Фазовая диаграмма системы Pt-V

К оглавлению: Другие диаграммы (Others phase diargams)

Pt-V (Platinum-Vanadium) J.F. Smith The assessed Pt-V phase diagram is based on [73Wat] with review of the data of [74Sta]. Temperatures in the assessed diagram for the invariant transformations have precisions on the order of с10 C. A superlattice near Pt8V, analogous to Ni8V in the Ni-V system [82Smi], has been reported by [86Sch] as part of an extensive investigation of ordering in the Pt-V system. This Pt8V phase may or may not be an equilibrium feature of the system; extensive anneals such as the 3-week or more periods found by [ 85Aok] to be necessary for the establishment of equilibrium in the Co-rich region of the Co-V system have not been made for the Pt-V system. Even if Pt8V proves to be an equilibrium feature, there is at present no basis for deciding whether it forms by first- or second-order transformation nor for associating temperatures and compositions with boundaries defining its range of existence. Therefore, the phase has not been included in the assessed diagram. [73Wat] showed a single congruent transformation near 1015 C from the terminal solution to a lower-temperature phase with the Al3Ti structure. [ 73Wat] was unable to form the ordered AuCu3 structure at ideal stoichiometry but was able to form the phase at Pt-rich compositions. However, the manner of its formation was such that [73Wat] suggested that the phase might be metastable rather than an equilibrium phase in the binary diagram. Recent work on the Co-V system [79Aok, 85Aok] supports the view that the AuCu3-type phase is likely to be metastable rather than stable. [86Sch] confirmed that the AuCu3 structure is metastable in Pt-V and that it does not occur at a stoichiometry of Pt3V. Instead, at ideal stoichiometry, the (Pt) terminal solution was found to transform to the Al3Ti structure via metastable intermediate structures that are associated with long-range antiphase boundaries. The observation [86Sch] of a superstructure with stoichiometry of Pt8V and related to the AuCu3 structure is unequivocal, and, in the composition region between Pt8V, there were found only substitutionally disordered derivatives of Pt8V and the AuCu3 structure without long-range periodicity. PtV3 becomes superconducting at low temperatures [61Mat, 63Mat]. Annealing [ 73Cox] tends to raise the superconducting transition temperature, but only slightly. At elevated temperatures, the phase exists from 66 to 82 at.% V [ 73Wat] and decomposes peritectically near 1800 C. It is in eutectic equilibrium with liquid and the Pt-rich terminal solution near 1720 C and in eutectoidal equilibrium with the Pt-rich terminal solution and PtV near 1410 C. AuCu3-type ordering has been observed in coexistence with the Cr3Si-type structure at compositions in the vicinity of 75 at.% V, but there is agreement [64Mal, 64Phi] that this results from contamination by interstitial elements, definitely oxygen and probably nitrogen and/or carbon. [56Gre] found that terminal solutions up to ~40 at.% V in (Pt) could be retained by rapid cooling and concluded that the terminal solubility at elevated temperatures exceeded 40 at.% V. [73Wat] and [74Sta] agree that the solubility limit at the ~1720 C eutectic reaction is near 60 at.% V. Both also agree that the ordered structures of the three Pt-rich phases all form from this terminal solution by congruent reactions during cooling. [73Wat] and [74Sta] show extensive terminal solubility of Pt in (V). According to [73Wat], this terminal solubility increases from a composition near 5 at.% Pt at 700 C to near 12 at.% Pt at the 1800 C peritectic temperature. The report by [55Gel] of more than 8 at.% Pt solubility in (V) at elevated temperature corroborates the general range of the assessed solvus. 55Gel: S. Geller, B.T. Matthias, and R. Goldstein, J. Am. Chem. Soc., 77, 1502- 1504 (1955). 56Gre: P. Greenfield and P.A. Beck, Trans. AIME, 206, 265-276 (1956). 61Mat: B.T. Matthias, V.B. Compton, and E. Corenzwit, J. Phys. Chem. Solids, 19, 130-133 (1961). 63Mat: B.T. Matthias, T.H. Geballe, and V.B. Compton, Rev. Mod. Phys., 35, 1- 22 (1963). 64Mal: A. Maldonado and K. Schubert, Z. Metallkd., 55, 619-626 (1964). 64Phi: H. von Philipsborn and F. Laves, Acta Crystallogr., 17, 213-214 (1964). 73Cox: J.E. Cox and R.M. Waterstrat, Phys. Lett. 46, 21-22 (1973). 73Wat: R.M. Waterstrat, Metall. Trans., 4, 455-466 (1973). 74Sta: E.A. Statnova and V.V. Kuprina, Vestn. Mosk. Univ., (2), 243-245 (1974) in Russian; TR: Moscow Univ. Chem. Bull., 29(2), 88-89 (1974). 79Aok: Y. Aoki, Y. Obi, and H. Komatsu, Z. Metallkd., 70, 436-440 (1979). 82Smi: J.F. Smith, O.N. Carlson, and P.G. Nash, Bull. Alloy Phase Diagrams, 3, 342-348 (1982). 85Aok: Y. Aoki and J. Echigoya, Scr. Metall., 19, 639-642 (1985). 86Sch: D. Schryvers and S. Amelinckx, Acta Metall., 34, 43-54 (1986). Published in Phase Diagrams of Binary Vanadium Alloys, 1989. Complete evaluation contains 1 figure, 2 tables, and 36 references. Special Points of the Pt-V System