Callister - An introduction - 8th edition

294 • Chapter 9 / Phase Diagrams
When the densities of the phases in a two-phase alloy differ significantly, there
will be quite a disparity between mass and volume fractions; conversely, if the phase
densities are the same, mass and volume fractions are identical.
9.9 DEVELOPMENT OF MICROSTRUCTURE IN
ISOMORPHOUS ALLOYS
VMSE
Isomorphous (Sb-Bi)
Equilibrium Cooling
At this point it is instructive to examine the development of microstructure that
occurs for isomorphous alloys during solidification. We first treat the situation in
which the cooling occurs very slowly, in that phase equilibrium is continuously
maintained.
Let us consider the copper–nickel system (Figure 9.3a), specifically an alloy of
composition 35 wt% Ni–65 wt% Cu as it is cooled from 1300C. The region of the
Cu–Ni phase diagram in the vicinity of this composition is shown in Figure 9.4.
Cooling of an alloy of this composition corresponds to moving down the vertical
dashed line. At 1300C, point a, the alloy is completely liquid (of composition
35 wt% Ni–65 wt% Cu) and has the microstructure represented by the circle inset
in the figure. As cooling begins, no microstructural or compositional changes
will be realized until we reach the liquidus line (point b, ~1260C). At this point,
Figure 9.4
Schematic
representation of the
development of
microstructure during
the equilibrium
solidification of a 35
wt% Ni–65 wt% Cu
alloy.
1300
(46 Ni)
L
(35 Ni)
a
L
(35 Ni)
L
+
L
Temperature (°C)
1200
L (24 Ni)
L (32 Ni)
b
c
d
e
(43 Ni)
L (32 Ni)
(35 Ni)
(46 Ni)
(43 Ni)
L (24 Ni)
(35 Ni)
1100
20
30
40
50
Composition (wt% Ni)