Phase-Field Study on the Segregation Mechanism of Additive Elements in NbSi 2 /MoSi 2 Duplex Silicide
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Phase-Field Study on the Segregation Mechanism of Additive Elements in NbSi2/MoSi2 Duplex Silicide Toshihiro Yamazaki1,2, Yuichiro Koizumi2, Akihiko Chiba2, Koji Hagihara3, Takayoshi Nakano4, Koretaka Yuge5, Kyosuke Kishida5 and Haruyuki Inui5 1 Department of Materials Processing, Tohoku University, 6-6-02 Aoba Aramaki, Aoba-ku, Sendai, 980-8579, Japan 2 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-0011, Japan 3 Department of Adaptive Machine Systems, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan 4 Division of Materials and Manufacturing Science, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan 5 Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT We have examined segregation behavior of various alloying elements at lamellar interfaces of C40-NbSi2/C11b-MoSi2 duplex silicide by a phase-field simulation, which takes into account not only bulk chemical free energy but also segregation energy evaluated by the first principles calculation to reflect interaction between solutes and interface. The simulation suggests that segregation behaviors greatly depend on additive elements. In the case of Craddition, the C40-phase becomes enriched with Nb and Cr, while the C11b-phase becomes enriched with Mo, which agrees with the equilibrium phase diagram. Slight segregation of Cr atoms is observed at the interface, whereas Nb and Mo concentrations monotonically change across the diffuse interface between C11b and C40 phases. Significant segregations of Zr and Hf are formed at static interfaces, which are attributed to the chemical interaction between solute atoms and the static interface. INTRODUCTION Duplex silicide composed of NbSi2 with C40-structure and MoSi2 with C11b-structure (Figure 1) is one of the candidates for ultrahigh-temperature structural material for future gas turbine power generation systems. In this material, an oriented lamellar structure composed of C11b-type Mo-rich phase and C40-type Nb-rich phase is formed by unidirectional solidification that is followed by appropriate annealing. This lamellar structure improves properties of lowtemperature toughness and high-temperature creep resistance [1, 2]. However, the lamellar structure collapses under prolonged annealing at high-temperature. Therefore, we need to improve the thermal stability of C40/C11b lamellar structure for practical use. One possible approach to achieve a better thermal stability of the lamellar structure is adding another element. In a previous study [3], it was found that a small amount of Cr-addition can improve thermal stability of the lamellar structure, and Cr atoms are found to segregate at the lamellar interface. Cr-segregation at the interface is suggested to alter the lattice misfit effectively in the vicinity of the lamellar interface, which significantly improves planarity and thermal stability of the lamellar structures. But the mechanism of Cr-segregation at the C40/C11b interface has not been
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