Фазовая диаграмма системы Ge-W

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Ge-W (Germanium-Tungsten) S.V. Nagender Naidu, A.M. Sriramamurthy, and P. Rama Rao The assessed Ge-W phase diagram is based on [71Pov]. The diagram is suggested to be a simple eutectic at atmospheric pressure. The eutectic reaction occurs at about 925 C with the eutectic composition slightly greater than 0.2 at.% W. There is indication of formation of an intermediate compound, W2Ge3, by a peritectoid reaction occurring at about 750 C. The solid solubility of W in ( Ge) is about 0.2 at.% W and that of Ge in (W) is about 2.5 at.% Ge at the eutectic temperature. At high pressures the system becomes more complex. [71Pov] reported the solubility of W in liquid Ge to be about 2.5 at.% W at 1700 C. Other than this limited information, no data are available to determine the shape of the liquidus. As such, the liquidus is shown as a dotted curve in the assessed diagram. [71Pov] concluded that the intermediate phase contains 60 at.% Ge and 40 at.% W, which corresponds to the chemical formula W2Ge3. However, the phase was not characterized by crystal structure analysis, nor has this phase been confirmed by any other group of investigators. The findings of [78Pop], [79Pop], [81Ago], [82Pop], and [83Pop], who used Ge of 99.99% and W of 99.91% purity and made high-pressure X-ray studies on Ge-W alloys containing 25 to 70 at.% Ge between 200 and 2000 C, can be summarized as follows. At a pressure of 30 kbar, aWGe2 is formed, which is isostructural to aMoGe2. At a pressure of 77 kbar, tungsten germanides of composition W5Ge3 in two modifications and also tungsten digermanide, bWGe2, having the same structure type as that of MoSi2, are formed. The two modifications of W5Ge3 have tetragonal crystal structures of W5Si3 and Cr5B3 types. No variation of the unit cell parameters with composition was found for these phases. Lattice parameter measurements indicate a homogeneity range for the bWGe2 phase. According to lattice parameter measurements, the system has high solubility of Ge in (W) at high temperatures and pressures. When the thermal stability of tungsten germanides was investigated at atmospheric pressure at 1000 C for 5 h, there was no change in the W5Ge3 phases; however, bWGe2 was found to decompose into elemental forms. 71Pov: K.B. Povarova and E.M. Savitskii, Diagrammy Sostoyaniya Metal. Sist. Mater. Vses, Soveshch., N.V. Agreev, Ed., Nauka Press, Moscow, 95-98 (1971) in Russian. 78Pop: S.V. Popova and L.N. Fomitcheva, Izv. Akad. Nauk SSSR, Neorg. Mater., 14(4), 684-686 (1978) in Russian. 79Pop: S.V. Popova and L.N. Fomitcheva, High Pressure Science and Technology, Vol. 1, Plenum Press, New York, 272-273 (1979). 81Ago: V.M. Agoshkov, V.D. Gorbatenko, S.V. Popova, and L.N. Fomitcheva, J. Less-Common Met., 78(2), 235-243 (1981). 82Pop: S.V. Popova, Phys. Scr., 131-133 (1982). 83Pop: S.V. Popova, Fiz. Tekh. Vys. Davleniy, 11, 3-12 (1983) in Russian. Published in J. Alloy Phase Diagrams, 5(3), Sep 1989. Complete evaluation contains 1 figure, 1 table, and 14 references. 1