Mechanisms of Cr segregation to C11b/C40 lamellar interface in (Mo,Nb)Si2 duplex silicide: A phase-field study to bridge experimental and first-principles investigations

Abstract

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.

Mechanisms of lamellar structure formation and Cr interfacial segregation in C11b-MoSi2/C40-NbSi2 dual phase silicide verified by a phase-field simulation incorporating elastic inhomogeneity

“…Gaussian functions are used to interpolate the energies of Cr atoms on the atomic plane to the position between the atomic layers smoothly. In our previous study, it was clarified that the Cr segregation at the lamellar interface was found to be the segregation energy rather than the relaxation of the lattice misfit [13].…”

“…Finally, f seg ðc Cr ; /Þ is the segregation energy function, which represents the chemical energy due to the interaction between the additive Cr atoms and the atomic layers is the diffuse interface. The details of the segregation energy function was described in the previous paper [13]. Here, only the essentials of the segregation energy is described.…”

“…MoSi 2 , NbSi 2 , CrSi 2 ). The values of the lattice sizes can be found in the previous paper [13]. The directions of coordinate used for the simulation space were defined as Direction …”

“…In our previous study, we primarily focused on the segregation mechanism of the additive element. We clarified that the Cr-segregation is largely dependent on the segregation energy, which is the chemical interaction between the solute atoms and the interface which was evaluated by using the first-principles calculation [12][13][14][15][16][17]. Further, we have examined the dominant factor for the formation of lamellar structure [18].…”

“…Therefore, here we developed a phase-field model for the MoSi 2 /NbSi 2 lamellar structure to verify the dominant factor for the formation of lamellar structure taking into account the effect of elastic inhomogeneity on the morphological evolution of the lamellar structure. We developed the model for elastically inhomogeneous system by modifying the phase-field model for elastically homogeneous system, which was used in our previous study on the segregation mechanism of additive element on lamellar boundaries [13]. In our other previous study on the lamellar structure formation [18], we examined only the qualitative correlation among the morphology, the anisotropy of interfacial energy, and the presence of elastic strain energy.…”