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

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

2nd Revised Pages

444 • Chapter 11 / Phase Transformations

(CuAl 2 )





700

600

500

400

300

1200

1000

800

600

Temperature (

°C)

Temperature (

°F)

0 5 10 20 30

(Al)

0 10 20 30 40 50

L

Composition (at % Cu)

Composition (wt % Cu)

+

+ L

+ L

Figure 11.43 The

aluminum-rich side

of the aluminum–

copper phase

diagram. (Adapted

from J. L. Murray,

International Metals

Review, 30 , 5, 1985.

Reprinted by

permission of ASM

International.)

solution of copper in aluminum, whereas the intermetallic compound CuAl 2 is des-

ignated theθphase. For an aluminum–copper alloy of, say, composition 96 wt% Al–4

wt% Cu, in the development of this equilibriumθphase during the precipitation heat

treatment, several transition phases are first formed in a specific sequence. The me-

chanical properties are influenced by the character of the particles of these transition

phases. During the initial hardening stage (at short times, Figure 11.42), copper atoms

cluster together in very small and thin discs that are only one or two atoms thick and

approximately 25 atoms in diameter; these form at countless positions within the

αphase. The clusters, sometimes called zones, are so small that they are really not

regarded as distinct precipitate particles. However, with time and the subsequent

diffusion of copper atoms, zones become particles as they increase in size. These

precipitate particles then pass through two transition phases (denoted asθ′′andθ′)

before the formation of the equilibriumθphase (Figure 11.44c). Transition-phase

(a) (b) (c)

Solvent (Al) atom Solute (Cu)

atom " Phase particle  Phase particle

Figure 11.44 Schematic depiction of several stages in the formation of the equilibrium

precipitate (θ) phase. (a) A supersaturatedαsolid solution. (b) A transition (θ′′) precipitate

phase. (c) The equilibriumθphase, within theα-matrix phase.