GTBL042-10 GTBL042-Callister-v3 October 4, 2007 11:56
2nd Revised Pages10.14 Eutectoid and Peritectic Reactions • 371+ L
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LComposition (at% Pb)Composition (wt% Pb)Temperature (°C)Temperature (°F)1200100080060040020070060050040030020010000(Mg) (Pb)51 0 2 030400 20 40 60 80 10070 100LML
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Mg 2 PbL
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Mg 2 Pb + Mg 2 Pb
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Mg 2 Pb
Mg 2 PbFigure 10.20 The magnesium–lead phase diagram. [Adapted fromPhase Diagrams of
Binary Magnesium Alloys,A. A. Nayeb-Hashemi and J. B. Clark (Editors), 1988. Reprinted
by permission of ASM International, Materials Park, OH.]Several other characteristics are worth noting for this magnesium–lead system.
First, the compound Mg 2 Pb melts at approximately 550◦C (1020◦F), as indicated by
pointMin Figure 10.20. Also, the solubility of lead in magnesium is rather extensive,
as indicated by the relatively large composition span for theα-phase field. On the
other hand, the solubility of magnesium in lead is extremely limited. This is evident
from the very narrowβterminal solid-solution region on the right or lead-rich side
of the diagram. Finally, this phase diagram may be thought of as two simple eu-
tectic diagrams joined back to back, one for the Mg–Mg 2 Pb system and the other
for Mg 2 Pb–Pb; as such, the compound Mg 2 Pb is really considered to be a compo-
nent. This separation of complex phase diagrams into smaller-component units may
simplify them and, furthermore, expedite their interpretation.10.14 EUTECTOID AND PERITECTIC REACTIONS
In addition to the eutectic, other invariant points involving three different phases
are found for some alloy systems. One of these occurs for the copper–zinc system
(Figure 10.19) at 560◦C (1040◦F) and 74 wt% Zn–26 wt% Cu. A portion of the phase
diagram in this vicinity appears enlarged in Figure 10.21. Upon cooling, a solidδ
phase transforms into two other solid phases (γand) according to the reactionδΔ
cooling
heatingγ+ (10.14)The eutectoid
reaction (per point
E, Figure 10.21)