Page 349 - Materials Science and Engineering An Introduction

9.12 Development of Microstructure in Eutectic Alloys • 321

Figure 9.12 Schematic
representations of the equi-
x librium microstructures for a
L
d L lead–tin alloy of composition
(C wt% Sn) C 2 as it is cooled from the
2
liquid-phase region.
300
L
e

+ L
Temperature (°C) 200 f C wt% Sn

2

Solvus
line
g

100

+

x
0 10 20 30 40 50
C 2
Composition (wt% Sn)

cooling, these particles grow in size because the mass fraction of the b phase increases
slightly with decreasing temperature.
The third case involves solidification of the eutectic composition, 61.9 wt% Sn (C 3 in
Figure 9.13). Consider an alloy having this composition that is cooled from a temperature
within the liquid-phase region (e.g., 250 C) down the vertical line yy¿ in Figure 9.13. As
the temperature is lowered, no changes occur until we reach the eutectic temperature,
: VMSE 183 C. Upon crossing the eutectic isotherm, the liquid transforms into the two a and b
Eutectic (Pb-Sn)
phases. This transformation may be represented by the reaction
cooling
L(61.9 wt% Sn) m a(18.3 wt% Sn) + b(97.8 wt% Sn) (9.9)
heating
in which the a- and b-phase compositions are dictated by the eutectic isotherm end points.
During this transformation, there must be a redistribution of the lead and tin com-
ponents because 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 ac-
complished 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, rep-
eutectic structure resented schematically in Figure 9.13, point i, is called a eutectic structure and is char-
acteristic of this reaction. A photomicrograph of this structure for the lead–tin eutectic
is shown in Figure 9.14. Subsequent cooling of the alloy from just below the eutectic to
room temperature results in only minor microstructural alterations.