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The electronic, lattice, and magnetic properties of CoSb3, Cr-doped CoSb3, and related compound thin films were calculated by using the total energy pseudopotential method. A vacuum region and slab as a thin film were periodically repeated in the supercell. We considered three slab structures. One is symmetric, another is asymmetric, and the third is unusual. They are free-standing in the supercell. The magnetic moment of doped Cr atom is large in all Cr-doped cases. Magnetic moments of several Co atoms are enhanced in the asymmetric and unusual cases. The calculation results in this study indicate that electronic states of most thin films are ferromagnetic and they are energetically more favorable than nonmagnetic cases.

We studied thin films of Fe2VAl, Fe2VAl/Si, and a related compound using the total energy pseudopotential method. The internal atoms in a supercell of the repeated slab model were fully relaxed, except for Fe2VAl/Si. The detailed electronic and magnetic properties of structurally relaxed thin films of Fe2VAl, Fe2VAl/Si, and a related compound were calculated. These films are free-standing in the supercell, except for Fe2VAl/Si, which comprises Fe2VAl and Si (substrate) layers in a supercell. A thin film comprising Fe2V, Al, and Si (substrate) layers in the supercell (denoted as Fe2V/Al/Si) was also studied. Although electronic states of bulk Fe2VAl (full-Heusler) are metallic with pseudo-gap states around the Fermi level and nonmagnetic, the calculation results for Fe2VAl, Fe2VAl/Si, and Fe2V/Al/Si thin films in this study indicate that the films have ferromagnetic properties and their ferromagnetic states are energetically more favorable than nonmagnetic states. The magnetic moments are large and enhanced in the thin films. The magnetic moments of Fe in Fe2VAl (Fe12V4Al4), Fe2VAl (Fe20V8Al8), Fe2VAl/Si, and Fe2V/Al/Si thin films are 2.07 μ B/per atom, 1.29 μ B/per atom, 1.83 μ B/per atom, and 2.22 μ B/per atom, respectively. Although thin films have net ferromagnetic properties, the magnetic moments of the up and down spin states of Fe and V are antiferromagnetically aligned.

The geometrical properties of the icosahedral ordered structure formed in liquid and glassy phases of metallic glasses are investigated by using molecular dynamics simulations. We investigate the Zr-Cu alloy system as well as a simple model for binary alloys, in which we can change the atomic size ratio between alloying components. In both cases, we found the same nature of icosahedral order in liquid and glassy phases. The icosahedral clusters are observed in liquid phases as well as in glassy phases. As the temperature approaches to the glass transition point Tg, the density of the clusters rapidly grows and the icosahedral clusters begin to connect to each other and form a medium-range network structure. By investigating the geometry of connection between clusters in the icosahedral network, we found that the dominant connecting pattern is the one sharing seven atoms which forms a pentagonal bicap with five-fold symmetry. From a geometrical point of view, we can understand the mechanism of the formation and growth of the icosahedral order by using the Regge calculus, which is originally employed to formulate a theory of gravity. The Regge calculus tells us that the distortion energy of the pentagonal bicap could be decreased by introducing an atomic size difference between alloying elements and that the icosahedral network would be stabilized by a considerably large atomic size difference.

The Gibbs energy functions of the phases in the Al-Cu binary system are taken from the CALPHAD-type thermodynamic assessment (Witusiewicz et al., 2004; Ansara et al., 1998) [1] , [2] , where the effect of the monovacancy (Va), divacancy (VaVa) and Va-solute atom pair are taken into account based on the formulation (Abe et al., In press). The divacancy is modeled as an associate, VaVa, in the FCC solid solution. The contributions from the Va-solute pair are included through the ternary excess Gibbs energy term. Using the Gibbs energy functions provided in this data article, the fractions of the monovacancies and divacancies, even in various metastable conditions, can be calculated. Since the Gibbs energy functions and phase descriptions are written in the TDB (Thermodynamic DataBase) format, one can use this file with various thermodynamic software packages, such as OpenCalphad [3] etc.

Thermal vacancies and vacancy complexes in solids have not been treated explicitly in the CALPHAD-type thermodynamic assessments. Since thermal vacancies have important roles in phase transformations, precipitations and diffusion processes, it is necessary to describe them in the thermodynamic models for various simulations e.g. phase field simulations. In the present work, a description of the monovacancy for pure metals was expanded to vacancy complexes in an FCC solid solution in alloys where the interaction energies between vacancy-vacancy and vacancy-solute were taken into account using the Gibbs energy functions.

We present theoretical calculations of the thermoelectric properties of a magnetic semi-conductor CuFeS2. The electrical contributions, namely the Seebeck coefficients, electrical conductances, and electronic thermal conductances, are investigated using first-principles calculation techniques based on density functional theory and the nonequilibrium Green's function formalism. The lattice thermal transport properties are analyzed using the nonequilbirium molecular dynamics simulation method. Using a combination of the calculated electronic contributions and the lattice thermal contribution, the thermoelectric figures of merit are calculated and analyzed in doped configurations for the theoretical prediction of the highest thermoelectic energy conversion efficiency. It is shown that dilute doping enhances the figure of merit by 2.5 times compared to 3% doping which was previously experimentally obtained to give the best thermoelectric performance among samples studied.

The thermoelectric properties of TiN/MgO surface nanostructures have been determined using first-principles calculations based on the nonequilibrium Green’s function (NEGF) method. Through structural modification of the surfaces at the atomistic level, we find that the metallic TiN thin-film layer becomes semiconducting with a small bandgap, which enhances the Seebeck coefficient, while the electrical conductivity remains high at room temperature. Hence, a much larger thermoelectric figure of merit is obtained compared with bulk. These findings indicate the possibility of designing thermoelectric devices with surface nanostructures.

The effect of short range ordering (SRO) on the Gibbs energy is discussed for the BCC phases within the framework of compound energy formalism (CEF). Although in the compound energy formalism, SRO is not explicitly described it is possible to include the contribution through the reciprocal parameter. Introducing the reciprocal parameter of the form L i , j : i , j : ⁎ : ⁎ for the four sublattice models, one can describe the contribution of SRO to the Gibbs energy in the CEF. Compared to the results from the MC simulations, the present estimate of the SRO, based on the CEF, provided reasonable agreement for SRO, although the CEF gives a slightly higher transition temperature. By introducing the positive value of the 3nn interaction parameter, the order of the transition is of the first order. This result suggests that for the assessment of the phase diagrams where the D03/B2 phase boundaries are well separated, the 3nn pair interaction parameter has to be considered.

Nanostructured TiN(001) dot/MgO(001)-4 × 4 and 5 × 5 superlattices based on a repeated slab model have been investigated. The electronic states of TiN(001) dot/MgO(001)-4 × 4 and 5 × 5 superlattices were obtained by using the total-energy pseudopotential method. The internal atomic coordinates in a unit cell were fully relaxed. We consider two types of TiN dot structures as rectangular and rectangular parallelepiped. They all correspond to semiconductor, although relaxed rectangular parallelepiped TiN(001) dot/MgO(001)-2 × 2 and 3 × 3 superlattices correspond to metallicity. A bandgap value of the rectangular TiN(001) dot/MgO(001) superlattice increases with increasing the size of the MgO substrate, although the difference in the band gap between 4 × 4 and 5 × 5 is small at approximately 0.01 eV. Their electronic properties depend on the shape of the TiN dot and the size of the MgO substrate. The bandgap value is corrected by using a generalized density-functional theory.

Amorphous/nanocrystalline formation process of TiNi alloys induced by severe plastic deformation was studied by using molecular dynamics simulations. During the simulations of the extrusion process of TiNi specimens, we have observed nanocrystalline formation as well as amorphous formation. The relation between the area reduction by extrusion and the volume fraction of the amorphous phases has been calculated and agrees with the experimental observations. We have also observed the formation of arrays of edge dislocations or partial wedge disclinations prior to nanocrystalline formation. [doi:10.2320/matertrans.MH201311]

The fragility of materials should be related to a local structure formed in supercooled liquid states. Especially, in the cases of metallic glasses, the icosahedral order formation is considered to play an important role. To clarify the relation between the local structure formation and the relaxation behavior (or fragility) in the supercooled liquid states of metallic glasses, molecular dynamics simulations were performed for a simple model of binary alloys, in which we can change the atomic size ratio. From the simulation results, we found that the icosahedral clusters exist in supercooled liquid states and form a medium-range network structure in strongly supercooled regime near the glass transition as well as in the glassy states. We also found that the number density of the clusters increases as the atomic size difference increases. Consequently, the calculated fragility parameter takes a lower value in a system with larger atomic size difference due to the richness of the icosahedral clusters.

Thermodynamic assessments of the Ni–Zr and the Cu–Zr binary systems were carried out using thermodynamic properties obtained in a wide range of temperatures. The associated solution model was applied to describe the short range ordering in the liquid. The critical cooling rates for the glass formation were estimated from the optimized parameter set, and compared with experimental data. In both the Cu–Zr and the Ni–Zr systems, the minimum critical cooling rate was found in alloys with compositions around 35 at.% Zr. Smaller critical cooling rates in the Cu–Zr system than those in the Ni–Zr alloys suggest that Cu–Zr alloys have higher GFA than Ni–Zr alloys.

Formation of the icosahedral order in supercooled liquids and glassy phases is investigated for a model alloy system by using molecular dynamics simulations. The simulation results show that the short-range icosahedral order grows in the supercooled liquids as well as in the glassy phases. Structural analyses reveal that the icosahedral clusters form a network in which the clusters are connected via the pentagonal-bicap sharing. Geometrical property of the network formed by the icosahedral clusters is an origin of medium-range order in the glassy phases

Relaxation behavior of structural change at the atomic cluster level in supercooled liquids during isothermal annealing processes is investigated for a model alloy system by using molecular dynamics simulations. The simulation results show that the short-range icosahedral order grows with the annealing time. Moreover, the spatial distribution of the icosahedral clusters shows a medium-range order of a few nanometers. This type of geometrical short-range order of icosahedral symmetry is drastically enhanced by alloying components with a large atomic size difference and negative heat of mixing. On the other hand, various types of chemical short-range order grow according to a combination of heats of mixing.

The thermal resistances of 1250 kinds of interface were computed at room temperature based on the phonon diffuse mismatch model. The result shows that the ratio of Debye temperature and the ratio of average sound velocity can be approximately used to characterize the difference of two materials in terms of interfacial thermal resistance. The high interfacial thermal resistances are composed of high and low Debye temperature materials. The low interfacial thermal resistances are composed of both similar Debye temperature materials, and their Debye temperatures are very high. The relation between the interfacial thermal resistance with the ratio of average sound velocity is similar to that of the ratio of Debye temperature. [doi:10.2320/matertrans.MAW200717]

By use of the molecular dynamics based on a simple 8-4 Lennard-Jones type potential, we study how the stability of B2 against the martensite, in other words, how A S depends drastically on one of six parameters in the potential, i.e., the interactive distance parameter. Then, we study how the same parameter is related to T C order–disorder critical temperature in the same alloy by use of the Monte-Carlo method. By combining these studies, we show how the aging in the martensite, i.e. rubber-like behavior, depends on the relative position of A S with respect to T C .

The liquid phase miscibility gap was calculated in ternary glass-forming systems using the sub-regular solution model. We found that liquid miscibility gaps exist at low temperature in most of the reported ternary bulk-forming metallic glass systems even if enthalpies of mixing of constituent elements are all negative. The glass-forming ranges do not overlap with the miscibility gaps except in the Cu–Ti–Zr system, suggesting that glass-forming ability is low in the phase separating compositions.

Thermodynamic assessment of the Cu–Zr binary system was carried out using thermodynamic data obtained over a wide range of temperatures. The associated solution model was applied to describe the short range ordering in a liquid. The glass transition was treated as a second order transition with Hillert–Jarl functions. The driving force for crystallization and time–temperature–transformation (TTT) curves were estimated from the optimized parameter set, and compared with experimental data and the results of previous thermodynamic assessments. Taking into account thermodynamic data at low temperatures, the calculated driving force was found to be lower than the results of previous assessments. Consequently, the nose of the TTT curve was shifted to a longer time.

It is well-known that encounter of difficult geological condition such as fault and fractured zones in underground construction projects leads to cost overrun and delay of construction term. Particularly, water inrushing from excavated face is a typical example of geotechnical risk factors which cause serious damage. However, cost variation caused by geotechnical risk factors mentioned above has not been investigated sufficiently until now, since it has been born by construction owners with deep pocket. From such a viewpoint, this paper focuses on estimation of cost variation caused by water inrushing from excavated face in a tunnel. In detail, this study adopts DFN (Discrete Fracture Network) model, which is often used for seepage analysis in discontinuous media, to simulate groundwater behavior due to tunnel excavation. Seepage analysis based on Monte Carlo Simulation, which select analytical parameters concerning information on fractures involved in DFN model as random variables, is conducted to evaluate discharge along excavated tunnel. And, in accordance with plan, which prescribes cost of countermeasures against water inrushing, the relationship between cost of countermeasures and corresponding exceedance probability, which is so-called Risk Curve, is obtained. Furthermore, Value at Risk (VaR), which is a popular index to evaluate the occurrence of unwanted events and/or losses based on Risk Curve in financial engineering, is adopted as an index to evaluate cost of countermeasures. The adoption of an index in accordance with financial engineering helps project participants, whose major is not engineering, to make decision on execution of underground construction projects. Furthermore, the methodology proposed in this paper is applied to cost estimation of tunnel countermeasures due to water inrushing in a tunnel to be excavated by TBM method in underground research facility. Finally, results make it clear that methodology proposed in this paper is very effective to investigate cost variation risk caused by encounter of fault and fractured zones during construction.

Relaxation behavior of structural change at the atomic cluster level in supercooled liquids during isothermal annealing processes is investigated for a model alloy system by using molecular dynamics simulations. The simulation results show that the number density of icosahedral clusters increases with the annealing time. Moreover, the number density of icosahedral clusters, the structural relaxation time, and the atomic diffusivity strongly correlate with the packing density of the supercooled liquids for many kinds of systems of alloying components with a variety of atomic size ratios and heats of mixing. This suggests that the packing density would be a good probe to estimate the stability of supercooled liquid phases and the glass-forming ability of the system.

Using thermodynamic data obtained over a wide range of temperatures, a thermodynamic assessment of the Ni–Zr system was carried out. The associated solution model was applied to describe the short range ordering in the liquid, and the glass transition was treated as a second order transition, using the Hillert–Jarl functions. The driving force of crystallization, and the time–temperature–transformation (TTT) curves were estimated from the optimized parameter set, and compared with experimental data and the results of previous thermodynamic assessments. Taking into account the low temperature thermodynamic data, the calculated driving force was found to have decreased compared to the previous assessments. Consequently, the nose of the TTT curve was shifted to higher times.

Criteria for estimating the glass-forming ability accessible by the molecular dynamics simulations are proposed. The enthalpy of the glassy phases and the density of the supercooled liquid phases can be the criteria, the former reflects the energetics and the latter reflects the dynamics. The relaxation behavior of the glassy phases suggests that the cooling rate dependence of the glass transition temperature also could be the criterion as to explore the ‘energy landscape’ and the phase stability for both supercooled liquid and glassy phases.

A difference between the isothermal and adiabatic elastic moduli of a crystal signals damping at finite frequencies of vibration. Isothermal and adiabatic elastic moduli of B2 alloys with complete and incomplete long range order are calculated by use of the fluctuation formula of molecular dynamics. The results of the calculation show a decrease of the shear elastic modulus ( c 11 − c 12 )/2 upon approaching M S . This temperature seems to decrease as the long range order decreases. An increase of damping as the temperature is lowered towards M S is not obtained from the present calculation.

Various nanostructured TiN(001)/MgO(001) superlattices based on a repeated slab model with a vacuum region have been investigated by the total energy pseudopotential method. They are rectangular and rectangular parallelepiped TiN(001) dot structures on MgO(001)-2×2 and 3×3 substrates. A rectangular TiN(001) structure on a MgO(001)-2×1 substrate has also been calculated. Their detailed electronic and internal lattice properties were investigated systematically. The internal atomic coordinates in a unit cell were fully relaxed. The rectangular TiN(001) structure on the MgO(001)-2×1 superlattice, which is not a dot owing to its periodicity, corresponds to metallicity. The electronic states of relaxed rectangular TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices are semiconducting. All relaxed rectangular parallelepiped TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices correspond to metallicity. The electronic properties depend on the shape of the TiN dot and the size of the MgO substrate.

We analyze the Seebeck coefficients of a magnetic semiconductor CuFeS2 using first-principles calculation methods based on density functional theory. The calculated temperature dependence of the Seebeck coefficient in the antiferromagnetic phase reproduces a distinctive behavior in a bulk CuFeS2, such as a peak structure at a low temperature and weak temperature dependence around room temperature. In doped systems, almost linear temperature dependence appears. Despite not including any effect beyond the conventional spin density functional theory in our calculations, the calculated results agree qualitatively with the experimental results. These agreements indicate that the behavior of the Seebeck coefficients in CuFeS2 is mainly determined by its electronic structure.

The mechanisms for martensitic transformation from fcc to bcc, those by Bain, Kurdjumow and Sachs (K–S), Frank or Bogers and Burgers, are reexamined on the basis of the comparison of the cohesive energy. The cohesive energy of the transient lattices formed by those mechanisms is estimated by use of the embedded atom method potential for Fe. In the process of the calculation, it has been realized that the shear deformation involved in all of the latter three mechanisms must be carried out in a reinterpreted sense slightly different from the strict sense of the word. In an ideal situation, where the formation of the martensite without any constraint from the surrounding austenite is allowed, the saddle point energy for the latter three mechanisms is equal to the maximum energy for the Bain deformation process between fcc and bcc. However, if the constraint that the distance between the close-packed planes is kept constant during the martensitic transformation is imposed, only the mechanism by K–S can bring fcc lattice to bcc lattice among the latter three.

Formation of the amorphous phase and its relaxation and crystallization processes in the Ti–Al binary system are investigated by using molecular dynamics techniques. For various Al concentrations ranging from 0.0 to 1.0, amorphous alloys are prepared by quenching from liquid state. By varying the cooling rates, we can obtain the critical cooling rate to get the amorphous state in each composition. The geometrical analysis shows that a structural inhomogeneity on nanometer scale exists in the amorphous state, which mainly consists of two regions. One is dominated by the icosahedral clusters and the other is filled with approximate crystalline packing. The excess of low frequency vibration modes called the boson peak is observed in the amorphous states, which is getting weaker during the annealing process and completely vanishes after crystallization.

The order-disorder transition in hcp binary alloys is investigated by Monte Carlo simulations and the renormalization-group methods. The inclusion of the interactions beyond the nearest neighbors induces qualitative changes of the phase boundaries. Especially, the contribution from second-neighbor interactions explains the shift along the concentration axis of the congruent point of order-disorder transition observed in the $\ensuremath{\alpha}/{\ensuremath{\alpha}}_{2}$ phase boundaries in the Ti-Al binary system.

Evolution of thermal lattice vibration in a small crystal of Fe into the displacement necessary for nucleation of FCC to BCC martensitic transformation is studied by use of the molecular dynamics based on the embedded atom method potential due to Johnson and Oh. Vortex like displacement on the surface of a dislocation-free small crystal with diameter in the range of nm is found to introduce the Bain deformation which is necessary for the martensitic transformation.

Vortices of trajectories of atoms on the surface of a crystal are found as a transient pattern in the numerical experiment to study martensitic transformation in an ideal crystal which has its surface as a sole lattice defect, but no other internal lattice defect. An empirical instability condition for the appearance of the vortex is interpreted in terms of the dependence of the excitation energy of a vortex on its size.

By using isobaric-isothermal molecular dynamics (MD) technique and many-body effective potential, we investigate the martensitic transformation in iron. To describe structural phase transitions, we adopt the Parrinello-Rahman scheme and its invariant modification, in which the MD cell with periodic borders is allowed to change in size and shape with time. The main observed path from fcc to bcc is the Bain transformation, but this process seems to be induced by the tetragonal distortion of the MD cell. Accordingly, the dynamical trajectory of the transformation does strongly depend on the size and shape of the MD cell and on the MD scheme itself. The unwanted contribution from the cell dynamics cannot be eliminated even in the modified schemes based on the Parrinello-Rahman method.

Interfacial thermal resistance is an important factor that has a considerable elect on the thermal conductivity of composites, especially nanocomposites, and must therefore be considered when developing new composites for various structural and nonstructural applications. However, reported data on interfacial thermal resistance are sparse as a result of a lack of efficient measurement methods. We developed a new analytical and measurement method for the determination of the interfacial thermal resistance between a metal and a dielectric material by using a technique involving periodic Joule (ohmic) heating and thermo-reflectance. The principle is based on a one-dimensional model of heat conduction in a two-layered system, taking into account the interfacial thermal resistance. By using this method, the interfacial thermal resistances between Au films and substrates of SiO 2 glass or sapphire single crystal were measured. The results were compared with values calculated by the diffusion mismatch model, and the experimental factors that might affect the interfacial thermal resistance are discussed.

We computed the thermal conductivity of silicon single crystal thin film with a thickness of 25 nm–134 nm at room temperature by nonequilibrium molecular dynamics simulation. The thermal conductivity was shown to depend on the thickness of the film, and is markedly lower than that in bulk silicon. The phonon classical thermal conductivity theory, incorporating the Boltzmann transport equation, was used to establish a phonon scattering model for size dependence. The results show that boundary scattering is very strong for phonon transport in silicon thin film. [doi:10.2320/matertrans.MAW200710]

Liquid-to-amorphous transition and amorphous-to-crystal transition processes of Ti-Al alloys were investigated by using constant-pressure, constant-temperature molecular dynamics techniques. Many-body interactions semi-empirically determined by the Embedded Atom Method were used. Thermodynamic properties obtained in the simulations were in good agreement with experimental measurements. Especially, the present simulation could reproduce the experimentally observed concentration range (40-85 at%Al) for amorphization. Structure analyses and the cnergetical consideration about the clusters found in the system revealed that the basic structure of amorphous states in the Ti-Al system is a network of icosahedral clusters stabilized by many-body effect.

We present the thermoelectric properties of TiN/MgO superlattices employing first-principles calculation techniques. The Seebeck coefficients, the electrical conductances, the thermal conductances, and the figure of merit are investigated employing electrical and thermal transport calculations based on density functional theory combined with the nonequilibrium Green's function and nonequilibrium molecular dynamics simulation methods. The TiN/MgO superlattices with a small lattice mismatch at the interfaces are ideal systems to study the way for an enhancement of thermoelectric properties in artificial nanostructures. We find that the interfacial scattering between the two materials in the metal/insulator superlattices causes the electrical conductance to change rapidly, which enhances the Seebeck coefficient significantly. We show that the figure of merit for the artificial superlattice nanostructures has a much larger value compared with that of the bulk material and changes drastically with the superlattice configurations at the atomistic level.

The Kaluza--Klein mechanism is introduced into three-dimensional Regge calculus. After compactification of one direction the three-dimensional Regge action splits into two parts: one is the two-dimensional Regge action and the other is the action of the discretized electromagnetic field on the two-dimensional lattice, which respectively yield the Einstein action and the electromagnetic action in the continuum limit.