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Phase-Field Simulation of Lamellar Structure Formation in MoSi2/NbSi2 Duplex Silicide Yuichiro Koizumi1, Toshihiro Yamazaki1,2, Akihiko Chiba1, Koji Hagihara3, Takayoshi Nakano4, Koretaka Yuge5, Kyosuke Kishida5 and Haruyuki Inui5 1 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai, Miyagi 980-8577, Japan 2 Department of Materials Processing, Tohoku University, 6-6-02 Aoba Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan 3 Department of Adaptive Machine Systems, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 4 Division of Materials and Manufacturing Science, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 5 Department of Materials Science and Engineering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT We conducted phase-field simulations of microstructural evolution in C11b-MoSi2 / C40NbSi2 dual phase alloy with and without Cr-addition to examine the factors responsible for the formation and stability of the lamellar structure on the basis of thermodynamics, micromechanics and first-principles calculations. The first principles calculation was used for evaluating the interfacial energy, segregation energy of solute Cr-atoms and lattice parameters of imaginary disilicides for estimating the effects of solute distribution on the lattice misfit. When both of lattice misfit and the anisotropy of interfacial energy is taken into account, a lamellar structure similar to that observed experimentally is formed. In the absence of Cr-addition, the straightness of lamellar structure decreased slightly. When an isotropic interfacial energy is assumed, lamellar structure is not formed. Instead, a microstructure with habit planes parallel to {1 0 q1 1} plane of C40-phase is formed. Thus, the anisotropy of interfacial energy is crucial for the lamellar structure formation rather than the elastic energy due to lattice misfit. INTRODUCTION MoSi2-based materials are promising candidates for ultrahigh-temperature structural applications of ultrahigh efficiency gas-turbine power generation system [1-3]. Recently, oriented lamellar structure formed in C11b-MoSi2/C40-NbSi2 duplex-silicide has been found to improve high temperature strength and room temperature toughness [4]. More recently, Hagihara et al. [5] demonstrated that Cr-addition is effective to improve the thermal stability of the lamellar structure, which is associated with Cr segregation at the lamellar interface. In practical use, however, the stability of the lamellar structure needs to be further improved. In order to improve the thermal stability of the lamellar structure, the deep understanding of the factors that govern the stability of microstructure is important. On the other hand, the phasefield method has become very powerful tool for simulating microstructural evolution in alloys and examining the factors which affect the morphology of microstructures [6]. In the present study, the effects of (i) lattice misfit between C40-marix phase and C11b-precipitate, (ii)

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anisotropy of