GTBL042-10 GTBL042-Callister-v2 August 13, 2007 18:16
374 • Chapter 10 / Phase DiagramsTemperature (°C)Temperature (°F)2600240022002000180015001400130012001100100090030 40 50 60 7030 40 50 60 70+ LL+ L+ L++Composition (wt% Ti)Composition (at% Ti)1310 °C
44.9 wt% TiFigure 10.22 A portion of
the nickel–titanium phase
diagram on which is shown
a congruent melting point
for theγ-phase solid
solution at 1310◦C 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.]the two components are compounds that share a common element, often oxygen.
These diagrams may have configurations similar to metal–metal systems, and they
are interpreted in the same way.The Al 2 O 3 –Cr 2 O 3 System
One of the relatively simple ceramic phase diagrams is that found for the aluminum
oxide–chromium oxide system, Figure 10.23. This diagram has the same form as the
isomorphous copper–nickel phase diagram (Figure 10.3a), consisting of single liquid
and single solid phase regions separated by a two-phase solid–liquid region having
the shape of a blade. The Al 2 O 3 –Cr 2 O 3 solid solution is a substitutional one in which
Al^3 +substitutes for Cr^3 +, and vice versa. It exists for all compositions below the
melting point of Al 2 O 3 inasmuch as both aluminum and chromium ions have the
same charge as well as similar radii (0.053 and 0.062 nm, respectively). Furthermore,
both Al 2 O 3 and Cr 2 O 3 have the same crystal structure.The MgO–Al 2 O 3 System
The phase diagram for the magnesium oxide–aluminum oxide system (Figure 10.24) is
similar in many respects to the lead–magnesium diagram (Figure 10.20). There exists
an intermediate phase, or better, a compound calledspinel,which has the chemical
formula MgAl 2 O 4 (or MgO–Al 2 O 3 ). Even though spinel is a distinct compound [of
composition 50 mol% Al 2 O 3 –50 mol% MgO (72 wt% Al 2 O 3 –28 wt% MgO)], it is
represented on the phase diagram as a single-phase field rather than as a vertical
line, as for Mg 2 Pb (Figure 10.20); that is, there is a range of compositions over which
spinel is a stable compound. Thus, spinel is nonstoichiometric (Section 5.3) for other
than the 50 mol% Al 2 O 3 –50 mol% MgO composition. Furthermore, there is limited
solubility of Al 2 O 3 in MgO below about 1400◦C (2550◦F) at the left-hand extremity