Фазовая диаграмма системы O-Ti
К оглавлению: Другие диаграммы (Others phase diargams)
Co-Ti (Cobalt-Titanium)
J.L. Murray
The equilibrium solid phases of the Ti-Co system are (1) the cph solid
solutions, (aTi) and (eCo), where (aTi) is stable below 882 C and (eCo) is
stable below approximately 422 C; (2) the bcc solid solution, (bTi), stable
in pure Ti above 882 C, with a maximum solubility of Co in (bTi) of 14.5 at.%
at 1020 C; (3) the fcc solid solution, (aCo), stable in pure Co above 422 C,
with a maximum solubility of Ti in (aCo) of 14.1 at.% at 1190 C; (4) Ti2Co,
an ordered fcc structure with a homogeneity range of not more than about 0.3
at.% near stoichiometry; (5) TiCo, with the CsCl structure, which melts
congruently at 1325 C and has a homogeneity range of 49 с 1 to 55 с 0.5 at.%
Co at 1200 C; (6) cubic (C15) and hexagonal (C36) Laves phases of approximate
stoichiometry TiCo2, designated in the present evaluation as TiCo2(c) and
TiCo2(h), respectively, where TiCo2(h) is slightly richer in Co, because the
homogeneity ranges of TiCo2(c) and TiCo2(h) are approximately 66.5 to 67.0 at.%
Co and 68.75 to 72 at.% Co, respectively; and (7) TiCo3, with the ordered fcc
AuCu3 structure and a maximum homogeneity range of 75.7 to 80.7 at.% Co.
The assessed phase diagram contains two "deep eutectics," in which amorphous
alloys can be formed, and the higher order ferromagnetic transition has a
pronounced effect on the solvus. However, some features of the diagram have
received only cursory examination or are the subject of controversy. Notably,
the liquidus has not been determined from 0 to 20 at.% Co, and there are
discrepancies between 20 and 80 at.% Co. There are conflicting reports about
which of the TiCo2 Laves phases are stable phases, and it has been suggested (
based on observation of polytypism) that this part of the diagram may be more
complex than had been imagined.
Conflicting reports suggest that only a hexagonal Laves phase TiCo2(h) [50Duw]
or only a cubic Laves phase TiCo2(c) [83Uhr] is stable. By X-ray diffraction, [
50Duw] found only TiCo2(h) in alloys containing 63 to 68 at.% Co. [83Uhr]
found only TiCo2(c) formed from the liquid at 71 at.% Co. [59Fou] found only
TiCo2(h) in an as-cast 66.1 at.% Co sample.
Using electron microscopy, [72All] found that Ti-Co exhibits polytypism: the
stable hexagonal form can be described as a 4H stacking and 6H and 12-layer
stackings were also found. These contradictory observations are at present
unresolvable.
The minimum Co content for which (bTi) can be retained metastably during
quenching ranges from 4.5 [59Yak] to 10 at.% Co [58Swa]. The composition
reported depends on the investigator's judgment of whether (bTi) was fully
retained, as well as on the quenching rate. [77Nik] found that whereas cph Co
martensite is formed from 100 to 98 at.% Co, a faulted structure appeared
between 98 and 96 at.% Co, and that from 96 to 93 at.% Co the martensite has a
126-layer rhombohedral structure.
[80Ino] produced mixed crystalline and amorphous phases near 23 and 88 at.% Co
by rapid solidification. Between 87 and 89 at.% Co, only the amorphous phase
was present without any trace of the crystalline phase.
50Duw: P. Duwez and J.L. Taylor, Trans. AIME, 188, 1173-1176 (1950).
58Swa: P.R. Swann and J.G. Parr, Trans. AIME, 212, 276-279 (1958).
59Fou: R.W. Fountain and W.D. Forgeng, Trans. AIME, 215, 998-1008 (1959).
59Yak: F.W. Yakymyshyn, G.R. Prudy, R. Taggart, and J.G. Parr, Trans. ASM, 53,
283-294 (1959).
72All: C.W. Allen, P. Delavignette, and Amelinckx, Phys. Status Solidi (a), 9,
237-246 (1972).
77Nik: B.I. Nikolin, Dokl. Akad. Nauk SSSR, 223, 587-590 (1977) in Russian; TR:
Sov. Phys. Dokl., 22(4), 226-228 (1977).
80Ino: A. Inoue, K. Kobayashi, C. Suryanarayana, and T. Masumoto, Scr. Metall.,
14, 119-123 (1980).
83Uhr: B. Uhrenius and K. Forsen, Z. Metallkd., 74(9), 610-615 (1983).
Published in Phase Diagrams of Binary Titanium Alloys, 1987. Complete
evaluation contains 4 figures, 8 tables, and 60 references.
Special Points of the Ti-Co System