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328 • Chapter 9 / Phase Diagrams

9.14 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 9.19) at
560 C (1040 F) and 74 wt% Zn–26 wt% Cu. A portion of the phase diagram in this vicinity
is enlarged in Figure 9.21. Upon cooling, a solid d phase transforms into two other solid
phases (g and P) according to the reaction

The eutectoid cooling
reaction (per point d m g + P (9.14)
heating
E, Figure 9.21)
eutectoid reaction The reverse reaction occurs upon heating. It is called a eutectoid (or eutectic-like)
reaction, and the invariant point (point E, Figure 9.21) and the horizontal tie line
at 560 C are termed the eutectoid and eutectoid isotherm, respectively. The feature
distinguishing eutectoid from eutectic is that one solid phase instead of a liquid
transforms into two other solid phases at a single temperature. A eutectoid reaction
found in the iron–carbon system (Section 9.18) is very important in the heat treating
of steels.
peritectic reaction The peritectic reaction is another invariant reaction involving three phases at
equilibrium. With this reaction, upon heating, one solid phase transforms into a
liquid phase and another solid phase. A peritectic exists for the copper–zinc system
(Figure 9.21, point P) at 598 C (1108 F) and 78.6 wt% Zn–21.4 wt% Cu; this reaction
is as follows:

The peritectic cooling
reaction (per point d + L m P (9.15)
P, Figure 9.21) heating

The low-temperature solid phase may be an intermediate solid solution (e.g., P in the
Tutorial Video: preceding reaction), or it may be a terminal solid solution. One of the latter peritectics
Eutectic vs. exists at about 97 wt% Zn and 435 C (815 F) (see Figure 9.19), where the h phase, when
Eutectoid Reactions heated, transforms into P and liquid phases. Three other peritectics are found for the
Cu–Zn system, the reactions of which involve b, d, and g intermediate solid solutions as
What’s the Difference
between a Eutecçtic the low-temperature phases that transform upon heating.
and Eutectoid Reaction?

Figure 9.21 A region of the
copper–zinc phase diagram + L
that has been enlarged to show 700
eutectoid and peritectic invariant
points, labeled E (560 C, 74 wt%
Zn) and P (598 C, 78.6 wt% Zn), + + L L 1200
respectively.
[Adapted from Binary Alloy Phase Temperature (°C) 600 P 598°C Temperature (°F)
Diagrams, 2nd edition, Vol. 2, T. B.
Massalski (Editor-in-Chief), 1990. 560°C +
Reprinted by permission of ASM E
International, Materials Park, OH.] 1000
+ L
+
500

60 70 80 90
Composition (wt% Zn)