The stability of Nb/Nb 5 Si 3 microlaminates at high temperatures
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THE next generation of gas-turbine jet engines are being designed to operate at temperatures approaching 1315 ⬚C, 200 ⬚C higher than is possible with the current Ni-based superalloys.[1,2] In addition, the turbine blades in these engines are being designed to have wall thicknesses of 500 m or less, considerably thinner than can be fabricated with current casting techniques. In order to achieve these operating temperatures and manufacturing specifications, new alloys and new manufacturing techniques are required. In this study we examine the high-temperature stability of Nb/Nb5Si3, one of the most promising alloy systems for the manufacture of the new turbine blades. The alloys examined were manufactured using a fabrication technique and an alloy geometry that are ideally suited to the preparation of thin-walled composite components, namely, vapor deposition of microlaminate foils. The Nb/Nb5Si3 system is attractive for high-temperature structural applications because it is chemically stable to 1660 ⬚C[3] and exhibits very good mechanical properties both at room temperature[4–7] and at elevated temperatures.[1,3,8,9] In addition, recent studies have shown that alloying and protective coatings can significantly reduce oxidation, which is known to limit high-temperature applications of Nb and its alloys.[1,10,11] Most of the Nb/Nb5Si3 composites studied to date have been so-called in-situ composites, which are D. VAN HEERDEN, Associate Research Professor, and T.P. WEIHS, Associate Professor, are with the Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218. P.R. SUBRAMANIAN, Research Scientist, is with GE Corporate Research & Development, Physical Metallurgy Laboratory, Schenectady, NY 123011072. T. FOECKE, Materials Scientist, is with the Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, MD 208998553. A.J. GAVENS, formerly Doctoral Student, Department of Materials Science and Engineering, Johns Hopkins University, is a Research Engineer with Knolls Atomic Power Laboratory, Schenectady, NY. Manuscript submitted August 25, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
prepared using directional solidification or hot extrusion.[2,4,12] More recently, Nb/Nb5Si3 microlaminate composites have been produced by vapor deposition and by hot pressing sheets of Nb and either Nb5Si3 or Si.[9,13,14] Initial investigations have indicated that microlaminates have comparable or superior properties to the bulk in-situ composites.[9,13–15] However, the phase, chemical, and microstructural stability, and the mechanisms by which microlaminates break down at high temperatures, are not well documented. In one of the few published studies of the high-temperature breakdown of microlaminates, Rowe et al.[16,17] and Cao and co-workers[18,19] examined the phase stability of Nb/Nb3Al and Nb/NbCr2 microlaminates. They found that these two sets of microlaminates broke down above 1000 ⬚C, primarily because the volume fraction of the two phases in each composite varie
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