GTBL042-10 GTBL042-Callister-v2 August 13, 2007 18:16
10.12 Development of Microstructure in Eutectic Alloys • 365300(^200183) °C
18.3 97.8
100
Temperature (
°C)
600
500
400
300
200
100
Temperature (
°F)
0 20 40
0
60 80 100
(Pb) (Sn)
Composition (wt% Sn)
C 3
(61.9)
- L
L h
y
y
+ L
+
i
L
(61.9 wt%
Sn)
(18.3 wt %
Sn)
(97.8 wt%
Sn)
Figure 10.13
Schematic
representations of
the equilibrium
microstructures for a
lead–tin alloy of
eutectic composition
C 3 above and below
the eutectic
temperature.
redistribution is accomplished by atomic diffusion. The microstructure of the solid
that results from this transformation consists of alternating layers (sometimes called
lamellae) of theαandβphases that form simultaneously during the transformation.
This microstructure, represented schematically in Figure 10.13, pointi, is called a
eutectic structure eutectic structureand is characteristic of this reaction. A photomicrograph of this
structure for the lead–tin eutectic is shown in Figure 10.14. Subsequent cooling of
the alloy from just below the eutectic to room temperature will result in only minor
microstructural alterations.
The microstructural change that accompanies this eutectic transformation is rep-
resented schematically in Figure 10.15; here is shown theα-βlayered eutectic growing
into and replacing the liquid phase. The process of the redistribution of lead and tin
occurs by diffusion in the liquid just ahead of the eutectic–liquid interface. The ar-
rows indicate the directions of diffusion of lead and tin atoms; lead atoms diffuse
toward theα-phase layers since thisαphase is lead-rich (18.3 wt% Sn–81.7 wt% Pb);
conversely, the direction of diffusion of tin is in the direction of theβ, tin-rich (97.8
Figure 10.14 Photomicrograph showing
the microstructure of a lead–tin alloy of
eutectic composition. This microstructure
consists of alternating layers of a lead-rich
α-phase solid solution (dark layers), and a
tin-richβ-phase solid solution (light
layers). 375×. (Reproduced with
permission fromMetals Handbook,9th
edition, Vol. 9,Metallography and
Microstructures,American Society for
Metals, Materials Park, OH, 1985.)