copper, copper–beryllium, copper–tin, and
magnesium–aluminum; some ferrous alloys are also precipitation hardenable.
Precipitation hardening and the treating of steel to form tempered martensite
are totally different phenomena, even though the heat treatment procedures
are similar; therefore, the processes should not be confused. The principal difference
lies in the mechanisms by which hardening and strengthening are achieved.
These should become apparent as precipitation hardening is explained.
Heat Treatments
Inasmuch as precipitation hardening results from the development of particles of
a new phase, an explanation of the heat treatment procedure is facilitated by use
of a phase diagram. Even though, in practice, many precipitation-hardenable alloys
contain two or more alloying elements, the discussion is simplified by reference to
a binary system. The phase diagram must be of the form shown for the hypothetical
A–B system in Figure 11.21.
Two requisite features must be displayed by the phase diagrams of alloy systems
for precipitation hardening: an appreciable maximum solubility of one component
in the other, on the order of several percent; and a solubility limit that rapidly
decreases in concentration of the major component with temperature reduction.
Both of these conditions are satisfied by this hypothetical phase diagram (Figure
11.21). The maximum solubility corresponds to the composition at point M.In
addition, the solubility limit boundary between the and phase fields
diminishes from this maximum concentration to a very low B content in A at point
N. Furthermore, the composition of a precipitation-hardenable alloy must be less
than the maximum solubility. These conditions are necessary but not sufficient for
precipitation hardening to occur in an alloy system. An additional requirement is
discussed below.
Solution Heat Treating
Precipitation hardening is accomplished by two different heat treatments. The first
is a solution heat treatment in which all solute atoms are dissolved to form a singlephase
solid solution. Consider an alloy of composition C 0 in Figure 11.21. The