Experimental and thermodynamic assessment of the Ge-Nb-Si ternary phase diagram

Niobium silicide-based in-situ composites have the potential to supersede nickel-based superalloys due to their excellent high temperature mechanical properties and low density. The addition of small amounts of germanium into these systems can significantly improve oxidation resistance. The effect of germanium on the phases formed in bulk niobium silicide-based in-situ composites is not particularly well understood, in particular the effect of introducing germanium on the formation of the Nb5Si3 intermetallic. Limited data is available in the literature. To provide coherent information on the effect of germanium on the phase equilibrium in the Nb-Si system, a comprehensive thermodynamic description of the Ge-Nb-Si system has been developed in the current paper using the CALPHAD method. Initially the Ge-Nb phase diagram was reassessed using the CALPHAD method to take into account recent ab initio data. To supplement limited information on the ternary system in the literature between 800 and 1820 °C, the pseudo binary between Nb5Si3 and Nb5Ge3 was studied experimentally between 1200 and 1500 °C. Experimental and modelling results indicate that the W5Si3 prototype of Nb5Si3 can be stabilised to lower temperatures on the addition of germanium. Ge contents in excess of 12.4 at. % at 1200 °C in stoichiometric Nb5(Ge,Si)3 stabilise the W5Si3 prototype. In non-stoichiometric Nb5(Ge,Si)3, where Nb < 62.5 at. %, lower amounts of Ge are required to stabilised the W5Si3 prototype. The liquidus projection suggests a ternary eutectic with Nb5(Ge,Si)3, Nbss and Nb3Si can form in Nb-Si rich alloys during solidification

Experimental and thermodynamic assessment of the Ge-Nb-Si ternary phase diagram

Niobium silicide-based in-situ composites have the potential to supersede nickel-based superalloys due to their excellent high temperature mechanical properties and low density. The addition of small amounts of germanium into these systems can significantly improve oxidation resistance. The effect of germanium on the phases formed in bulk niobium silicide-based in-situ composites is not particularly well understood, in particular the effect of introducing germanium on the formation of the Nb5Si3 intermetallic. Limited data is available in the literature. To provide coherent information on the effect of germanium on the phase equilibrium in the Nb-Si system, a comprehensive thermodynamic description of the Ge-Nb-Si system has been developed in the current paper using the CALPHAD method. Initially the Ge-Nb phase diagram was reassessed using the CALPHAD method to take into account recent ab initio data. To supplement limited information on the ternary system in the literature between 800 and 1820 °C, the pseudo binary between Nb5Si3 and Nb5Ge3 was studied experimentally between 1200 and 1500 °C. Experimental and modelling results indicate that the W5Si3 prototype of Nb5Si3 can be stabilised to lower temperatures on the addition of germanium. Ge contents in excess of 12.4 at. % at 1200 °C in stoichiometric Nb5(Ge,Si)3 stabilise the W5Si3 prototype. In non-stoichiometric Nb5(Ge,Si)3, where Nb < 62.5 at. %, lower amounts of Ge are required to stabilised the W5Si3 prototype. The liquidus projection suggests a ternary eutectic with Nb5(Ge,Si)3, Nbss and Nb3Si can form in Nb-Si rich alloys during solidification

Laser melting of uranium carbides

J. Nucl. Mater. 2009, 385, 443-44