Theoretical study of oxygen insertion and diffusivity in the g-TiAl L10 system.

This work is a first-principles study of the insertion and diffusivity of oxygen in the [Formula: see text]-TiAl L10 system. Five interstitial positions were identified as stable. One, however, the 2h site a pyramid composed of a Ti square topped by an Al atom, was found more stable than the others. The oxygen interactions with the TiAl system were thus studied and analyzed in detail using vibrational, elastic and electronic properties. The results show that the O atom prefers to be surrounded by Ti atoms and tries to minimize the number of bonds with aluminum. The diffusion mechanism is subsequently studied at the atomic scale, by analyzing displacements between stable interstitial sites. The oxygen diffusivity is found to be anisotropic and the components in the x and z direction, D x and D z , are then calculated and compared with those of O diffusion into other Ti-Al alloys. The analysis of results shows two effects. First, the stability of sites is related to the number of O-Al bonds, the fewer there are, the more stable the site is, and second, the diffusion is faster when the content of interstitial sites composed of many Ti atoms is low.

Diffusion of interstitials in metallic systems, illustration of a complex study case: aluminum.

While diffusion mechanisms of interstitial elements in fcc systems are generally well-known, especially in the case of H atoms, we show in this work that even in the case of a simple metallic system (aluminum), the diffusion of interstitials exhibits a wide variety of paths and mechanisms that depend on the specie. We used an approach based on first-principles calculations associated with kinetic Monte-Carlo simulations and a multi-state diffusion formalism to compute the diffusion coefficients of five interstitial elements: hydrogen, boron, carbon, nitrogen and oxygen. For instance, at the atomic scale, whilst we find that C atoms prefer to be located in octahedral sites (labeled o) rather than in tetrahedral positions (labeled t), we find one additional stable position in the lattice (M). The diffusion through these three stable positions are thus studied in detail. In the case of B atoms, for which the tetrahedral site is found unstable, the diffusion path is between o-o sites. Similarly, in the case of oxygen, t positions are found to be the only stable positions (o are unstable) and the path of migration, along t-t direction, is found through a twice degenerated asymmetric transition state. In the case of H and N atoms for which t and o sites are stable, we explain why the only path is along the t-o direction. Finally, we discuss explicit formulas to compute coefficients of diffusion of interstitials in fcc structures.

Comparative study of metallic silicide-germanide orthorhombic MnP systems.

We present a comparative study of the structural, energetic, electronic and elastic properties of MX type MnP systems (where X=Si or Ge, and M=Pt, Pd or Ni) using first-principles calculations. The optimized ground state properties of these systems are in excellent agreement with the experimental values. A detailed comparative study of the elastic properties of polycrystalline structures is also presented. We analyze the relationship between the composition and the properties of the systems. Finally, we present the properties of NiSi1-xGex alloys. We show that these properties depend linearly on the Ge content of the alloy. This work has important consequences for semiconductor devices in which silicides, germanides and alloys thereof are used as contact materials.

Diffusion and segregation of niobium in fcc-nickel.

Niobium is one of the major alloying elements, among the refractory elements, contributing to the strengthening of superalloys. Consequently, data about its behavior and its migration mechanism in fcc-Ni are essential knowledge to understand and control the strengthening in such alloys. We present in this work Nb interactions, solubility and diffusion in Ni performed by using the GGA approximation of the density functional theory. The substituted site is found to be the most favorable configuration in comparison to the tetrahedral and octahedral sites. The effect of temperature on solubility is discussed taking into account the thermal expansion of the lattice parameter and the vibrational contribution. Its diffusion mechanism is also discussed and compared to the literature. We finally discuss the segregation of Nb atoms on a Σ(5)-(012) symmetric tilt grain boundary.

First-principles study of diffusion and interactions of vacancies and hydrogen in hcp-titanium.

We present a study of the stability of n-vacancies (V (n)) and hydrogens in the hexagonal close-packed titanium system computed by means of first-principles calculations. In this work, performed by using the generalized gradient approximation of density functional theory, we focused on the formation energies and the processes of migration of these defects. In the first part, the calculated formation energy of the monovacancy presents a disagreement with experimental data, as already mentioned in the literature. The activation energy is underestimated by almost 20%. The stability of compact divacancies was then studied. We show that a divacancy is more stable than a monovacancy if their migration energies are of the same order of magnitude. We also predict that the migration process in the basal plane of the divacancy is controlled by an intermediate state corresponding to a body-centered triangle (BO site). The case of the trivacancies is finally considered from an energetic point of view. In the second part, the insertion of hydrogen and the processes of its migration are discussed. We obtain a satisfactory agreement with experimental measurements. The chemical nature of the interactions between hydrogen and titanium are discussed, and we show that the H-atom presents an anionic behavior in the metal. The trapping energy of hydrogen in a monovacancy as a function of the number of hydrogen atoms is finally presented.