Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59

2nd Revised Pages

11.11 Mechanism of Hardening • 443

Solution heat

treatment

Quench

Precipitation

heat treatment

Ti m e

Temperature

T 0

T 2

T 1

Figure 11.41 Schematic

temperature-versus-time plot

showing both solution and

precipitation heat treatments for

precipitation hardening.

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 11.41. The character of theseβparticles, and

subsequently the strength and hardness of the alloy, depend on both the precipitation

temperatureT 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 precipitateβparticles on time and temper-

ature under isothermal heat treatment conditions may be represented by C-shaped

curves similar to those in Figure 11.18 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 temperatureT 2. The behavior for a typical precipitation-

hardenable alloy is represented schematically in Figure 11.42. 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 overaging.The influence of temperature is incorporated by the superposition, on a

single plot, of curves at a variety of temperatures.

11.11 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 ex-

tensively for the aluminum–copper alloys. Figure 11.43 presents the aluminum-rich

portion of the aluminum–copper phase diagram. Theαphase is a substitutional solid

Strength or hardness

Zones

Overaging

Logarithm of aging time



" '

Figure 11.42

Schematic diagram

showing strength and

hardness as a

function of the

logarithm of aging

time at constant

temperature during

the precipitation heat

treatment.