Callister - An introduction - 8th edition

9.12 Development of Microstructure in Eutectic Alloys • 307
Figure 9.13
Schematic
representations of
the equilibrium
microstructures for a
lead–tin alloy of
eutectic composition
C 3 above and below
the eutectic
temperature.
Temperature (°C)
300
200
+ L
L
183°C
+ L
18.3 i
97.8
h
y
L
(61.9 wt%
Sn)
600
500
400
300
Temperature (°F)
100
0
0
(Pb)
+
20 40
60 80 100
C 3
(Sn)
(61.9)
Composition (wt%Sn)
y
(18.3 wt%
Sn)
(97.8 wt%
Sn)
200
100
temperature, 183C. Upon crossing the eutectic isotherm, the liquid transforms to the
two and b phases. This transformation may be represented by the reaction
L161.9 wt% Sn2 Δ cooling
heating
a118.3 wt% Sn2 b197.8 wt% Sn2
(9.9)
eutectic structure
in which the a- and b-phase compositions are dictated by the eutectic isotherm end
points.
During this transformation, there must necessarily be a redistribution of the lead
and tin components, inasmuch as the a and b phases have different compositions
neither of which is the same as that of the liquid (as indicated in Equation 9.9). This
redistribution is accomplished by atomic diffusion. The microstructure of the solid
that results from this transformation consists of alternating layers (sometimes called
lamellae) of the a and b phases that form simultaneously during the transformation.
This microstructure, represented schematically in Figure 9.13, point i, is called a
eutectic structure and is characteristic of this reaction. A photomicrograph of this
structure for the lead–tin eutectic is shown in Figure 9.14. Subsequent cooling of the
50 m
Figure 9.14 Photomicrograph showing
the microstructure of a lead–tin alloy of
eutectic composition. This microstructure
consists of alternating layers of a leadrich
a-phase solid solution (dark layers),
and a tin-rich b-phase solid solution (light
layers). 375 . (Reproduced with
permission from Metals Handbook, 9th
edition, Vol. 9, Metallography and
Microstructures, American Society for
Metals, Materials Park, OH, 1985.)