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Callister - An introduction - 8th edition

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12.7 Ceramic Phase Diagrams • 479<br />

Figure 12.26 A<br />

portion of the<br />

zirconia–calcia phase<br />

diagram; ss denotes<br />

solid solution.<br />

(Adapted from V. S.<br />

Stubican and S. P.<br />

Ray, “Phase<br />

Equilibria and<br />

Ordering in the<br />

System ZrO 2 –CaO,”<br />

J. Am. Ceram. Soc.,<br />

60 [11–12] 535<br />

(1977). Reprinted by<br />

permission of the<br />

American Ceramic<br />

Society.)<br />

Temperature (°C)<br />

0 10 20 30<br />

3000<br />

2500<br />

2000<br />

1500<br />

Cubic<br />

ZrO 2 (ss)<br />

Cubic +<br />

Tetragonal<br />

Composition (mol% CaO)<br />

Cubic<br />

+ Liquid<br />

Liquid<br />

Cubic ZrO 2 (ss)<br />

+<br />

CaZrO 3<br />

40 50<br />

Liquid +<br />

CaZrO 3<br />

5000<br />

4000<br />

3000<br />

Temperature (°F)<br />

Tetragonal<br />

ZrO 2 (ss)<br />

Monoclinic<br />

ZrO 2 (ss)<br />

Cubic ZrO 2 (ss)<br />

1000<br />

+<br />

2000<br />

CaZr 4 O 9<br />

CaZr<br />

Cubic +<br />

4 O 9<br />

+<br />

Monoclinic<br />

CaZrO 3<br />

CaZr 4 O 9 CaZr 4 O 9<br />

Monoclinic<br />

+<br />

500<br />

1000<br />

0<br />

(ZrO 2 )<br />

10 20<br />

Composition (wt% CaO)<br />

30<br />

CaZrO 3<br />

The SiO 2 –Al 2 O 3 System<br />

Commercially, the silica–alumina system is an important one because the principal<br />

constituents of many ceramic refractories are these two materials. Figure 12.27<br />

shows the SiO 2 –Al 2 O 3 phase diagram. The polymorphic form of silica that is stable<br />

at these temperatures is termed cristobalite, the unit cell for which is shown in<br />

Figure 12.10. Silica and alumina are not mutually soluble in one another, which is<br />

evidenced by the absence of terminal solid solutions at both extremities of the phase<br />

diagram.Also,it may be noted that the intermediate compound mullite, 3Al 2 O 3 –2SiO 2 ,<br />

exists, which is represented as a narrow phase field in Figure 12.27; furthermore,<br />

mullite melts incongruently at 1890C (3435F). A single eutectic exists at 1587C<br />

(2890F) and 7.7 wt% Al 2 O 3 . In Section 13.5, refractory ceramic materials, the prime<br />

constituents for which are silica and alumina, are discussed.<br />

Concept Check 12.4<br />

(a) For the SiO 2 –Al 2 O 3 system, what is the maximum temperature that is possible<br />

without the formation of a liquid phase? (b) At what composition or over what<br />

range of compositions will this maximum temperature be achieved?<br />

[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]

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