GTBL042-10 GTBL042-Callister-v2 August 13, 2007 18:16
372 • Chapter 10 / Phase Diagrams+ L+ L+ L++
+
Composition (wt % Zn)Temperature (°C)Temperature (°F)60 70 80 90700600500598 °C560 °C
ELP12001000Figure 10.21 A region of the copper–zinc phase diagram that has been enlarged to show
eutectoid and peritectic invariant points, labeledE(560◦C, 74 wt% Zn) andP(598◦C, 78.6
wt% Zn), respectively. [Adapted fromBinary Alloy Phase Diagrams,2nd edition, Vol. 2,
T. B. Massalski (Editor-in-Chief), 1990. Reprinted by permission of ASM International,
Materials Park, OH.]eutectoid reaction The reverse reaction occurs upon heating. It is called aeutectoid(or eutectic-like)
reaction,and the invariant point (pointE, Figure 10.21) and the horizontal tie line
at 560◦C are termed theeutectoidandeutectoid 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
is found in the iron–carbon system (Section 10.19) that is very important in the heat
treating of steels.
peritectic reaction Theperitectic reactionis yet 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
10.21, pointP) at 598◦C (1108◦F) and 78.6 wt% Zn–21.4 wt% Cu; this reaction is as
follows:δ+LΔ
cooling
heating(10.15)
The peritectic
reaction (per pointP,
Figure 10.21)The low-temperature solid phase may be an intermediate solid solution (e.g.,in the
above reaction), or it may be a terminal solid solution. One of the latter peritectics
exists at about 97 wt% Zn and 435◦C (815◦F) (see Figure 10.19), wherein theηphase,
when heated, transforms toand liquid phases. Three other peritectics are found
for the Cu–Zn system, the reactions of which involveβ,δ, andγintermediate solid
solutions as the low-temperature phases that transform upon heating.10.15 CONGRUENT PHASE TRANSFORMATIONS
Phase transformations may be classified according to whether or not there is any
change in composition for the phases involved. Those for which there are no com-
congruent positional alterations are said to becongruent transformations.Conversely, forin-
transformation congruent transformations,at least one of the phases will experience a change in