From Figure, the phase diagram for the MgO–Al2O3 system, it

From Figure, the phase diagram for the MgO–Al2O3 system, it may be noted that the spinel solid solution exists over a range of compositions, which means that it is nonstoichiometric at compositions other than 50 mol% MgO–50 mol% Al2O3.


(a) The maximum nonstoichiometry on the Al2O3-rich side of the spinel phase field exists at about 2000°C (3630°F) corresponding to approximately 82 mol% (92 wt%) Al2O3. Determine the type of vacancy defect that is produced and the percentage of vacancies that exist at this composition.

(b) The maximum nonstoichiometry on the MgO-rich side of the spinel phase field exists at about 2000°C (3630°F) corresponding to approximately 39 mol% (62 wt%) Al2O3. Determine the type of vacancy defect that is produced and the percentage of vacancies that exist at thiscomposition.


Composition (mol% Al,0,) 40 60 80 2800 5000 Liquid Al 203 4500 MgAl 0, (ss) + Liquid 2400 Mg0 (ss) Liquid Liquid 4000 Mg

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