Microstructure Control of Nb-Si Alloy Doped with Zr and Mg through Eutectic and Eutectoid Reactions and its Deformation
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0980-II05-33
Microstructure Control of Nb-Si Alloy Doped with Zr and Mg Through Eutectic and Eutectoid Reactions and its Deformation Behavior Seiji Miura1, Yuki Murasato1, Kenji Ohkubo1, Yoshisato Kimura2, Nobuaki Sekido3, Yoshinao Mishima2, and Tetsuo Mohri1 1 Mat.Sci. and Eng., Hokkaido Univ., Kita-13, Nishi-8, Sapporo, 060-8628, Japan 2 Mat.Sci. and Eng, Tokyo Institute of Technology, Yokohama, 4259, Japan 3 Mat.Sci. and Eng, University of Wisconsin-Madison, Madison, WI, 53706
ABSTRACT In order to overcome the brittleness of Nb-Si intermetallic compounds, novel microstructure control through eutectic solidification and eutectoid decomposition reactions has been proposed for obtaining Nb matrix alloys with a Nb-silicide dispersion by the present authors. As the additions of Zr and Mg accelerate the eutectoid decomposition rate and spheroidization of Nb-silicide during heat treatment, the effects of these additives on the eutectic microstructure are investigated. The effect of the growth rate during the uni-directional solidification on the microstructure of a selected alloy was examined and it was found that the following eutectoid reaction rate is strongly affected by the growth rate. INTRODUCTION A new class of metallic and ceramic materials has been intensively investigated by many research groups because of increasing demands for high temperature materials. These materials are required to possess high melting point, low density, good oxidation resistance, high strength at high temperature, and high toughness at low temperature. In terms of the melting-point and density, Nb-based material is one of the most promising materials because the melting point of Nb is about 1000 oC higher than that of Ni, while its density is lower than those of Ni and any other refractory metals. Recently Nb-Si based alloys have attracted a broad attention [1-3]. However, in order to improve the mechanical properties of the alloys, further microstructural control is inevitable. Figure 1 shows a Nb-rich side of the Nb-Si binary phase diagram [4]. The high melting point of Nb is attractive for high temperature use, but it is also a barrier to applying conventional techniques that are practically applied to single crystal growth of advanced Nibased superalloys. Moreover, the maximum solid solubility of Si in Nb is as small as 3.5 at.%, which provides a limited amount of α-Nb5Si3 dispersoids in the Nb matrix, while the volume fraction of precipitates in recent Ni-base superalloys is over 60 %. In order to overcome these limitations of Nb alloys, the present authors have proposed a new technique based on the solid-solid transformation combined with eutectic solidification for Nb-Si alloys [5, 6]. The process of the microstructure evolution is summarized in Figure 2. According to the Nb-Si binary phase diagram, there are two invariant reactions. One is a eutectic reaction, L -> Nb + Nb3Si (1)
Fig.1 The Nb-Si binary phase diagram [4].
Fig.2 Schematic drawing of the microstructure evolution of Nb-Si alloys. and the other is a eutectoid
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