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

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2nd Revised Pages

354 • Chapter 10 / Phase Diagrams

Composition (wt% Ni)

a

b

c

d

e

f

L

1300 + L

1200

1100

20 30 40 50 60

Temperature (

°C)

L (21 Ni)

L (21 Ni)

L (24 Ni)

L (29 Ni)

L (29 Ni)

L (35 Ni)

(46 Ni)

(40 Ni)

(35 Ni)

L (24 Ni)

(46 Ni)

(40 Ni)

(35 Ni)

(31 Ni)

(31 Ni)

(40 Ni) (46 Ni)

(42 Ni)

(46 Ni)





(46 Ni)

(40 Ni)

(35 Ni) (38 Ni)

(46 Ni)

(40 Ni)

(35 Ni)

(31 Ni)

L

(35 Ni)

Figure 10.5

Schematic

representation of the

development of

microstructure

during the

nonequilibrium

solidification of a 35

wt% Ni–65 wt% Cu

alloy.

at 1240◦C)—and is represented by the dashed line in Figure 10.5. There is no com-

parable alteration of the liquidus line inasmuch as it is assumed that equilibrium is

maintained in the liquid phase during cooling because of sufficiently rapid diffusion

rates.

At pointd′(∼ 1220 ◦C) and for equilibrium cooling rates, solidification should be

completed. However, for this nonequilibrium situation, there is still an appreciable

proportion of liquid remaining, and theαphase that is forming has a composition of

35 wt% Ni [α(35 Ni)]; also, theaverageα-phase composition at this point is 38 wt%

Ni [α(38 Ni)].

Nonequilibrium solidification finally reaches completion at pointe′(∼ 1205 ◦C).

The composition of the lastαphase to solidify at this point is about 31 wt% Ni; the

averagecomposition of theαphase at complete solidification is 35 wt% Ni. The inset

at pointf′shows the microstructure of the totally solid material.

The degree of displacement of the nonequilibrium solidus curve from the equi-

librium one will depend on rate of cooling. The slower the cooling rate, the smaller

this displacement; that is, the difference between the equilibrium solidus and aver-

age solid composition is lower. Furthermore, if the diffusion rate in the solid phase

is increased, this displacement will be diminished.