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

GTBL042-10 GTBL042-Callister-v2 August 13, 2007 18:16

10.10 Mechanical Properties of Isomorphous Alloys • 355

There are some important consequences for isomorphous alloys that have solid-

ified under nonequilibrium conditions. As discussed previously, the distribution of

the two elements within the grains is nonuniform, a phenomenon termedsegregation;

that is, concentration gradients are established across the grains that are represented

by the insets of Figure 10.5. The center of each grain, which is the first part to freeze,

is rich in the high-melting element (e.g., nickel for this Cu–Ni system), whereas the

concentration of the low-melting element increases with position from this region to

the grain boundary. This is termed acoredstructure, and gives rise to less than the

optimal properties. As a casting having a cored structure is reheated, grain boundary

regions will melt first inasmuch as they are richer in the low-melting component.

This produces a sudden loss in mechanical integrity due to the thin liquid film that

separates the grains. Furthermore, this melting may begin at a temperature below

the equilibrium solidus temperature of the alloy. Coring may be eliminated by a ho-

mogenization heat treatment carried out at a temperature below the solidus point for

the particular alloy composition. During this process, atomic diffusion occurs, which

produces compositionally homogeneous grains.

10.10 MECHANICAL PROPERTIES

OF ISOMORPHOUS ALLOYS

We shall now briefly explore how the mechanical properties of solid isomorphous

alloys are affected by composition as other structural variables (e.g., grain size) are

held constant. For all temperatures and compositions below the melting tempera-

ture of the lowest-melting component, only a single solid phase will exist. Therefore,

each component will experience solid-solution strengthening (Section 8.10), or an

increase in strength and hardness by additions of the other component. This ef-

fect is demonstrated in Figure 10.6aas tensile strength versus composition for the

copper–nickel system at room temperature; at some intermediate composition, the

curve necessarily passes through a maximum. Plotted in Figure 10.6bis the ductility

(%EL)–composition behavior, which is just the opposite of tensile strength; that is,

ductility decreases with additions of the second component, and the curve exhibits a

minimum.

Tensile strength (MPa) Tensile strength (ksi)

Elongation (% in 50 mm [2 in.])

400

300

200

0

(Cu)

20 40 60 80 100

Composition (wt% Ni) (Ni) Composition (wt% Ni)

(a) (b)

60

50

40

30

60

50

40

30

20

0

(Cu)

20 40 60 80 100

(Ni)

Figure 10.6 For the copper–nickel system, (a) tensile strength versus composition, and

(b) ductility (%EL) versus composition at room temperature. A solid solution exists over all

compositions for this system.