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GE Corporate Research and Development, Schenectady, NY 12301, USA. of Engineering, Brown University, Providence, RI 02912, USA.

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ABSTRACT This paper will discuss the relationship between microstructure and creep behavior in hightemperature niobium-silicide based in-situ composites. The creep behavior of composites generated from binary Nb-Si alloys, and higher order alloys containing Mo, Hf and Ti additions, will be described. In-situ composites were tested in compression at temperatures up to 12000 C and stress levels in the range 70 to 280MPa. It was found that the Hf concentration can be increased to 7.5 with little increase in creep rate, over that for the binary Nb3Si-Nb composite, but at higher concentrations the creep rate is increased at stress levels higher than 21OMPa. At stresses less than 21OMPa the Ti concentration can be increased to 21 without a detrimental effect on creep performance, but at higher concentrations there is a substantial increase in the creep rate. INTRODUCTION Niobium silicide based in-situ composites are potential candidates for use as structural materials at very high temperatures [1-4]. These composites consist of Nb 5Si3/Nb3Si toughened with a Nb solid solution (abbreviated by (Nb) in the present paper). Previous investigations of in-situ composites generated from binary Nb-Si alloys have shown a promising combination of high-temperature strength, creep resistance, and room-temperature fracture toughness [1-3]. The present paper focuses on creep behavior, and in particular the effect of Ti, Hf, and Mo additions on the secondary creep rates of Nb3Si-(Nb) in-situ composites. The composites described in this paper were produced by directional solidification, which gives a microstructure consisting of (Nb) dendrites together with Nb3Si aligned with the growth direction. The present approach for investigating creep is separated into two stages: first compression creep tests at temperatures from 1000 0C to 1200'C were conducted, and second, in selected composites tensile creep tests were performed. Previous work indicated that creep deformation in Nb5 Si3 is controlled by bulk diffusion of Nb in the Nb5Si3 [5]. Mo and Hf have a relatively large atomic size and it was therefore considered that they may be effective in improving the intrinsic creep performance of both the silicide and the composite. Mo and Hf additions were also considered because they increase the strength of the (Nb). Ti and Hf are added to these composites to improve room temperature fracture toughness and high-temperature oxidation performance [1, 4]. The aim of the present paper is to describe high-temperature creep in directionally solidified (DS) in-situ composites, and the effect of Ti, Hf and Mo additions on the secondary creep rate. EXPERIMENTAL The samples were prepared using cold crucible directional solidification [1,3] after triple melting the starting charges from high purity elements (>99.99%). Induction melting in a segmented watercooled copper crucible was used. The directional solidification