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a b s t r a c tCr segregation at lamellar interfaces in the MoSi 2 /NbSi 2 duplex silicide was examined using a newly developed phase-field model to elucidate the mechanism of interfacial segregation, which is believed to improve the thermal stability of lamellar structures as well as creep resistance. This is because lamellar structures can improve the high-temperature strength, and the stabilization of the lamellar structures improves creep resistance. The model takes into account the segregation energy determined using firstprinciples calculations to reflect the chemical interaction between the solute atoms and the interface, in addition to the elastic interaction. Cr segregation occurs at the interface when the segregation energy is considered, whereas no segregation occurs in the case where only the elastic interaction is considered. However, the extent of segregation was much smaller than that observed experimentally when the segregation energy was evaluated using first-principles calculations without considering lattice vibrations (i.e., the calculations were performed for 0 K). A simulation that took into consideration the segregation energy with the lattice vibrations at 1673 K resulted in segregation similar to that observed experimentally, where the Cr-added MoSi 2 /NbSi 2 duplex silicide was equilibrated at 1673 K, namely, the temperature at which the segregation energy was calculated. Thus, it was revealed that the soluteinterface chemical interaction and its temperature dependence are responsible for the interfacial segregation of Cr. These results suggest that the segregation energy needs to be taken into account in the search for more effective additive elements for improving the thermal stability of lamellar structures as well as the creep resistance.

MoSi2–based alloys are attracting attention as ultra-high temperature structural material for super-high efficiency gas turbine power generation systems. In this study, the effects of Cr-and Zr-addition on interface migration in MoSi2/NbSi2 lamellar silicide were examined by phase field simulations employing the segregation energies evaluated by the first principles calculation in addition to thermodynamic free energy in order to take into account the chemically-driven interfacial segregation. The simulation results indicate that both Cr and Zr can segregate at the lamellar interface to suppress its migration, and the Zr-addition is more effective to lower the interface migration rate than the Cr-addition owing to its higher segregation energy.

a b s t r a c tWe developed a phase-field model of C11 b -MoSi 2 /C40-NbSi 2 duplex silicide incorporating elastic inhomogeneity, and simulated microstructure formation and interface segregation. We examined the effect of elastic inhomogeneity on the morphology, volume fraction of the C11 b -precipitate, stress distribution, and solute partitioning. In the simulations, parameters evaluated by first-principles calculation are used for the experimentally unknown parameters. The lamellar structure was not formed in the case incorporating the elastic strain energy only and ignoring the anisotropy of the interfacial energy. When the anisotropy of the interfacial energy was taken into account, the lamellar structure was formed parallel to (0 0 0 1) C40 as observed in the experiment. It was also found that the elastic strain energy changes the equilibrium concentrations by >0.2 at%, but the difference between the equilibrium concentrations in homogeneous and inhomogeneous systems was <0.1 at%. The interfacial segregation was also hardly affected by the elastic inhomogeneity. These results confirm that elastic inhomogeneity can be neglected in the simulation of MoSi 2 /NbSi 2 lamellar silicide.

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 Cr-addition, 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.