GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59
2nd Revised Pages442 • Chapter 11 / Phase TransformationsComposition (wt% B)TemperatureA
B + L + LL + T 1C C 0NM
T 2T 0CFigure 11.40 Hypothetical
phase diagram for a
precipitation-hardenable alloy
of compositionC 0.binary system. The phase diagram must be of the form shown for the hypothetical
A–B system in Figure 11.40.
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 these
conditions are satisfied by this hypothetical phase diagram (Figure 11.40). The maxi-
mum solubility corresponds to the composition at pointM. In addition, the solubility
limit boundary between theαandα+βphase fields diminishes from this maximum
concentration to a very low B content in A at pointN. Furthermore, the composition
of a precipitation-hardenable alloy must be less than the maximum solubility. These
conditions are necessary butnotsufficient 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 solution heat treatmentin which all solute atoms are dissolved to form a single-phase
treatment solid solution. Consider an alloy of compositionC 0 in Figure 11.40. The treatment
consists of heating the alloy to a temperature within theαphase field—say,T 0 —and
waiting until all theβphase that may have been present is completely dissolved. At
this point, the alloy consists only of anαphase of compositionC 0. This procedure is
followed by rapid cooling or quenching to temperatureT 1 , which for many alloys is
room temperature, to the extent that any diffusion and the accompanying formation
of any of theβphase are prevented. Thus, a nonequilibrium situation exists in which
only theα-phase solid solution supersaturated with B atoms is present atT 1 ; in this
state the alloy is relatively soft and weak. Furthermore, for most alloys diffusion rates
atT 1 are extremely slow, such that the singleαphase is retained at this temperature
for relatively long periods.Precipitation Heat Treating
precipitation heat For the second orprecipitation heat treatment,the supersaturatedαsolid solution is
treatment ordinarily heated to an intermediate temperatureT 2 (Figure 11.40) within theα+β
two-phase region, at which temperature diffusion rates become appreciable. The
βprecipitate phase begins to form as finely dispersed particles of compositionCβ,
which process is sometimes termed “aging.” After the appropriate aging time atT 2 ,