Callister - An introduction - 8th edition

438 • Chapter 11 / Applications and Processing of Metal Alloys
treatment consists of heating the alloy to a temperature within the phase field—
say, T 0 —and waiting until all of the phase that may have been present is completely
dissolved. At this point, the alloy consists only of an phase of composition
C 0 . This procedure is followed by rapid cooling or quenching to temperature T 1 ,
which for many alloys is room temperature, to the extent that any diffusion and the
accompanying formation of any of the phase are prevented. Thus, a nonequilibrium
situation exists in which only the -phase solid solution supersaturated with
B atoms is present at T 1 ; in this state the alloy is relatively soft and weak. Furthermore,
for most alloys diffusion rates at T 1 are extremely slow, such that the single
phase is retained at this temperature for relatively long periods.
precipitation heat
treatment
overaging
Precipitation Heat Treating
For the second or precipitation heat treatment, the supersaturated solid solution
is ordinarily heated to an intermediate temperature T 2 (Figure 11.21) within the
two-phase region, at which temperature diffusion rates become appreciable.
The precipitate phase begins to form as finely dispersed particles of composition
C , which process is sometimes termed aging. After the appropriate aging time
at T 2 , the alloy is cooled to room temperature; normally, this cooling rate is not an
important consideration. Both solution and precipitation heat treatments are represented
on the temperature-versus-time plot, Figure 11.22. The character of these
particles, and subsequently the strength and hardness of the alloy, depend on
both the precipitation temperature T 2 and the aging time at this temperature. For
some alloys, aging occurs spontaneously at room temperature over extended time
periods.
The dependence of the growth of the precipitate particles on time and
temperature under isothermal heat treatment conditions may be represented by
C-shaped curves similar to those in Figure 10.18 for the eutectoid transformation
in steels. However, it is more useful and convenient to present the data as
tensile strength, yield strength, or hardness at room temperature as a function
of the logarithm of aging time, at constant temperature T 2 . The behavior for a
typical precipitation-hardenable alloy is represented schematically in Figure
11.23. With increasing time, the strength or hardness increases, reaches a
maximum, and finally diminishes. This reduction in strength and hardness that
occurs after long time periods is known as overaging. The influence of temperature
is incorporated by the superposition, on a single plot, of curves at a variety
of temperatures.
Temperature
Solution heat
Figure 11.22 Schematic
treatment
temperature-versus-time plot
T 0
showing both solution and
Quench
precipitation heat treatments
for precipitation hardening.
Precipitation
heat treatment
T 2
T 1
Time