A systematic study of the atomic and electronic structure of the γ-TiAl(111)/α-Al2O3(0001) interface with intermediate metal (Nb, Mo, Ni, Re) and oxide (Nb2O5, MoO3) layers has been performed by the projector augmented-wave method within density functional theory. The work of separation at the interfaces in dependence on the cleavage plane has been calculated. It is shown that a high adhesion energy obtained at the interface with the O-terminated α-Al2O3 is decreased at the γ-TiAl/Me interface but it remains enough high at the Me/α-Al2O3(0001)O interface due to a large ionic contribution to the chemical bonding. The influence of formation of intermediate impurity oxide layers on the adhesive properties of the alloy/oxide interface is discussed as well. The obtained results indicate that the fracture will occur inside the impurity oxide or its interface with the alloy.

The influence of interstitial B and C impurities on grain boundary cohesion in the series of B2-TiMe alloys, where Me=Fe, Co, Ni or Pd, was investigated using the plane-wave pseudopotential method within density functional theory. The most preferential sites for interstitial impurity atoms at the TiMe Sigma 5(310) symmetrical tilt grain boundary were determined. It was shown that the impurities' sorption energies at the grain boundary depend strongly on their local environment. Analysis of the electronic properties allows us to reveal the microscopic nature of the chemical bonding of B and C at the grain boundary. It was shown that, in contrast with hydrogen, both impurities decrease the grain boundary energy more significantly than the surface one. This results in an increase in the Griffith work which also indicates the strengthening of the grain boundary. Our estimation of the Griffith work for the TiMe alloy containing both B and H atoms shows an increase in comparison with the undoped alloy, but the effect of carbon on grain boundary strengthening in the presence of hydrogen seems to be negligible. The contributions of the chemical and elastic mechanisms to the Griffith work are discussed.

The hydrogen diffusion pathways were studied in B2-TiFe alloy within density functional theory (DFT) using the plane-wave pseudo-potential method. Our results confirm that the hydrogen diffusion between octahedral interstices where it is surrounded by two Fe and four Ti atoms along [10-1] direction is most preferential in TiFe bulk. The estimated hydrogen diffusion barrier of 0.62 eV differs insignificantly from values of barriers in pure Ti. The influence of substitutional transition and simple metal impurities on the energy barriers is discussed. It was found that impurities such as V, Cr, Mn decrease the hydrogen diffusion barriers along both considered pathways whereas Pd impurity decreases considerably the barrier along [00-1] direction that leads to the change of the diffusion mechanism. In general, the competition of structural and electronic factors strongly influences the hydrogen diffusion barriers. The present results provide a deep understanding of H behavior in TiFe bulk. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The atomic and electronic structure of the interfaces between metals with body-centered cubic (bcc) and face-centered cubic (fcc) structures and zirconium dioxide is studied systematically using the ab initio methods of the electron density functional theory (DFT). It is shown that high adhesion properties can be attained at the nonstoichiometric polar Me(001)/ZrO2(001) interface with bcc metals from the middle of the 4d–5d periods (Mo, Ta, W, and Nb). Charge transfer from the metal to the oxide substrate ensures the strong ionic chemical bond on the metal-ceramic interfaces. The structural and electronic factors responsible for lowering of adhesion at differently oriented interfaces are analyzed. It is shown that a decrease of adhesion at the (110) nonpolar stoichiometric interface is due to an increase in the interfacial spacing as well as a decrease in the number of metal-oxygen bonds. The effect of doping with oxides (CaO, MgO, and Y2O3) stabilizing zirconium dioxide at low temperatures on the adhesion energy at the Me(001)/ZrO2(001) interface is analyzed.

The hydrogen sorption in intermetallic B2 TiM (M = Ni, Co, Pd) with a symmetric Σ5(310) tilt grain boundary and a (310) surface is studied by density functional theory methods. The effect of hydrogen on the electronic characteristics of the alloys is analyzed as a function of a sorption position at the interfaces. The hydrogen sorption energy is shown to depend on the local environment of hydrogen; on the whole, hydrogen at the interfaces prefers titanium-rich positions. The hydrogen sorption energy in metal-rich positions decreases when the d shell of the second alloy component is filled with electrons. The grain-boundary energy, the surface energy, and the hydrogen segregation energies to the interfaces are calculated. Hydrogen sorption in titanium alloys is shown to decrease Griffith work and to favor brittle fracture along tilt grain boundaries.

A comparative theoretical study of metal-zirconia interfaces with BCC and FCC metals was performed using pseudopotential approach with LDA and GGA approximation for exchange-correlation functional. It was shown that the high adhesion can be achieved at the O-terminated Me/ZrO2(001) interface with BCC metals that is related to large charge transfer from metal film to substrate and increase of an ionic contribution in the chemical bonding. The structural and electronic factors which are responsible for decrease of adhesion at differently oriented metal-zirconia interfaces are discussed. The influence of CaO, MgO and Y2O3 doping on the work of separation (W-sep) at Me(001)/c-ZrO2(001) is analyzed.

We present a density functional theory (DFT) study of the hydrogen-metal interaction in the B2-TiFe alloy with Σ5(310) symmetrical tilt grain boundary (GB) and (310) free surface (FS). The influence of hydrogen on the electronic properties of alloy with GB and FS is analyzed for different hydrogen sorption sites. The hydrogen absorption/adsorption, binding and segregation energies are calculated at GB and FS. Our calculations reveal that H segregates more strongly to the surface than to the GB that results in decrease in the Griffith work, i.e., H makes the fracture of the GB easier.

We present a comparative ab-initio study of atomic and electronic properties of Al/TiC(N) and Al/VC(N) interfaces performed using DFT with the projector-augmented-wave method. The most stable configuration of metal film on the ceramic substrate was determined. The work of separation of metallic films in dependence on cleavage plane was calculated. The analysis of the electronic properties confirms a stronger interaction in the case of the Al top position over metalloid which indicates the dominant role of the covalent contribution in the chemical bonding at these interfaces. We demonstrate that point defects at the interface (metal, carbon and nitrogen vacancies) change the adhesion at the metal-ceramic interfaces significantly and cause redistribution of the electron properties across the interface.

Molecular dynamics simulations are used to study brittle/ductile interfaces Ni/B2-NiAl under mechanical loading. Uniaxial tensile tests perpendicular to the interface are performed. It is shown that interfaces have influence on strain induced material failure by nucleation of defects. Crack propagation in the interface is investigated by applying load via fixed displacement boundary conditions. Determined crack velocities in the interface are found to be clearly above those in each of the materials.

This paper reports on the results of the systematic analysis of the atomic and electronic structure of the Me /α-Al 2 O 3 (0001) interfaces for two series of isoelectronic metals ( Me = Cu, Ag, Au and Ni, Pd, Pt), depending on the termination of the oxide substrate and the configuration of oxide films. The calculations have been performed by the pseudopotential method in the plane-wave basis set. The adhesion energy of metal films has been calculated depending on the cleavage plane. It has been shown that the adhesion energy is maximum at the oxygen interface, which is caused by the ion component in chemical bonding at this interface. The aluminum and aluminum-enriched interfaces are characterized by the metallic type of bonding. The local densities of states and the charge distribution near the interface have been analyzed. It has been demonstrated that oxygen vacancies at the interface substantially weaken the adhesion due to the partial breaking of Me -O bonds.

The atomic and electronic structures of Me/ZrO2(001) interfaces, where Me is Ni, Fe or a Ni–Fe alloy, are investigated by the plane wave pseudopotential method within density-functional theory. The work of separation of metal films from oxide substrate for the O- and Zr-terminated Me/ZrO2(001) interfaces is calculated. High adhesion at both Me/(ZrO2)O and Me/(ZrO2)Zr interfaces is found. The effect of oxygen vacancies on the adhesion at the metal–ceramic interfaces is also investigated. It is shown that Ni(Fe)–O interaction at the O-terminated interface weakens in the presence of interfacial oxygen vacancies. At interfaces with Ni–Fe alloys the adhesion depends strongly on the composition of the interfacial layers and their magnetic properties.

The atomic and electronic structure of Me/α-Al2O3(0 0 0 1) interfaces, where Me=Al, Ag, Cu, are investigated by the plane wave pseudopotential method within density functional theory. The work of separation of metal films from oxide substrates is calculated for three terminations of the oxide surface. The work of separation at the Me/(Al2O3)O interface is found to be several times larger than that at the Me/(Al2O3)Al interface. The effect of oxygen and metal vacancies on the adhesion at the metal–ceramic interfaces is investigated. It is shown that the Me–O interaction at the considered interfaces weakened due to presence of surface oxygen vacancies.

As the order of a quasicrystal is quasiperiodic, it can be described as an irrational cut through a periodic structure in a higher-dimensional space. This mathematical trick has important consequences for the low energy excitations that can occur. Translating the cut space to a different position is a symmetry operation, which changes the quasicrystal structure, but not its energy. A small breaking of this symmetry, by chosing a cut of small and slowly varying slope (with respect to the ideal orientation) therefore leads to low-energy Goldstone modes, called phasons. In many respects, phasons are analogous to phonons, which are small and slowly varying distortions of a structure in physical space. The distortions related to phasons rest in the complementary, internal space needed for the embedding of the quasicrystal in the higher-dimensional crystal. In much the same way as there is an elastic energy for phonon type distortions of a solid, there is an effective elasticity theory for the phason degrees of freedom, which moreover is coupled to the phonon elasticity. A non-zero phason strain, i.e., a non-zero slope of the cut space, will cost energy. At higher temperatures, however, phason excitations, which correspond to a fluctuating phason strain, will become possible. This is analogous to phonons. There is one important difference, however. Whereas phonons are usually propagating modes, phasons are believed to be diffusive. Phason strain in a quasicrystal is connected with a rearrangement of certain local atomic configurations. In an elementary form these rearrangements are called phason flips. Understanding the dynamics of phason flips and other atomic rearrangements of the structure of a quasicrystal is essential for the understanding of the formation and stability of quasicrystals, and also for many of their physical properties. Perfect quasicrystals are usually obtained by high temperature annealing after solidification. During this process, many defects initially present are eliminated. It is therefore necessary that phason flips, atomic diffusion, and other dynamical processes are possible and effective at these temperatures. Atomic diffusion and phason mobility are also important for the mobility of dislocations. Unlike in a crystal, in a quasicrystal a moving dislocation leaves a phason wall in its wake. This phason wall must be smoothed out and finally eliminated by phason flips and diffusion processes, for otherwise the material would harden very quickly. Mobile phason flips are therefore necessary for the ductility of quasicrystals observed at high temperatures. There are several other interesting consequences …

The atomic dynamics of decagonal Al-Ni-Co and Al-Cu-Co quasicrystals is investigated by molecular dynamics simulations. Depending on the local environments, the mobility of the atoms varies greatly from site to site. Above two thirds of the melting temperature, a large fraction of the aluminium atoms become so mobile, that their diffusion can be measured directly in the simulation. As aluminium diffusion is hard to access experimentally, molecular dynamics simulations therefore provide a unique way to study aluminium diffusion in these complex systems.

Aluminium is the majority element in many quasicrystals and expected to be the most mobile element, but its diffusion properties are hardly accessible to experiment. Here we investigate aluminum diffusion in decagonal Al-Ni-Co and Al-Cu-Co quasicrystals by molecular dynamics simulations, using classical effective pair potentials. Above two-thirds of the melting temperature, strong aluminum diffusion is observed. The diffusion constant is measured as a function of temperature and pressure, from which the activation enthalpies and activation volumes are determined. As there are no vacancies in the samples, the diffusion, which is anisotropic, must use a direct mechanism. The high mobility of aluminium is also relevant for structure determination, and will contribute to diffuse scattering. The qualitative behavior of the dynamics is confirmed by ab initio simulations.

The atomic dynamics of decagonal Al–Ni–Co quasicrystals are investigated by molecular dynamics simulations. The model structures used consist of an alternating stacking of two layers, which are decorations of a hexagon-boat-star tiling. Apart from small relaxations, these model structures are essentially stable up to the melting point. Above two thirds of the melting temperature, a large fraction of the aluminium atoms becomes mobile, and long-range atomic diffusion sets in, without destroying the overall structure. The diffusion of aluminium is analyzed as a function of temperature and pressure, and the microscopic diffusion processes are discussed.