The Effect of Ge and Ti Additions on the Microstructures and Properties of Nb-18Si Based Alloys
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The Effect of Ge and Ti Additions on the Microstructures and Properties of Nb-18Si Based Alloys Zifu Li and Panos Tsakiropoulos IMMPETUS, Department of Materials Science and Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, England, UK ABSTRACT The effects of Ge and Ti additions on the microstructure, hardness and oxidation behaviour of the alloys Nb–18Si–5Ge (ZF1) and Nb–24Ti–18Si–5Ge (ZF3) were studied. The as cast microstructure of the alloy ZF1 consisted of Nbss (cI2), and ȕNb5Si3 (tI32) with the latter being the primary phase and the two phases forming high volume fractions of Nbss + ȕNb5Si3 eutectic. The Ge addition stabilised the ȕNb5Si3 (tI32), and destabilised the Nb3Si (tP32) and the Nbss + Nb3Si eutectic. After heat treatment at 1200 °C for 100 h the ȕNb5Si3 (tI32) was partially transformed to the ĮNb5Si3 (tI32), and equilibrium was reached after heat treatment at 1500 °C for 100 h. The phases present in the as cast alloy ZF3 were the Nbss (cI2), and the Nb3Si (tP32), ȕNb5Si3 (tI32) and Ti5Si3 (hP16) silicides, with the latter forming a eutectic with the solid solution. The same phases were present after heat treatment at 1200 °C for 100 h but only the Nbss, and the Nb3Si and Nb5Si3 silicides were present after 100 h at 1500 °C where TiO2 was also formed. The Ge addition increased the microhardness of the Nb5Si3. The synergy of Ti with Ge resulted in a strong hardening effect and a remarkable retention of the hardness of the alloy ZF3. The additions of Ge and Ti to the Nb-18Si alloy improved the oxidation resistance at 800 °C, but pest oxidation behaviour was not eliminated.
INTRODUCTION In advanced gas turbine engines metal surface temperatures at the hottest locations of airfoils have approached about 1150 oC, which is close to the maximum temperature capability of Ni based superalloys. Nb-silicide based alloys could replace Ni-based superalloys in certain applications in gas turbine engines owing to their lower densities, high melting temperature and high temperature strength [1]. The new alloys will need coatings, like the Ni based superalloys, and thus should have oxidation resistance to enable them to provide adequate protection in case of coating failure. Alloy development has focused on improving mechanical behaviour at room, intermediate and high temperatures as well as oxidation resistance. Alloying additions in Nb – silicide based alloys include transition metals and free electron metals. Alloying with Ti has been shown to significantly improve oxidation resistance [2] and to affect mechanical properties [3]. Germanium is known to have a significant beneficial effect on high temperature oxidation resistance of coatings on refractory metals owing to the formation of a GeO2·SiO2 glass [4]. There is limited work on the effects of Ge and Ti on their own and simultaneously on phase equilibria and oxidation in Nb-Si-X alloy systems. In this work the effects of the synergy of Si with Ge and Ti on phase selection and stability and alloy hardness a
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