Фазовая диаграмма системы Au-Ti
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Au-Ti (Gold-Titanium)
J.L. Murray
The equilibrium solid phases of the Ti-Au system are (1) the bcc (bTi) solid
solution, with a maximum Au solubility of about 15 at.%; (2) the cph (aTi)
solid solution, with a maximum Au solubility of about 1.7 at.%; (3) the fcc (
Au) solid solution, with a maximum Ti solubility of 12 at.%; (4) the
essentially stoichiometric compound Ti3Au, with the A15 structure and a
congruent melting point; (5) the TiAu compounds, with a maximum homogeneity
range of 38 to 52 at.% Au; (6) the essentially stoichiometric compound TiAu2,
with the MoSi2 structure and a congruent melting point; and (7) the compound
TiAu4, with a homogeneity range of 79 to 82 at.% Au and the Ni4Mo structure,
which melts by a peritectic reaction with (Au).
The main outlines of the Ti-Au system are based on the results of two studies [
56Pie, 62Pie]. Thermodynamic calculations of the diagram agree very well with
the experimental data, within the uncertainties expected for a Ti-based system
for which there are no experimental thermodynamic data, i.e., within с35 C
for three-phase equilibria.
The assessed (Au) boundaries were drawn from the thermodynamic calculation
because they agreed with the assessment within 2 C. The (bTi) liquidus and
solidus were based on calculations as well as experimental data, because the
experimental observations do not conform with thermodynamic constraints. The
remainder of the diagram was drawn from the experimental data.
The Ti-rich region of the diagram has been studied extensively, but the data
of [78Pli] are preferred, because their results are least likely to be
influenced by interstitial impurities. The (bTi) transus is difficult to
determine metallographically due to the massive transformation. (bTi) that has
transformed to (aTi) during quenching differs very little in appearance from
equilibrium (aTi). The (bTi) structure cannot be retained metastably during
quenching for alloys containing 0 to 6 at.% Au [54Mcq]. For cooling rates less
than about 103 C/s, the mechanism of the transformation was shown to be of
the massive type [78Pli].
There are at least two, perhaps three, equilibrium polymorphic forms of TiAu.
The forms are designated gTiAu, bTiAu, and aTiAu, proceeding from high to low
temperature and high to low Ti content. There are discrepancies concerning the
structures of the allotropic forms. However, it is generally agreed that the
room-temperature phase on the Ti-rich side of the phase field has the B19
structure.
[70Don] splat quenched alloys between 40 and 52.6 at.% Au to prepare
metastable single-phase TiAu alloys and to examine the martensitic
transformation. The low-temperature phase was found to have the B19 structure
for all compositions studied, and it transformed martensitically from the high-
temperature CsCl structure.
TiAu4 has been identified as TiAu3 [43Wal] and as TiAu6 [52Rau]. The latter
identification was based on interpretation of a microstructure as single-phase
at that composition; the former identification is clearly the result of large
interstitial impurity content. The observation of a two-phase TiAu2 + TiAu4
assemblage at 75 at.% Au [62Pie, 69Sin] appears to preclude the occurrence of
TiAu3 as an equilibrium binary phase.
Thermal analysis (cooling) data were reported for Au-rich alloys by [52Rau],
and incipient melting data were reported by [62Pie]. For alloys containing
more than 67 at.% Au, reported liquidus and invariant temperatures are in
agreement.
43Wal: H.J. Wallbaum, Naturwissenschaften, 31, 91-92 (1943) in German.
52Rau: E. Raub, P. Walter, and M. Engel, Z. Metallkd., 43, 112-118 (1952) in
German.
54Mcq: M.K. McQuillan, J. Inst. Met., 82, 511-512 (1954).
56Pie: P. Pietrokowsky, E.P. Frink, and P. Duwez, Trans. AIME, 206, 930-935 (
1956).
62Pie: P. Pietrokowsky, J. Inst. Met., 90, 434-438 (1962).
69Sin: A.K. Sinha, Trans. AIME, 245, 237-240 (1969).
70Don: H.C. Donkersloot and J.H.N. Van Vucht, J. Less-Common Met., 20, 83-91 (
1970).
78Pli: M.R. Plichta, H.I. Aaronson, and J.H. Perepezko, Acta Metall., 26, 1293-
1305 (1978).
Published in Phase Diagrams of Binary Gold Alloys, 1987, Phase Diagrams of
Binary Titanium Alloys, 1987, and Bull. Alloy Phase Diagrams, 4(3), Nov 1983.
Complete evaluation contains 3 figures, 6 tables, and 16 references.
Special Points of the Ti-Au System