On the Nb-Ge Binary System
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THE Ni-based superalloys currently used in the most advanced gas turbine engines are exposed to temperatures around 1423 K (1150 C). As engine temperatures are increased to improve efficiency, these materials are reaching the limits of their temperature capabilities, and thus there is a need for new alloys that can be used at higher temperatures in the next generation of gas turbine technology. Niobium silicide-based intermetallic alloys are candidate materials which combine desirable creep properties, strength, and high melting points with low density.[1] These alloys typically consist of ductile Nb solid solution (Nbss) and high strength intermetallic particles (e.g., Nb5Si3). The oxidation resistance of these alloys, however, can be poor. Improvements in oxidation resistance can be made through alloying additions such as Ge,[2] and therefore an accurate description of the Nb-Ge phase diagram is needed. Germanium additions have been shown to dissolve in the Nbss and to enter the Nb5Si3 phase.[3,4] Germanium is reported to improve oxidation of these alloys through formation of a glassy layer on the surface of the samples at high temperatures.
IOANNIS PAPADIMITRIOU, Ph.D. Student, CLAIRE UTTON, Post-doctoral Research Associate, and PANOS TSAKIROPOULOS, Professor, are with the Department of Materials Science and Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, England, UK. Contact e-mail: p.tsakiropoulos@sheffield.ac.uk Manuscript submitted February 23, 2015. Article published online September 23, 2015 5526—VOLUME 46A, DECEMBER 2015
Although experimental and computational assessments of the Nb-Ge phase diagram exist, many publications report differing results. Geng et al.[5] recently performed a Calphad assessment and reported four intermetallic phases, namely Nb3Ge (cP8 Cr3Si-type) stable up to 2176 K (1903 C), Nb5Ge3 (tI32 W5Si3-type, D8m) stable up to 2452 K (2179 C), Nb3Ge2 (hP16 Mn5Si3-type, D88), stable up to 2313 K (2040 C), and NbGe2 (hP9 CrSi2-type, C40) stable up to 1953 K (1680 C) (Figure 1(a)). The Nb3Ge2 phase, which is also known in the literature as Nb10Ge7, Nb5Ge3.5, or bGe3Nb5, has been reported to crystallize in the hexagonal D88 symmetry.[6–10] In the current study, the Nb3Ge2 will be referred to as Nb10Ge7. Nowotny et al.,[6] Pan et al.,[11] and Richter et al.[12] reported the latter as a stable phase in the Nb-Ge binary system, while Jorda et al.[13] and Kloska and Haase[10] suggested that it is either stabilized by light elements or metastable. Geng et al.[5] produced two thermodynamic descriptions: one containing the Nb10Ge7 phase and one without. A zoomed in region of the phase diagram containing Nb10Ge7 is shown in Figure 1(b). Okamoto[14] published a review of the phase diagram following Geng et al.[5] and reported the same phases and temperature stability range only for the 3:1 and 1:2 stoichiometry. Okamoto[14] included the Nb10Ge7 phase. But data for the Nb10Ge7 and also Nb5Ge3 phases conflict with Geng et al.[5] Okamoto[14] repor
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