Callister - An introduction - 8th edition

9.15 Congruent Phase Transformations • 315
Temperature (°C)
1500
1400
1300
1200
1100
30 40 50 60 70
+ L
Composition (at% Ti)
1310°C
44.9 wt% Ti
+ L
L
+ L
2600
2400
2200
2000
Temperature (°F)
Figure 9.22 A portion of
the nickel–titanium phase
diagram on which is
shown a congruent
melting point for the -
phase solid solution at
1310C and 44.9 wt% Ti.
[Adapted from Phase
Diagrams of Binary Nickel
Alloys, P. Nash (Editor),
1991. Reprinted by
permission of ASM
International, Materials
Park, OH.]
1000
900
+
1800
+
30 40 50 60 70
Composition (wt% Ti)
9.15 CONGRUENT PHASE TRANSFORMATIONS
Phase transformations may be classified according to whether there is any change
congruent
transformation
in composition for the phases involved. Those for which there are no compositional
alterations are said to be congruent transformations. Conversely, for incongruent
transformations, at least one of the phases will experience a change in composition.
Examples of congruent transformations include allotropic transformations (Section
3.6) and melting of pure materials. Eutectic and eutectoid reactions, as well as the
melting of an alloy that belongs to an isomorphous system, all represent incongruent
transformations.
Intermediate phases are sometimes classified on the basis of whether they melt
congruently or incongruently.The intermetallic compound Mg 2 Pb melts congruently
at the point designated M on the magnesium–lead phase diagram, Figure 9.20. Also,
for the nickel–titanium system, Figure 9.22, there is a congruent melting point for
the g solid solution that corresponds to the point of tangency for the pairs of liquidus
and solidus lines, at 1310C and 44.9 wt% Ti. Furthermore, the peritectic reaction
is an example of incongruent melting for an intermediate phase.
Concept Check 9.6
The following figure is the hafnium–vanadium phase diagram, for which only singlephase
regions are labeled. Specify temperature–composition points at which all eutectics,
eutectoids, peritectics, and congruent phase transformations occur. Also, for
each, write the reaction upon cooling. [Phase diagram from ASM Handbook, Vol.
3, Alloy Phase Diagrams, H. Baker (Editor), 1992, p. 2.244. Reprinted by permission
of ASM International, Materials Park, OH.]