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376 • Chapter 10 / Phase Diagramsof Figure 10.24, which is due primarily to the differences in charge and radii of the
Mg^2 +and Al^3 +ions (0.072 versus 0.053 nm). For the same reasons, MgO is virtually
insoluble in Al 2 O 3 , as evidenced by a lack of a terminal solid solution on the right-
hand side of the phase diagram. Also, two eutectics are found, one on either side of
the spinel phase field, and stoichiometric spinel melts congruently at about 2100◦C
(3800◦F).The ZrO 2 –CaO System
Another important binary ceramic system is that for zirconium oxide (zirconia) and
calcium oxide (calcia); a portion of this phase diagram is shown in Figure 10.25. The
horizontal axis extends to only about 31 wt% CaO (50 mol% CaO), at which com-
position the compound CaZrO 3 forms. It is worth noting that one eutectic (2250◦C
and 23 wt% CaO) and two eutectoid (1000◦C and 2.5 wt% CaO, and 850◦C and 7.5
wt% CaO) reactions are found for this system.
It may also be observed from Figure 10.25 that ZrO 2 phases having three different
crystal structures exist in this system—namely, tetragonal, monoclinic, and cubic.
Pure ZrO 2 experiences a tetragonal-to-monoclinic phase transformation at about
1150 ◦C (2102◦F). A relatively large volume change accompanies this transformation,Composition (wt% CaO)0
(ZrO 2 )3000250015001000500200050004000200010003000Liquid010 20 3010 20 3040 50Cubic
+ LiquidCubic +
MonoclinicCubic
ZrO 2 (ss)
Cubic ZrO 2 (ss)
+
CaZrO 3Monoclinic
+
CaZr 4 O 9CaZr 4 O 9
+
CaZrO 3Temperature (°C)Temperature (°F)Composition (mol% CaO)Liquid +
CaZrO 3Cubic +
TetragonalTetragonal
ZrO 2 (ss)Monoclinic
ZrO 2 (ss)
CaZr 4 O 9Cubic ZrO 2 (ss)
+
CaZr 4 O 9CaZrO 3Figure 10.25 A
portion of the
zirconia–calcia phase
diagram;ssdenotes
solid solution.
(Adapted from V. S.
Stubican and S. P.
Ray, “Phase
Equilibria and
Ordering in the
System ZrO 2 –CaO,”
J. Am. Ceram. Soc.,
60 [11–12] 535
(1977). Reprinted by
permission of the
American Ceramic
Society.)