Effects of Zr on the eutectoid decomposition behavior of Nb 3 Si into (Nb)/Nb 5 Si 3
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I. INTRODUCTION
ALLOYS based on Nb-silicides have been investigated because they exhibit superior high-temperature strength to the commercial nickel-base superalloys.[1,2] However, lack of room-temperature ductility and high-temperature oxidation resistance are the major drawbacks for further development. In order to improve the toughness of these alloys, microstructure control is a key method by which the alloys based on brittle intermetallic compounds are endowed with ductile-phase toughening. The eutectoid decomposition from Nb3Si to Nb/Nb5Si3 two-phase structure was reported to be so sluggish that it takes about 100 hours to finish the decomposition even at the nose temperature of about 1500 °C.[1] These slow kinetics may be relevant to its high melting point, and is an advantage for high-temperature applications in a practical viewpoint. According to a partial Pettifor map on Nb and related silicides,[3] Zr also forms a M3Si-type silicide (M is a transition metal) with the Ti3P-structure such as Nb, Ti, and Ta in the neighborhood of Zr. It was found that both Ti and Ta stabilize Nb3Si by replacing Nb and form continuous solid solutions (Nb, Ti)3Si and (Nb, Ta)3Si.[3–6] Sekido et al. found that addition of the 10 at. pct of Ti increases the decomposition rate of the Nb3Si, but the details are still not fully understood.[6–10] Despite the similarity among Ti, Ta, and Zr as the Ti3P-silicide former element, no continuous solid solution between Nb3Si and Zr3Si phases is SEIJI MIURA, Associate Professor, MIKI AOKI and YASUHIKO SAEKI, Graduate Students, KENJI OHKUBO, Staff Member, and TETSUO MOHRI, Professor, are with the Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan. Contact e-mail: [email protected] YOSHINAO MISHIMA, Professor, is with the Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502, Japan. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
reported in the Nb-Zr-Si ternary system.[11] We found that Zr also accelerates the decomposition of Nb3Si, like Ti does.[3] Therefore, in this article, we attempt to understand the eutectoid decomposition reaction of Nb3Si by comparing it with the decomposition reaction of binary and Tiadded alloys. Also, we investigate the effect of the Nb and/or Nb5Si3 phases formed during solidification by focusing on whether they act as preferential nucleation sites for the decomposition. II. EXPERIMENTAL PROCEDURES Alloys were arc-melted in Ar atmosphere on a water-cooled copper hearth. The binary alloy conta
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