First-principle atomistic thermodynamic study on the early-stage corrosion of NiCr alloy under fluoride salt environment.

The atomic morphology change in the NiCr alloy surface induced by fluorine-chemisorption was investigated by the ab initio atomistic thermodynamic method to elucidate early-stage corrosion processes of nickel-based alloys in strong oxidizing environment. The surface phase diagrams of Cr-doped Ni(111) surface as a function of fluorine chemical potential were obtained to track the surface structures that are most likely to be fostered in various temperature and pressure conditions. The adsorption of fluorine on the top site of Cr in the alloy surface was the most energetically favorable one. With increasing fluorine chemical potential, more fluorine atoms started to agglomerate in the trapping sink of Cr. Fluorine-fluorine repulsion interaction coupled with strong F-Cr bonding could facilitate a decided morphology modification of the metal substrate. Moreover, an insight into the desorption pathways for potential species revealed that in the presence of fluorine, the dissociation of Cr predominantly stems from the relatively easy desorption in the form of CrF2/CrF3 molecules from the non-passivated Ni-based alloy surface.

First-Principles Study of Vacancies in Ti3SiC2 and Ti3AlC2.

MAX phase materials have attracted increased attention due to their unique combination of ceramic and metallic properties. In this study, the properties of vacancies in Ti3AlC2 and Ti3SiC2, which are two of the most widely studied MAX phases, were investigated using first-principles calculations. Our calculations indicate that the stabilities of vacancies in Ti3SiC2 and Ti3AlC2 differ greatly from those previously reported for Cr2AlC. The order of the formation energies of vacancies is VTi(a) > VTi(b) > VC > VA for both Ti3SiC2 and Ti3AlC2. Although the diffusion barriers for Ti3SiC2 and Ti3AlC2 are similar (~0.95 eV), the properties of their vacancies are significantly different. Our results show that the vacancy-vacancy interaction is attractive in Ti3AlC2 but repulsive in Ti3SiC2. The introduction of VTi and VC vacancies results in the lattice constant c along the [0001] direction increasing for both Ti3SiC2 and Ti3AlC2. In contrast, the lattice constant c decreases significantly when VA are introduced. The different effect of VA on the lattice constants is explained by enhanced interactions of nearby Ti layers.

Piezoelectric, Mechanical and Acoustic Properties of KNaNbOF5 from First-Principles Calculations.

Recently, a noncentrosymmetric crystal, KNaNbOF5, has attracted attention due to its potential to present piezoelectric properties. Although α- and ß-KNaNbOF5 are similar in their stoichiometries, their structural frameworks, and their synthetic routes, the two phases exhibit very different properties. This paper presents, from first-principles calculations, comparative studies of the structural, electronic, piezoelectric, and elastic properties of the α and the ß phase of the material. Based on the Christoffel equation, the slowness surface of the acoustic waves is obtained to describe its acoustic prosperities. These results may benefit further applications of KNaNbOF5.