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Effects of additives on the phase stability of Nb3Si intermetallic compound and mechanical properties of Nb-Si alloy Tatsuichi Tanahashi*1, Seiji Miura1 and Tetsuo Mohri1 1

Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University,

Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Hokkaido, Japan *Graduate Student, Graduate School of Engineering, Hokkaido University ABSTRACT Recently, Nb-Si alloys have attracted attentions as substitutional materials of Ni-based superalloys because of its low density and high melting point. For attaining good room temperature toughness of Nb-Si alloys, proposed is a microstructure-control technique by combining eutectic reaction (L->Nb+Nb3Si) and eutectoid reaction (Nb3Si->Nb+ Nb5Si3) for spheroidizing Nb5Si3 strengthening phase embedded in Nb matrix [1]. For the solid solution strengthening of Nb matrix phase W and Mo are very effective, but Nb3Si phase disappears by adding these elements of as small as 3 at%. In contrast, Ti and Ta stabilize Nb3Si phase. For a further alloy development, establishment of an alloy design based on the control of phase stability of Nb3Si is needed. In the previous study [2], it was revealed that the phase stability of Nb3Si can be controlled by selecting appropriate Ta/Mo ratio. In the present study, this approach is expanded to other combinations of stabilizing and destabilizing elements of Nb3Si, such as Ti and W, Ta and W, and Ti and Mo. Vickers hardness tests were conducted on the heat-treated samples to reveal effects of additives on mechanical properties of Nb-Si alloy. INTRODUCTION In order to enhance the fuel efficiency of gas turbine engines, the development of high temperature materials is demanded. At the present day, Ni-based superalloys have been used as the turbine foils in gas turbine engines, but their temperature at the hottest locations reached 1150oC which exceeds 80 % of the melting point of Ni. Nb has attracted attentions as one of the candidates for Ni-based superalloys. It is because its melting point (2469oC) is 1000oC higher than that of Ni and its density (8.6Mg/m3) is lower than those of Ni (8.9Mg/m3) and other refractory metals (10-20Mg/m3). A Nb-Si intermetallic compound Nb5Si3 has potential for use at high temperature because of its high melting temperature (2515oC), low density (ȡ=7.16 Mg/m3) [3], high strength, excellent creep resistance

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[4] and superior oxidation resistance. However its brittleness at room temperature draws back Nb-Si based alloys from practical applications. For improving toughness of Nb-Si based alloys, in-situ composites composed of Nb solid solution (Nbss) and Nb5Si3 intermetallic phase have been studied. [5] For attaining good room temperature toughness of Nbss/Nb5Si3 two-phase alloys with spheroidized Nb5Si3 strengthening phase embedded in Nb matrix, proposed is a microstructure-control technique by combining eutectic reaction (L->Nb+Nb3Si) and eutectoid decomposition reaction (Nb3Si->Nb+Nb5Si3) [1, 6]. Fine Nb5Si3 lamellar plates formed through the eutectoid