Фазовая диаграмма системы Al-O
К оглавлению: Другие диаграммы (Others phase diargams)
Al-O (Aluminum-Oxygen)
H.A. Wriedt
The equilibrium phases of the Al-O system are (1) the liquid, which exists as
two immiscible liquids-liquid (Al) when Al-rich and liquid Al2O3 when O-rich; (
2) the fcc terminal solid solution, (Al), in which the solubility of oxygen is
unknown but small; (3) the hexagonal aluminum oxide, aAl2O3, a alumina, or
corundum, for which deviations from the exact stoichiometric composition are
unknown and small; and (4) the gas.
The assessed diagram is based on review of the work of [34Bau], [66Git], and [
68Yan], who prepared diagrams for the condensed phases at compositions ranging
from 0 to 60 at.% O. The studies of [78Cha], [81Ots], [81She], and [82Lia]
were also taken into account.
No stable condensed phase exists that is richer in oxygen than aAl2O3. At all
temperatures up to its melting point, aAl2O3 has a hexagonal structure, which
may also be regarded as rhombohedral. In condensed systems up to the
monotectic temperature, aAl2O3 can coexist stably only with solid or liquid (
Al), that is, oxygen-saturated aluminum. Above the O2 fugacities for
coexistence with (Al), aAl2O3 exists alone. The present diagram is primarily
qualitative, because of the lack of data for the composition ranges of the
solid and liquid phases. The existence of a eutectic near the melting point of
Al is speculative; however, the existence of a monotectic near the melting
point of Al2O3 is fairly well established. Displacements in temperature of
these three-phase equilibria from the melting points of pure Al and
stoichiometric Al2O3 are small and are probably not measurable for the
eutectic.
At low hydrostatic pressures, the terminal (Al) solid solution with a very
limited range exhibits only the fcc structure up to the melting point. No
reliable measurement of the solubility of oxygen in (Al) has been reported.
The concentration of dissolved oxygen in solid (Al) saturated with respect to
aAl2O3 is probably smaller than that in liquid (Al) and not measurable with
existing techniques.
Liquid (Al) exists stably only at low oxygen O2 fugacities. At extremely low
O2 fugacities, a short liquidus terminating at the postulated eutectic is
considered to exist for liquid (Al) in equilibrium with solid (Al). No
experimental data on this liquidus or the probable eutectic have been reported.
Liquid (Al) with very low concentrations of dissolved oxygen is in
equilibrium with solid aAl2O3 from the temperature of the eutectic reaction to
that of the monotectic reaction.
[68Yan] found no measurable lowering of the melting point of aAl2O3 with
Al addition, but [66Git] found a 15 C lowering. They agree on the existence
of two liquid phases in samples heated slightly below or above the melting
point of aAl2O3.
In addition to amorphous aAl2O3, there are at least five varieties of
crystalline Al2O3 that are well established, but many more have been reported.
All crystalline forms of Al2O3 are constructed of stacked, close-packed layers
of oxygen ions with Al ions and vacancies distributed on the tetrahedral and
octahedral sites among these oxygen ions. Polymorphism arises from the
possibilities for different oxygen layer stacking sequences and disorder
therein, from variations in the distribution of Al ions on their two types of
sites, and from ordering among the Al ions and vacancies on those sites.
At least eight other solid Al oxides have been reported: Al2O, Al4O3, AlO,
Al2O2, Al8O9, Al3O4, Al4O6, and AlO2. It is possible that none of these phases
exists even metastably. Oxide species occur in vapors of the Al-O system, as
well as single-element oxygen and Al species.
34Bau: E. Baur and R. Brunner, Z. Elektrochem., 40(3), 154-158 (1934) in
German.
66Git: G. Gitlesen, O. Herstad, and K. Motzfeldt, Selected Topics in High
Temperature Chemistry, T. Forland, Ed., Universitetsforlaget, Oslo, Norway,
179-196 (1966).
68Yan: H. Yanagida and F.A. Kroger, J. Am. Ceram. Soc., 51(12), 700-706 (1968).
78Cha: M.W. Chase, Jr., J.L. Curnutt, R.A. McDonald, and A.N. Syverud, J. Phys.
Chem. Ref. Data, 7(3), 793-940 (1978).
81Ots: S. Otsuka and Z. Kozuka, J. Jpn. Inst. Met., 22(8), 558-566 (1981).
81She: V.E. Shevtsov, Izv. Akad. Nauk SSSR, Met., (1), 60-65 (1981) in Russian;
TR: Russ. Metall., (1), 52-57 (1981).
82Lia: W.W. Liang, Z. Metallkd., 73, 369-375 (1982).
Published in Bull. Alloy Phase Diagrams, 6(6), Dec 1985. Complete evaluation
contains 1 figure, 6 tables, and 46 references.
Special Points of the Al-O System
