Callister - An introduction - 8th edition

296 • Chapter 9 / Phase Diagrams
Figure 9.5
Schematic
representation of the
development of
microstructure
during the
nonequilibrium
solidification of a 35
wt% Ni–65 wt% Cu
alloy.
1300
(46 Ni)
L
L (35 Ni)
a
L
(35 Ni)
+ L
b
L (29 Ni)
(40 Ni)
(46 Ni)
Temperature (°C)
1200
c
L (24 Ni)
d
L (21 Ni)
(35 Ni)
(31 Ni)
e
(42 Ni)
(38 Ni)
L (29 Ni)
(46 Ni)
(40 Ni)
(46 Ni)
(40 Ni)
(35 Ni)
(31 Ni)
f
L (24 Ni)
(46 Ni)
(40 Ni)
(35 Ni)
L (21 Ni)
(46 Ni)
(40 Ni)
(35 Ni)
(31 Ni)
1100
20 30 40 50 60
Composition (wt% Ni)
Upon further cooling to point c (about 1240C), the liquid composition has
shifted to 29 wt% Ni–71 wt% Cu; furthermore, at this temperature the composition
of the phase that solidified is 40 wt% Ni–60 wt% Cu [(40 Ni)]. However, because
diffusion in the solid phase is relatively slow, the phase that formed at point b
has not changed composition appreciably—that is, it is still about 46 wt% Ni—and
the composition of the grains has continuously changed with radial position, from
46 wt% Ni at grain centers to 40 wt% Ni at the outer grain perimeters. Thus, at
point c, the average composition of the solid grains that have formed would be
some volume weighted average composition, lying between 46 and 40 wt% Ni. For
the sake of argument, let us take this average composition to be 42 wt% Ni–58 wt%
Cu [(42 Ni)]. Furthermore, we would also find that, on the basis of lever-rule
computations, a greater proportion of liquid is present for these nonequilibrium
conditions than for equilibrium cooling. The implication of this nonequilibrium
solidification phenomenon is that the solidus line on the phase diagram has been
shifted to higher Ni contents—to the average compositions of the phase (e.g., 42
wt% Ni at 1240C)—and is represented by the dashed line in Figure 9.5. There is
no comparable alteration of the liquidus line inasmuch as it is assumed that