GTBL042-10 GTBL042-Callister-v3 October 4, 2007 11:56
2nd Revised Pages366 • Chapter 10 / Phase DiagramsLiquidEutectic
growth
directionSnPbPbSn PbFigure 10.15 Schematic representation of
the formation of the eutectic structure for
the lead–tin system. Directions of diffusion
of tin and lead atoms are indicated by blue
and red arrows, respectively.wt% Sn–2.2 wt% Pb) layers. The eutectic structure forms in these alternating layers
because, for this lamellar configuration, atomic diffusion of lead and tin need occur
over only relatively short distances.
The fourth and final microstructural case for this system includes all composi-
VMSEEutectic
(Pb–Sn)tions other than the eutectic that, when cooled, cross the eutectic isotherm. Consider,
for example, the compositionC 4 , Figure 10.16, which lies to the left of the eutectic;
as the temperature is lowered, we move down the linezz′, beginning at pointj.The
microstructural development between pointsjandlis similar to that for the sec-
ond case, such that just prior to crossing the eutectic isotherm (pointl), theαand
liquid phases are present, having compositions of approximately 18.3 and 61.9 wt%
Sn, respectively, as determined from the appropriate tie line. As the temperature is
lowered to just below the eutectic, the liquid phase, which is of the eutectic com-
position, will transform to the eutectic structure (i.e., alternatingαandβlamellae);
insignificant changes will occur with theαphase that formed during cooling through
theα+Lregion. This microstructure is represented schematically by the inset at
pointmin Figure 10.16. Thus, theαphase will be present both in the eutectic struc-
ture and also as the phase that formed while cooling through theα+Lphase field.
To distinguish oneαfrom the other, that which resides in the eutectic structure is
eutectic phase calledeutecticα, while the other that formed prior to crossing the eutectic isotherm
primary phase is termedprimaryα; both are labeled in Figure 10.16. The photomicrograph in Fig-
ure 10.17 is of a lead–tin alloy in which both primaryαand eutectic structures are
shown.
In dealing with microstructures, it is sometimes convenient to use the term
microconstituent microconstituent—that is, an element of the microstructure having an identifiable
and characteristic structure. For example, in the pointminset, Figure 10.16, there
are two microconstituents—namely, primaryαand the eutectic structure. Thus,
the eutectic structure is a microconstituent even though it is a mixture of two
phases, because it has a distinct lamellar structure, with a fixed ratio of the two
phases.
It is possible to compute the relative amounts of both eutectic and primaryα
microconstituents. Since the eutectic microconstituent always forms from the liquid
having the eutectic composition, this microconstituent may be assumed to have a