Фазовая диаграмма системы Cu-Nb

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Cu-Nb (Copper-Niobium) D.J. Chakrabarti and D.E. Laughlin The equilibrium phases of the Cu-Nb system are (1) the liquid, L; (2) a, the fcc terminal solid solution (Cu), with limited solubility of Nb; and (3) b, the bcc terminal solid solution (Nb), with limited solubility of Cu. The solid solubility of Nb in (Cu) at 1080 C is 0.10 at.%, and the solid solubility of Cu in (Nb) at 1080 C is ~1.2 at.% [79Pet]. The assessed phase diagram is based on the work of [69All] and [79Pet]. The Cu-Nb system is typical of a system in which the strong effect of compositional impurities in modifying the nature and extent of the distribution of coexisting phases is observed. The S-shaped, near-horizontal appearance of the liquidus indicates that the Gibbs energy curve of the liquid is flat over a range of composition and temperature, and it implies that a miscibility gap may be observed. However, the miscibility gap does not appear in the assessed Cu-Nb diagram. It is well known that impurities in the starting materials, or those encountered during processing, can influence the relative shape and positioning of the Gibbs energy curves of the coexisting phases and, thus, the relative stability of the phases. Oxygen has a strong effect in stabilizing this miscibility gap. Other interstitials, such as carbon and nitrogen, are suspected to have similar effects. A higher rate of solidification of the melt also favors the formation of the miscibility gap by suppressing the precipitation of the stable bcc (Nb) terminal solid solution phase. Similar effects of oxygen in causing liquid immiscibility were reported by [ 71Sch] in Cu-Nb alloys prepared from pressed bars of metal powders by arc melting. The phenomenon occurred in alloys of 10 to 40 wt.% Nb and was attributed to the relatively high oxygen content in the metal powders, because alloys made from bulk material using 99.9 wt.% nominal purity Nb were free from liquid immiscibility. Impurities in Nb stabilize the Nb-based bcc phase (b) and may be responsible for the peritectic transformation that had been reported at the Cu end of the phase diagram. Conversely, lowering the impurity level in Nb is commensurate with an upward shift of the b Gibbs energy curve and could lead to a eutectic transformation with a reduced solubility of Cu in (Nb). The lattice parameter of bcc Nb metal is very sensitive to oxygen content and decreases progressively even as the last traces of oxygen are removed. No compounds or intermediate phases occur in this system. Below 1080 C, a wide two-phase field exists, consisting of an fcc terminal solution, (Cu), and a bcc terminal solution, (Nb). 69All: C. Allibert, J. Driole, and E. Bonnier, C.R. Hebd. S‚ances Acad. Sci., Ser. C., Sci. Chem., 268, 1579-1581 (1969) in French. 71Sch: R.F. Schelle, thesis, Ames Laboratory, USAEC, Iowa State University, Ames, IA (1971). 79Pet: V.T. Petrenko, M.A. Tikhonovskii, A.P. Berdnik, A.I. Somov, M.M. Oleksienko, and V.M. Arzhavitin, Vopr. At. Nauki Tekh., Ser: Obshch. Yad. Fiz., 9, 20-24 (1979) in Russian. Published in Bull. Alloy Phase Diagrams, 2(4), Mar 1982. Complete evaluation contains 3 figures, 5 tables, and 23 references. 1