Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 21
follows the line of the liquidus as the temperature falls. At 144°C, the bismuth
crystals will be in equilibrium with liquid which has achieved eutectic
composition: the liquid then freezes to form a eutectic mixture of crystals,
giving the microstructure illustrated in Fig. 1.15(b). Figure 1.15(c) illustrates
the microstructure of alloy 3 in Fig. 1.14.
Limited mutual solid solubility
A eutectic system
Soft solders are based on lead and tin and this system forms a eutectic
system of this type, as shown in Fig. 1.16. Here, the liquidus ecf shows a
eutectic minimum at c, which means that an alloy containing 38 wt% lead
will remain liquid to a relatively low temperature (183°C), and this is the
basis of tinman’s solder, which may be used for assembling electrical circuits
with less risk of damaging delicate components through overheating them.
In Fig. 1.16, ea and fb are the solidus lines; the lead-rich solid solution is
labelled the α phase and the tin-rich solid solution is termed the β phase. In
interpreting the microstructures produced when alloys of various compositions
are allowed to solidify, the reasoning will be a combination of those presented
above.
Alloys with a tin content between 0 and a in Fig. 1.16 and between b and
100 will simply freeze to the single phase α and β solid solutions respectively
when the temperature falls slowly. Following the previous reasoning, for a
given alloy composition, solidification will start when the liquidus is crossed
and be completed when the appropriate solidus is crossed. An alloy of
composition c will solidify at the eutectic temperature (183°C) to form a
finely divided mixture of the α and β crystals.
However, considering the solidification of alloy d (Fig. 1.16), at temperature
T 1 , β crystals of composition x will nucleate and as the temperature falls
Eutectic mixture of Bi + Cd Eutectic Primary Bi Eutectic Primary Cd
(a) (b) (c)
1.15 Sketches of microstructures of Cd–Bi alloys of composition (a)
40% Cd, (b) 20% Cd and (c) 80% Cd.