MATERIALS TRANSACTIONS
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ONLINE ISSN: 1347-5320
PRINT ISSN: 1345-9678

MATERIALS TRANSACTIONS Vol. 46 (2005), No. 6

  • Enhanced Phenomena in Metals with Electric and Magnetic Fields: I Electric Fields

    pp. 1083-1087

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    DOI:10.2320/matertrans.46.1083

    The effects of an externally-applied electric field on the equilibria and kinetics of solid state transformations in metals and alloys are reviewed. Regarding equilibria, electric fields have been found to affect the solubility of solutes and the composition as well as volume fraction of phases present. Regarding kinetics, electric fields have been shown to affect recovery and recrystallization, precipitation, phase coarsening, hardenability and sintering. Electric fields thus offer an additional means of controlling microstructure and in turn properties. Our understanding of the effects of an electric field on solid state transformations in metals and alloys is very meager. It appears that most of the observed effects on kinetics are through its influence on vacancies.
  • Enhanced Phenomena in Metals with Electric and Magnetic Fields: II Magnetic Fields

    pp. 1088-1092

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    DOI:10.2320/matertrans.46.1088

    Magnetic field effects were first noticed in texture formation in ferrous alloys more than half a century ago, when the available field intensity was an order of or less than one Tesla. With increasing availability of very intense magnetic fields of an order of ten Tesla, the potential of magnetic field as an additional controllable external parameter for microstructure control is widely recognized, not only in ferromagnetic, but also in non-ferromagnetic metals and alloys. As a companion paper to Part I on electric field effects the influence of magnetic fields on microstructure, morphology and kinetics of transformations in solid metallic materials are reviewed in Part II.
  • Electronic and Lattice Properties of Layered Hexagonal Compounds Under Anisotropic Compression: A First-Principles Study

    pp. 1094-1099

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    DOI:10.2320/matertrans.46.1094

    We have investigated the wurtzite and hexagonal compounds (w- and h-BN, AlN, ZnO) under various compression conditions using the first-principles molecular dynamics (FPMD) method. Applying anisotropic compression is an important approach for the investigation of novel material properties. We found remarkable changes in the internal parameters u of all calculated wurtzite compounds under uniaxial c-axis compression (Pz) within the symmetry constraint. The internal parameter u of the wurtzite structure increased as the pressure increased and finally became 0.5, resulting in a phase transformation into a hexagonal structure. Transition pressures for BN, AlN and ZnO under c-axis compression are 300–325, 15–20, 5–10 GPa, respectively. The value of the transition pressure of BN (wurtzite → hexagonal) was found to be significantly higher than those of the other two compounds (AlN, ZnO) in which the crystal structure of BN could be broken under the large uniaxial compression. The changes in the electronic band structure and the lattice properties (lattice constant, ca ratio, volume of the unit cell) of BN in the wurtzite-to-hexagonal transformation were also quite large and unusual. These results imply that the wurtzite-to-hexagonal transformation of BN is unlikely to occur. In contrast, the changes in the lattice properties of AlN and ZnO were small because of their low transition pressures (5–20). These lower transition pressures are mainly due to larger ionicity of AlN and ZnO.
  • Ab Initio All-Electron GW Calculation of Lithium Chloride Crystal

    pp. 1100-1102

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    DOI:10.2320/matertrans.46.1100

    In order to determine the band gap and quasiparticle energies of lithium chloride crystal accurately, we employ the state-of-the-art GW approximation on the basis of many-body-perturbation-theory at the ab initio level. Our method is based on the all-electron mixed-basis approach. We use the 2×2×2 supercell in which four lithium and four chlorine atoms exist. We demonstrate the importance of the q point sampling for the momentum transfer q of the Coulomb matrix elements. The result for the direct band gap at the Γ point compares well with experiment and the previous calculations.
  • Instability of Dianions of Alkali-Metal Clusters

    pp. 1103-1105

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    DOI:10.2320/matertrans.46.1103

    In this paper, we discuss the instability of dianions of small alkali-metal clusters (Li2, Na2, and K2) starting from the state-of-the-art GW approximation and taking account of multiple scattering of two particles (hole–hole or electron–electron). The present approach is based on the first principles T-matrix theory in the many-body perturbation theory.
  • Local Geometry and Energetics of Hydrogen in Orthorhombic SrZrO3

    pp. 1106-1111

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    DOI:10.2320/matertrans.46.1106

    The local geometry around hydrogen and activation energies for hydrogen transfer in undoped, Y- and Al-doped SrZrO3 have been studied using the density functional theory under the generalized gradient approximation. It is shown that strong O–H bond is formed and orientated towards the outside of the ZrO6 octahedron in SrZrO3. The hydrogen tends to approach the dopant ion in the doped oxides. It is found that large local distortion is induced around hydrogen and dopant ion, and affects the activation energy of hydrogen transfer largely. The calculated activation energies are in agreement with the experimental values in the doped oxides.
  • First Principles Calculation of Defect Structure in Non-stoichiometric CoAl and CoTi

    pp. 1112-1116

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    DOI:10.2320/matertrans.46.1112

    First-principles electronic structure calculations have been performed for defect structure in non-stoichiometric CoAl and CoTi. In order to determine the type of constitutional defects, the compositional dependence curves both of formation energies and of lattice parameters are obtained by the calculations employing supercells in various sizes. The defect formation energies are calculated with taking into account the compositional dependence of the chemical potential. The calculated results suggest that the Co vacancy is the dominant thermal-excitation defect even in the Co-rich side near the stoiciometry in CoTi.
  • Elastic Constants of AlLi from First Principles

    pp. 1117-1121

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    DOI:10.2320/matertrans.46.1117

    The elastic stiffness coefficients of single crystal AlLi with cubic NaTl (B32) structure were calculated at 0 K from the first principles. The obtained elastic stiffness coefficients, in units of GPa, were c11=66.9, c12=38.2 and c44=51.7. Then the bulk modulus, Young’s modulus, shear modulus and Poison’s ratio were estimated for polycrystalline AlLi from the elastic stiffness coefficients. The Young’s modulus for single crystal AlLi was the highest in the ⟨111⟩ direction. The formation of the sp3-like bond connecting the nearest-neighbor Al atoms was confirmed from the charge density distribution. The elastic anisotropy of AlLi was compared with those of the sp3 bonded semiconductors such as Si and GaAs.
  • First-Principles Study of the Electronic Properties of γ⁄γ′ Interface in Ni Based Superalloys

    pp. 1122-1126

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    DOI:10.2320/matertrans.46.1122

    Electronic properties of the H-, B- and C-doped γ⁄γ′ interface in Ni based Superalloys are studied by means of the self-consistent full-potential linearized augmented-plane-wave method under generalized gradient approximation. It is shown that all the three impurities, H, B and C prefer to occupy the Ni-rich octahedral interstitial sites at the γ⁄γ′ interface. The calculated charge transfer suggests that the effect of H, B and C on the γ⁄γ′ interface is localized. For the B and C cases, due to the hybridization between the p states of the impurity and the d states of the nearest-neighbor nickel atoms, the effect of both B and C is to strengthen the γ⁄γ′ interface. In contrast, for the H case, the H-s/Ni-d hybridization results in a strong weakening of interplanar bonding and a slight enhancement of intraplanar Ni–Ni bonding on the interface planes, so that the effect of H is to decrease the cohesive strength of γ⁄γ′ interface.
  • Thermodynamic Properties of Transition Metals Using Face-Centered-Cubic Lattice Model with Renormalized Potentials

    pp. 1127-1130

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    DOI:10.2320/matertrans.46.1127

    The thermodynamic properties of transition metals are studied by introducing face-centered cubic (FCC) lattice model. In order to treat actual systems as quantitatively as possible, empirical second moment approximation (SMA) potentials proposed by Rosato et al. and by Cleri et al., which have been used widely for molecular dynamics (MD) simulations, are employed. To overcome shortcomings of lattice-gas models such as neglecting internal entropy of the system, the potential is mapped onto FCC lattice using the renormalization technique. It is found that the computed linear thermal expansion coefficients agree well with the results of MD simulations.
  • Local Electronic Structure and Protonic Conductivity in Perovskite-Type Oxide, SrZrO3

    pp. 1131-1139

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    DOI:10.2320/matertrans.46.1131

    Local electronic states around hydrogen and acceptor ions in SrZrO3 are simulated by the DV-Xα molecular orbital method to examine their effects on protonic conductivity. The calculated ioncities of the acceptor dopant ion, M, and the surrounding six oxygen ions, O(i) (i=1–6) are found to change largely with M in the doped oxide, where M’s are Yb, Y, In, Al and Ga. There is a clear tendency that the protonic conductivity decreases as these ionocities around the dopant ion, M, deviate further from the ones around the Zr ion in un-doped oxide. Also, in a geometrical viewpoint, the bond order between M and O(i) (i=1–6) ions is another indication to control the protonic conductivity. The presence of the slightly weaker M–O(i) bond than the Zr–O bond causes small expansion of the MO6 octahedron, and then gives a nearly symmetrical position for proton to move readily to the neighboring oxygen sites. These results are also found in the other oxides, BaZrO3 and CaZrO3. Both the ionicity and the bond order are indeed useful parameters for the design of perovskite-type oxides with high protonic conductivity.
  • Interaction between Substitutional and Interstitial Elements in α iron Studied by First-principles Calculation

    pp. 1140-1147

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    DOI:10.2320/matertrans.46.1140

    Interaction energies between substitutional 3d transition metal elements and an interstitial carbon atom in α iron are estimated using the first-principles calculation. Calculated interaction energies are in good agreement with the experimental values reported for Co, Ni and Cu, showing a repulsive interaction experimentally. However, the interaction for such elements as Ti, V, Cr and Mn are also estimated to be repulsive, although the interaction between these elements and a carbon atom is known to be attractive experimentally. This apparent contradiction may be due to a difference in the formation energy of carbide precipitation from the atomic pair interaction energy.
  • Atomic Bonding and Properties of Al–Mg–Sc Alloy

    pp. 1148-1153

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    DOI:10.2320/matertrans.46.1148

    Atomic bonding of Al–Mg alloy with minor Sc is calculated according to the “Empirical electron theory in solid” (EET). The results show that the particles Al3Sc precipitate firstly in melting during solidification is owing to the strong interaction of Al with Sc atom. The strong interaction of Al with Sc atom can also cause to form the Al–Sc and Mg–Sc segregation regions in solid solution matrix of Al–Mg alloy in further process of solidification. So in following homogenization treatment, finer dispersed Al3Sc second-particles owing to its strong Al–Sc covalent bonds are easily precipitated in these segregation regions. These secondary tiny Al3Sc particles coherent with the matrix can hinder the recrystallization of the alloy under high temperature by pinning the grain boundary. The reason, that the large Al3Sc particles can improve the strength and hardness of the alloy, is attributed to the strong Al–Sc covalent bond net in larger Al3Sc particles which are difficultly sheared by dislocation.
  • Compressive Deformation Simulation of Regularly Cell-Structured Materials with Various Column Connectivity

    pp. 1154-1160

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    DOI:10.2320/matertrans.46.1154

    A finite element method is developed to understand the in-plane, quasi–static compressive deformation mechanism and to predict elasto–plastic stress–strain responses of regularly cell-structured honeycombs. Temporal evolution of geometric configuration is also observed in series in order to understand its deformed pattern. Elasto–plastic model predicts quantitatively the compression behavior of copper cell-structured materials. Fairly good agreement with experimental results assures the validity of the present approach. Four different cell geometries are employed to discuss the effect of column-connectivity in a unit cell on the initial and shear localization behavior of cellular materials. In the case of cellular materials with four-edge connectivity, their initial and post-yielding behaviors are governed by buckling and bending deformation of one selected column. In the case of six-edge connected cellular materials, a set of columns is selected among six in the unit cell to make buckling and bending deformation in the dependent manner on loading directions. This leads to plastic anisotropy of this type of cellular materials.
  • High-Pressure Elasticity and Auxetic Property of α-Cristobalite

    pp. 1161-1166

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    DOI:10.2320/matertrans.46.1161

    The structural variations with pressure in α-cristobalite, a low-density polymorph of SiO2, have been studied through first-principles calculations using the projector-augmented-wave (PAW) method, with particular emphasis on its elastic and auxetic properties. We provide theoretical ab initio results for the volume compressibility and a complete set of independent elastic constants of cristobalite under hydrostatic pressures up to 10–15 GPa. Our calculated structural and elastic properties under pressure are in good agreement with the experimental data. In addition, the corresponding results of the molecular-dynamics simulations with the interatomic potential are also presented for comparison. The dominant mechanism of compression is the reduction of the Si–O–Si angles within the α-cristobalite structure, whereas the SiO4 tetrahedron undergoes only a slight distortion. α-Cristobalite is more compressible than other SiO2 polymorphs as shown by their volume compressibilities, because of its characteristic framework structure similar to re-entrant honeycombs. With increasing pressure, the rotational motions of the rigid SiO4 tetrahedra play an important role in the compressive behavior of cristobalite. The present simulations confirm that the system undergoes transformation from auxetic to non-auxetic under hydrostatic pressure of ca. 2 GPa, while retaining strong elastic anisotropy.
  • Equilibrium between Fluorite and Pyrochlore Structures in the ZrO2–Nd2O3 System

    pp. 1167-1174

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    DOI:10.2320/matertrans.46.1167

    The phase diagram of the ZrO2–Nd2O3 system has been characterized showing isolated two-phase regions for a cubic fluorite-type ZrO2 solid solution and Nd2Zr2O7 with a pyrochlore-type structure. A thermodynamic analysis was carried out to elucidate the origin of this interesting phase equilibrium. A compound energy model with the formula (Zr4+,Nd3+)0.5(Nd3+,Zr4+)0.5(O2−,va)2 was applied to describe the Gibbs energy for these phases in consideration of the ordering of the cation sites in the structure. The ordering arrangement on the anion sites was not taken into account. The Gibbs energy for the liquid was described using an ionic solution model, while the binary compounds, such as tetragonal and monoclinic ZrO2, and cubic and hexagonal Nd2O3, were treated as stoichiometric solid phases. The thermodynamic assessment was based on the experimental phase boundaries as well as the evaluated formation energy for the stoichiometric Nd2Zr2O7 phase. The phase diagram calculations showed that the peculiar feature of this phase diagram was reproduced well in our work. The results strongly suggest that the two-phase boundaries between the cubic fluorite-type ZrO2 solid solution and the pyrochlore-type structure occur due to the ordering of the Zr4+ and Nd3+ cations.
  • Development of New Equilibrium Calculation Software: CaTCalc

    pp. 1175-1179

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    DOI:10.2320/matertrans.46.1175

    Thermodynamic equilibrium calculation is sometimes difficult in a system when minor but important species are present, when only stoichiometric condensed phases are stable and the gas phase is absent, and when multiple minima are present in the Gibbs energy of some non-ideal solution phases. A new program is being developed to overcome these problems. The Gibbs energy minimization method has been implemented to determine the thermodynamic equilibrium similar to other conventional programs. When the system is singular, however, the chemical potentials of the system components are uniquely determined by taking additional minimization of the molar Gibbs energy of the gas phase. Accordingly, the vapor pressure, for instance, is always determined even when only pure condensed phases are active and the gas phase is absent at equilibrium. Furthermore, an automatic routine that introduces multiple phases of different compositions and checks their stability has been implemented to detect possible phase splitting in non-ideal solution phases.
  • Glass Transition within Cluster Variation and Path Probability Methods

    pp. 1180-1186

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    DOI:10.2320/matertrans.46.1180

    Thermodynamic framework of Crystal-Glass transition is described within the Cluster Variation Method (CVM). Free energy is calculated as a function of order parameter at various temperatures and T0 diagram is obtained. It is demonstrated that the glass transition temperature can be interpreted as the spinodal ordering temperature in the order-disorder transition. The advantage of the present description is that the kinetic behavior can be investigated within the same framework by employing Path Probability Method (PPM). Therefore, the combination of the CVM and PPM provides a unique theoretical tool to study Crystal-Glass transition in a consistent manner covering thermodynamics and kinetics.
  • Computer Simulation of Phase Decomposition in Fe–Cu–Mn–Ni Quaternary Alloy Based on the Phase-Field Method

    pp. 1187-1192

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    DOI:10.2320/matertrans.46.1187

    Recently, the phase-field method is becoming a powerful tool to simulate and predict complex microstructure evolutions in interdisciplinary fields of materials science. In this study, the phase-field simulations are demonstrated on the phase decomposition in the α (bcc) phase during isothermal aging in Fe–Cu–Mn–Ni quaternary system, which is a base alloy system of the light-water reactor pressure vessel. Since the CALPHAD method based on a thermodynamic database of equilibrium phase diagrams is used for the evaluation of a chemical free energy in this simulation, the calculated microstructure changes are directly linked to the phase diagram of the Fe–Cu–Mn–Ni system. At the early stage of phase decomposition, the Cu-rich zone with bcc structure begins to nucleate, and the component X (=Mn, Ni) is partitioned to the Cu-rich phase. When the Cu composition in the precipitate reaches almost the equilibrium value, the component X inside the precipitates moves to the interface region between the precipitate and the matrix. Finally, there appears the shell structure that the Cu precipitates are surrounded by the thin layer with high concentration of component X. This microstructure change is reasonably explained by considering the local equilibrium at the compositionally diffused interface region of Cu-rich nano-particles surface.
  • Thermodynamic Study of Phase Equilibria in the Ni–Fe–B System

    pp. 1193-1198

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    DOI:10.2320/matertrans.46.1193

    The phase equilibria in the Ni–Fe–B ternary system have been studied experimentally and using thermodynamic calculations. The Gibbs energy of the individual phases was described by the regular solution approximation and the two-sublattice model. The thermodynamic parameters for each phase were evaluated using the experimental data on phase boundaries obtained from differential scanning calorimetry (DSC) and other available literature data. The evaluated parameters enabled us to obtain reproducible calculations of the isothermal section diagrams. The calculated isopleth at the 26 mol% B section agreed with the DSC data obtained in this study, whereas the calculated liquidus and solidus temperatures were higher than the reported values at the Ni2B–Fe2B pseudo-binary section.
  • Experimental Verification of Magnetically Induced Phase Separation in αCo Phase and Thermodynamic Calculations of Phase Equilibria in the Co–W System

    pp. 1199-1207

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    DOI:10.2320/matertrans.46.1199

    Phase equilibria of solid phases in the Co–W system were investigated using two-phase alloys and the diffusion couple technique. In addition, systematic studies of magnetic and martensitic transitions were conducted by Differential Scanning Calorimetry (DSC), Vibrating Sample Magnetomety (VSM) and dilatometric measurement. Phase separation into ferromagnetic αCo and paramagnetic αCo was confirmed on the Co-rich side. The Curie temperature and saturation magnetization decreased with increasing W content and vanished at around 25 at%W. Furthermore, thermodynamic assessment by the CALPHAD approach was performed. A set of thermodynamic values describing the Gibbs energy of the Co–W system was in good agreement with the experimental phase diagram. Calculation of metastable magnetically induced phase separation in the hcp phase along the Curie temperature was also performed.
  • Phase Field Simulation of the Effect of Anisotropy in Grain Boundary Energy on Growth kinetics and Morphology of Grain Structure

    pp. 1208-1213

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    DOI:10.2320/matertrans.46.1208

    Effect of anisotropy in grain boundary energy on kinetics of grain growth and topological properties of grain structure in two dimensions was simulated by the phase field model. Misorientation distribution in a system with anisotropic grain boundary energy is found to be time-dependent. Fraction of low angle grains boundaries increases with time and high angle grains disappear fast. The average area is found to be proportional to time in both isotropic and anisotropic cases. The anisotropy in grain boundary energy delays the growth rate. The scaled grain size and the edge number distributions become time-independent in both isotropic and anisotropic cases. Anisotropy in grain boundary energy broadens the scaled grain size and the edge number distributions. The characteristics of the size distribution can be represented by the variation in a parameter, called microstructural entropy. The nearest neighbor face correlations obtained by the simulated grain structures with isotropic and anisotropic grain boundary energies are quite similar to the Aboav–Weaire relation.
  • Computer Simulation of Grain Growth in Three Dimensions by the Phase Field Model and the Monte Carlo Method

    pp. 1214-1220

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    DOI:10.2320/matertrans.46.1214

    Temporal evolution and morphology of grain structure in three dimensions were simulated by the phase field and the Monte Carlo simulations. In order to prevent impingement of grain of like orientation, a new algorithm was adopted for both simulations. Excluding the initial stage, the average area is found to be proportional to time in the phase field and the Monte Carlo simulations. The scaled grain size and the face number distributions become time-independent in both simulations. The scaled grain size and the face number distributions obtained by the phase field simulation are in good agreement with those by the Monte Carlo method. The nearest neighbor face correlation similar to the Aboav–Weaire relation is observed in simulated grain structures by both methods. The nearest neighbor face correlation for the phase field model is quite similar to that for the Monte Carlo method. The Allen–Cahn type equation for the phase field simulation can be derived from the master equation of the Monte Carlo Model.
  • Computational Study of Compressive Mechanical Response in Two-dimensional Cellular Solids under Microstructural Control

    pp. 1221-1229

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    DOI:10.2320/matertrans.46.1221

    A new computational mechanics model is proposed to describe the compression response of two-dimensional cellular materials with consideration of microstructural development. In this modeling, processing conditions to fabricate cellular materials are taken into account as a cell-growth mechanism. Representative volume elements (RVE) are generated by the phase-field model. Numerical simulations of compressive deformation are performed by finite element analysis for the selected RVE. The cell size distribution especially affects on the limit stress and the densification process.
  • Experimental and Computational Investigation of Formation of Precipitate Free Zones in an Al–Cu Alloy

    pp. 1230-1234

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    DOI:10.2320/matertrans.46.1230

    Formation mechanisms of precipitate free zones (PFZ) in an artificially aged Al–1.74 mol%Cu alloy have been clarified using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and a Monte Carlo computer simulation. The vacancy depletion mechanism caused by the annihilation of quenched-in excess vacancies was found to work predominantly in the early stage of aging at 433 K, whereas the solute depletion mechanism caused by the grain boundary precipitation followed thereafter. The simulation model taking into account such a vacancy depletion effect well reproduced the obtained experimental results, suggesting that the difference of size distribution of Cu clusters after quenching is responsible for the initial formation of PFZ in the subsequent aging stage.
  • Molecular Dynamics Simulation on the Single Particle Impacts in the Aerosol Deposition Process

    pp. 1235-1239

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    DOI:10.2320/matertrans.46.1235

    Single nano-particle impacts in the aerosol deposition (AD) method were simulated by the molecular dynamics calculation. By changing the incident speed, angle, and the crystal orientation of the aerosol particle, structural variation in the particle and the substrate was examined. It was found that the structural changes of the particle by the impact are classified into three cases. The particle maintains the original structure when the incident speed is low. If the incident speed is intermediate and if the particle is properly oriented, the particle was divided into a few grains. If the speed is high enough, the atomistic structure of the particle is composed of a disordered phase and small crystal regions.
  • Thermodynamic and Structural Properties for the FeO–SiO2 System by using Molecular Dynamics Calculation

    pp. 1240-1247

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    DOI:10.2320/matertrans.46.1240

    Molecular dynamics (MD) simulation has been widely used as a very useful method for the calculation of thermodynamic, structural and transport properties for the molten slags and fluxes at high temperatures. In this study, MD simulation using the Born-Mayer-Huggins type pairwise potential with partial ionic charges has been used to calculate the thermodynamic, structural and transport properties for the FeO–SiO2 system. The calculated structural properties such as pair distribution functions and fractions of bonding types of oxygen (bridging, non-bridging and free oxygen) with silicon atoms in FeO–SiO2 melts were in good agreement with previously measured and estimated results, and also the self-diffusion coefficients of iron, silicon and oxygen have been calculated at various temperatures and compositions. The enthalpy, entropy and Gibbs energy of mixing for the FeO–SiO2 system were calculated based on the thermodynamic and structural parameters obtained from MD simulation. The phase diagram for the FeO–SiO2 system estimated by calculated Gibbs energy of mixing shows good agreement with observed result in the range from pure iron to fayalite, and the liquid–liquid immiscibility region in the FeO–SiO2 system has also been assessed by MD calculation.
  • Computer Simulation of Multiphase Binary Diffusion in Gas–Solid Type Couples

    pp. 1248-1254

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    DOI:10.2320/matertrans.46.1248

    Model 1 for describing multiphase diffusion and Model 2 for determining the interdiffusion coefficient of each phase in binary gas–solid couples were developed by modifying models previously presented. No restrictions on the number of phases, the values of interdiffusion coefficients, or the homogeneity ranges of phases were placed on the present models.
    Composition profiles, mass changes per unit area of surface and dimensional changes normal to the surface for N–Cr, N–Fe, and N–Ti couples were numerically calculated from diffusivity and phase equilibrium data with Model 1. A good agreement between experimental and calculated results was found. It was confirmed that Model 1 is compatible with Model 2, which can be easily applied and is useful when no references are available for diffusivity data.
  • Dynamic Simulation of Multiplier Effects of Helium Plasma and Neutron Irradiation on Microstructural Evolution in Tungsten

    pp. 1255-1260

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    DOI:10.2320/matertrans.46.1255

    Plasma-facing and high heat flux materials in a fusion reactor suffer two types of damage: displacement damage caused mainly by high-energy neutrons and surface damage, such as erosion, sputtering, and blistering, caused by hydrogen and helium plasma. Usually, these two kinds of damage are investigated separately. In the present study, multiplier effects of helium plasma and neutron irradiation on microstructural evolution in tungsten were investigated using computer simulations based on a rate theory. Neutron irradiation with 10−6 dpa·s−1 at 873 K and a helium flux of 1018 m−2·s−1 with 10-keV energy were used as typical irradiation conditions. The effects of irradiation temperature and defect production rate on the interaction between helium and defects were also investigated. The simulation results revealed the rapid diffusion of helium in tungsten. The helium concentration was saturated in tungsten 0.067 mm thick within 0.01 s at 873 K, and the formation of helium-vacancy clusters was nearly uniform in the matrix except at the surfaces facing towards and away from the irradiation after 0.01 s. This meant that the 10-keV helium plasma could enhance formation of helium-vacancy clusters in the region without damage produced by helium. The concentration of 6He-V clusters reached a very high level (10−6) even after a 1-s irradiation with a defect production rate of 10−6 dpa·s−1 at 873 K. The concentration of helium-vacancy clusters decreased with increasing irradiation temperature and decreasing defect production rate. The accumulation of helium and the formation of helium-vacancy clusters depended on not only the vacancy concentration, which was determined by the irradiation temperature and defect production rate, and helium concentrations, but also irradiation time.
  • Sample Length Dependence of Giant Magnetoimpedance in Fe–Zr–Nb–Cu–B Nanocrystalline Ribbons

    pp. 1261-1263

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    DOI:10.2320/matertrans.46.1261

    In the present work the sample length dependence of giant magnetoimpedance for Fe84Zr2.08Nb1.92Cu1B11 nanocrystalline ribbons was investigated. With a reduction of sample length L, the peak field HP of positive magnetoimpedance increases, due to an enhancement of demagnetization effect along the sample length direction. Meanwhile, a shortening of ribbon length brings about a decrease of the permeability change under field, which results in a drop of negative magnetoimpedance. In addition, the frequency fmax where the maximum of negative (ΔZZ0)max occurs, shifts to higher frequencies with reduction of sample length. The giant magnetoimpedance in nanocrystalline ribbons depends not only on the field and frequency, but also sensitively on the sample length, which should be considered in designing of GMI electronic circuits.
  • Characterization of Nanocrystal Dispersed Cu60Zr30Ti10 Metallic Glass

    pp. 1264-1270

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    DOI:10.2320/matertrans.46.1264

    The microstructures of Cu60Zr30Ti10 metallic glass prepared by injection casting and melt-spinning have been investigated by transmission electron microscopy (TEM), and three dimensional atom probe (3DAP). The as-cast 2 mm diameter rod sample contained Cu-rich nanocrystals in the amorphous matrix, while the melt-spun ribbon was fully amorphous. The composition of the Cu-rich nanocrystals was around Cu76Zr19Ti5 with the fcc structure (lattice parameter a=0.37 nm), which is a metastable Cu phase supersaturated with Zr and Ti. Annealing the as-cast rod at 713 K (Tg) for 10 min led to a decrease in compressive plastic strain from 1.8 to 0.6% accompanied by an increase in the volume fraction of the Cu-rich crystal.
  • Elastic Properties of Sn-Based Pb-Free Solder Alloys Determined by Ultrasonic Pulse Echo Method

    pp. 1271-1273

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    DOI:10.2320/matertrans.46.1271

    The Young’s modulus (E) and Poisson’s ratio (ν) of Sn-based lead-free solders as Sn–3.5Ag, Sn–58Bi and Sn–9Zn (compositions in mass%) were measured at various temperatures between −50 and 100°C using an ultrasonic pulse echo method. The values of Young’s modulus and Poisson’s ratio were obtained at 25°C. These values coincide with the room temperature values in the literature. The Young’s modulus and Poisson’s ratio between −50 and 100°C were also measured for Sn–58Bi, Sn–9Zn, and Sn–37Pb.
  • Effect of Deformation on Damping Capacity and Microstructure of Fe–22%Mn–8%Co Alloy

    pp. 1274-1277

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    DOI:10.2320/matertrans.46.1274

    Effect of deformation on damping capacity and microstructure of Fe–22%Mn–8%Co alloy has been investigated. The γ→ε stress-induced martensitic transformation occurs during deformation in the alloy. The amount of ε martensite increases rapidly up to 5% deformation, and gradually increases with further deformation. The damping capacity of the alloy exhibits a maximum at around 5% deformation, and decreases with further deformation in spite of the increase in ε martensite content. The deterioration of damping capacity beyond 5% is ascribed to the dislocations introduced during deformation, which obstruct the movement of damping sources of the alloy.
  • Characterisation of Quasicrystalline Particles in an Isothermally Aged Al–10Mg–0.5Ag (mass%) Alloy

    pp. 1278-1287

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    DOI:10.2320/matertrans.46.1278

    The quasicrystalline structure found in the isothermally aged microstructure in an Al–10Mg–0.5Ag (mass%) alloy after solution treated, water quenched and then aged during the time between 20 and 40 min at 240°C has been characterised using transmission electron microscopy, electron microdiffraction and energy dispersive x-ray spectroscopy. The morphology of the quasicrystalline precipitate particles is rhombohedral in shape and those precipitate particles are homogeneously nucleated, and finely and uniformly dispersed in the matrix. The orientation relationship between the quasicrystalline phase and the α-Al matrix is as follows; i5||⟨011⟩α and i3||⟨111⟩α. The quasilattice constant aR of the icosahedral quasicrystalline phase is estimated to be 0.505±0.01 nm from the present 5-fold electron microdiffraction patterns. The lattice parameter ac of the corresponding crystalline cubic approximant is thus calculated to be 1.390±0.028 nm. This is in good agreement with the lattice parameter of the crystalline T phase (Mg32(Al,Ag)49, a=1.416 nm). The morphology of the quasicrystalline precipitate particle is consistent with that predicted from the intersection point group \\bar3, which was defined by symmetry elements common to the two lattices in the observed orientation relationship. The quasicrystalline particles contain elements of Al, Mg and Ag. The quasicrystalline precipitate particles, which are the metastable phase, appear to be the primary strengthening phase in the Al–10Mg–0.5Ag (mass%) alloy aged at 240°C.
  • Identification of Metastable Rod-Like Particles in an Isothermally Aged Al–10Mg–0.5Ag (mass%) Alloy

    pp. 1288-1294

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    DOI:10.2320/matertrans.46.1288

    The structure and morphology of a precipitation product in an Al–10Mg–0.5Ag (mass%) alloy aged isothermally at 240°C have been characterised using transmission electron microscopy and microbeam electron diffraction. A metastable T phase (Mg32(Al, Ag)49, b.c.c., a=1.41 nm) which is identified by the electron microdiffraction patterns, has been found in the Al–10Mg–0.5Ag (mass%) alloy after solution treatment, water quenched and then aged for 2 h at 240°C. The orientation relationship between the T phase and matrix α-Al phase was of the form (010)T||(11\\bar1)α and [001]T||[1\\bar10]α. Qualitative microanalysis suggested that the metastable crystalline T phase was a ternary compound that contained all three elements Al, Mg and Ag. The morphology of the metastable T phase is the ⟨110⟩α rod-like in shape and those precipitate particles are homogeneously nucleated, and finely and uniformly dispersed in the Al matrix. The icosahedral quasicrystalline phase was observed in the early stages of ageing (0.5 h at 240°C), and it was to be replaced by the metastable crystalline T phase Mg32(Al, Ag)49 after the alloy is aged for 2 h at 240°C. The T phase formed as faceted rods parallel to ⟨110⟩α directions of the α-Al matrix appeared to be the primary strengthening constitute exhibiting maximum hardness in the Al–10Mg–0.5Ag (mass%) alloy aged at 240°C.
  • Intermetallic Compound Formation and Growth Kinetics in Flip Chip Joints Using Sn–3.0Ag–0.5Cu Solder and Ni–P under Bump Metallurgy

    pp. 1295-1300

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    DOI:10.2320/matertrans.46.1295

    The interfacial microstructure of Sn–3.0Ag–0.5Cu solder with ENIG (electroless Ni/immersion Au) UBM was studied using scanning electron microscopy and transmission electron microscopy. (Cu,Ni)6Sn5 intermetallic compound layer was formed at the interface between the solder and Ni–P under bump metallization upon reflow. However, after isothermal aging, some AuSn4 but with a certain amount of Ni dissolved in it, i.e., (Au,Ni)Sn4, appeared above the (Cu,Ni)6Sn5 layer. Two distinctive layers, P-rich and Ni–Sn–P, were additionally found from the TEM observation. The analytical studies using EDS equipped in TEM revealed that the averaged composition of the P-rich layer was close to that of a mixture of Ni3P and Ni, while that of the Ni–Sn–P layer was analogous to the P-rich layer containing a small amount of Sn in it. The thickness of the (Cu,Ni)6Sn5 layer increased with increasing aging time and temperature. The layer growth of the intermetallic compound satisfied a parabolic law in the given temperature range. The increase of the intermetallic compound layer thickness was mainly controlled by a diffusion mechanism in the temperature range studied, because the values of time exponent (n) were approximately equal to 0.5. The apparent activation energy for the growth of (Cu,Ni)6Sn5 intermetallic compound layer was 55 kJ/mol.
  • Evaluation of Grain Boundary Effect on Strength of Fe–C Low Alloy Martensitic Steels by Nanoindentation Technique

    pp. 1301-1305

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    DOI:10.2320/matertrans.46.1301

    The grain boundary effect on the strength was evaluated through nanoindentation technique for Fe–0.4C–Cr–Mo steels that were produced by the ausform-tempered (AF) and conventional quench-tempered (QT) processes. A semiquantitative Hall–Petch plot was made to determine the locking parameter k for the two alloys using nanohardness, micro-Vickers hardness, and grain size. The k value for the QT sample is significantly larger than that for the AF sample and is attributed to the film-like carbides on the grain boundaries of the QT sample. The lower k value of the AF sample is one of the factors for the improved delayed fracture property in the AF compared to that of the QT sample.
  • Effect of Hydrogen on Damping Capacity of Ti50Ni25Cu25 Alloy

    pp. 1306-1310

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    DOI:10.2320/matertrans.46.1306

    Ternary Ti–Ni–Cu alloy is known to exhibit higher damping capacity than binary Ti–Ni alloy. Hydrogen doping was used to further improve the damping capacity of this ternary alloy. Considering ease of manufacture, the Ti50Ni25Cu25 alloy was produced by a lamination process. As a result of hydrogen doping, a new damping peak appeared at around 260 K. The peak temperature shifted with changes in oscillation frequency. The activation energy (H) and the relaxation time constant (τ0) of the damping peak were calculated according to the Arrhenius plot as H=57.5 kJ/mol, and τ0=2.2×10−13 s, respectively. The damping capacity due to hydrogen doping increased with increasing hydrogen concentration up to 0.45 at%, showing the maximum peak value of tanφ=0.05, but thereafter decreased. On the other hand, the peak temperature increased monotonically with increasing hydrogen concentration, and the peak shape was broader than the single Debye relaxation peak. From these results, we concluded that this damping peak was not the Snoek peak but the Snoek-Koester peak caused by hydrogen-dislocation interaction. The decrease of the Snoek-Koester peak with increasing hydrogen concentration is explained by formation of hydrogen clusters or hydrides. Schoeck’s theory for the Snoek-Koester peak may be applicable. On further hydrogen doping, both the Snoek-Koester peak and the transformation peak were hardly visible above 5 at% hydrogen. This was found to be due to destruction of the martensitic phase by excess hydrogen doping.
  • Theoretical Analysis on Crystal Alignment of Feeble Magnetic Materials under High Magnetic Field

    pp. 1311-1317

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    DOI:10.2320/matertrans.46.1311

    Since superconducting magnet was developed, high magnetic field has been used as one of the effective ways to get textured structures by aligning crystals in materials. It is well known that the high magnetic field can align crystals having magnetic anisotropy. However, effects of Brownian motion of crystals in liquid medium and gravity force on the crystal alignment are not well known. In this study, it has been found that there is a size range of crystal particles in which the crystals can be aligned by high magnetic field under actions of Brownian motion of crystals and gravity force. Moreover, by taking account of these factors, theoretical analysis of crystal alignment under high magnetic field has been carried out to elucidate the phenomena of the crystal alignment in both feeble magnetic materials having well or poor electric conductivity.
  • Thermal Cycle Resistance of Yttria Stabilized Zirconia Coatings Prepared by MO-CVD

    pp. 1318-1323

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    DOI:10.2320/matertrans.46.1318

    Yttria stabilized zirconia (YSZ) coatings containing 0 to 9.5 mol% Y2O3 were prepared on Hastelloy-XR alloys by metal-organic chemical vapor deposition (MO-CVD), and the effect of Y2O3 content on thermal cycle resistance was investigated in the temperature intervals from 600 to 1273 K and from 600 to 1373 K in air. In the 600–1273 K interval, delamination after 1200 cycles had not occurred for any of the Y2O3 contents of YSZ coatings. In the 600–1373 K interval, the YSZ coatings containing 0, 0.5 and 9.5 mol% Y2O3 suffered partial delamination after 1200 cycles, while no delamination was observed for the YSZ coatings containing 4 mol% Y2O3. During the thermal cycles, thermally grown oxide (TGO) layers consisting of inner Cr2O3 and outer (Ni,Mn,Fe)(Fe,Cr)2O4 spinel were formed at the YSZ coating/Hastelloy-XR substrate alloy interface. The delamination of YSZ coatings occurred at the alloy side near the TGO layer/Hastelloy-XR alloy interface.
  • Prediction of Granulated Slag Properties Produced from Spinning Disk Atomizer by Mathematical Model

    pp. 1324-1330

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    DOI:10.2320/matertrans.46.1324

    A mathematical model of a spinning disk atomizer (SDA) was developed to produce glass beads from high-temperature molten slag. The model comprises three parts: 1) fluid flow model of molten slag on the spinning disk, 2) physical model of ligament formation of slag, and 3) heat transfer model of slag drops dispersed from the ligament. First, a 2-D fluid flow model was developed to evaluate the film thickness of slag at the edge of the disk and was calculated using the scalar equation method (SEM). Next, the number and diameter of the ligaments formed were evaluated using the physical model. Finally, the heat transfer model was employed to evaluate the quenching rate and temperature distribution within the drop. The model developed was experimentally validated by comparing the calculated and observed values that were in good agreement. Most significantly, the model also estimated the quenching rate required for slag vitrification.
  • Oxidation of Pd–Al(111) Epitaxial Films in Ultra-High Vacuum

    pp. 1331-1334

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    DOI:10.2320/matertrans.46.1331

    Epitaxial Pd–Al alloy films were grown on α-Al2O3(0001) substrates by sputter-deposition. The composition of the films was approximately Pd0.8Al0.2. The films were completely oriented with (111) plane parallel to the surface without any trace of other intermetallic compounds. The oxidation behavior of the films in ultra-high vacuum at elevated temperature was studied. It was revealed that only aluminum was oxidized and the growth of alumina film stopped quickly at 0.5 nm in thickness. There were some indications that the alumina film was crystalline.
  • Thermodynamic Property of B in Molten Si and Phase Relations in the Si–Al–B System

    pp. 1335-1340

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    DOI:10.2320/matertrans.46.1335

    The thermodynamic property of B in molten Si, which is of importance for optimizing or designing the B removal process from Si, was determined as the following equation by equilibrating molten Si with Si3N4 and BN at 1693 and 1773 K. lnγ^°_B(l)in molten Si=1.19(±0.25)+\\frac289(±450)T (1693–1923 K) Also, the standard Gibbs energy change for N2 dissolution into molten Si was determined as the following equation. & \\frac12N2(g)=N(mass%, in molten Si)
    & ΔG^°=-36,200(±23,500)+39.8(±13.6)T  (J/mol) (1693–1773 K) On the other hand, to understand the B behavior in the solidification refining of Si with Si–Al melt, phase relations in the Si–Al–B system were investigated at 1373–1573 K. The boride in equilibrium with Si–Al melt was clarified as AlB12 solid solution and the thermodynamic property of that solid solution was discussed.
  • Laser Cladding of Fe–Cr–C Alloys on A5052 Aluminum Alloy Using Diode Laser

    pp. 1341-1347

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    DOI:10.2320/matertrans.46.1341

    In order to improve a wear resistance of aluminum alloy, we proposed a diode laser cladding of iron-based alloy on the surface of an aluminum alloy. In the first part of this research, an applicability of diode laser to laser cladding was evaluated. In this experiment, irradiation conditions were varied to investigate the effect of process parameters on the formation of clad layers. From this result, application of diode laser made it possible to obtain stable beads in low heat input compared with CO2 laser, which has been conventionally used for laser cladding. Secondly, we investigated effects of the irradiation conditions on the dilution ratio and the microstructure of a Fe–Cr–C clad layer. It was confirmed that decrease in laser power and increase in traverse speed made the dilution ratio suppressed. According to the increase in aluminum content in the clad layer, the microstructure of the clad layer changed as γ(8–20%)→γ+α(10–30%)→Fe3Al(30%–). At the interface between the clad layer and the aluminum alloy substrate, the reaction layer consisting of Fe2Al5 and FeAl3 formed. The obtained complex frequently included cracks within the clad layer or in the reaction layer, each of which was caused by different factors. The cracks in the clad layer decreased with decreasing the hardness of the clad layer and formation of the ferrite in the austenite phase, which was achieved by controlling the dilution ratio and the carbon content of the cladding material. With respect to the interface cracks, it was found that the addition of copper for reduction of the thermal stress arising at the interface had a beneficial effect on suppressing the interface cracks. In the abrasion Fe–Cr–C and the Fe–Cr–Cu–C clad layers exhibited a higher wear resistance compared with the aluminum alloy.
  • Vapor Pressure Measurements for Metal Chloride Systems by the Knudsen Effusion Method

    pp. 1348-1353

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    DOI:10.2320/matertrans.46.1348

    Thermodynamic knowledge of metal chlorides, such as vapor pressure and activity in salt/slag, are of remarkable industrial interest, particularly in fields related to the recovery/recycle/reuse of metal materials. In this study, a new apparatus for measuring the vapor pressure of metal chlorides from molten salt/slag was designed and tested with reference compounds. The results are in a good agreement with the available literature data. Using the developed apparatus, vapor pressure measurements for a KCl–NaCl–CaCl2 system have been performed. Using the measured data the activities of components were obtained. Simultaneously, the activities of components were calculated using a thermodynamic code, Factsage 5.2 wherein the Modified Quasi-chemical Model was employed to obtain the necessary thermodynamic parameters. The both data agree reasonably well with each other. The results suggested that the KCl–NaCl system exhibits similar behavior with ideal solution; while in the KCl–CaCl2 and NaCl–CaCl2 systems there are possible interactions between CaCl2 and KCl/NaCl. For the ternary system KCl–NaCl–CaCl2, in some concentration range (mole fraction of NaCl 0.25–0.50) NaCl behaves as in an ideal solution, suggesting a larger affinity between KCl and CaCl2 than that between NaCl and CaCl2.
  • Preparation of Mg–Li–Al–Zn Master Alloy in Air by Electrolytic Diffusing Method

    pp. 1354-1359

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    DOI:10.2320/matertrans.46.1354

    Electrolytic diffusing method was conducted at 500°C to prepare Mg–Li–Al–Zn master alloy in air. It was also explored for the melting and casting of Mg–Li alloy in air, as the Mg–Li–Al–Zn master alloy used as raw material. A mixture of 45 mass% lithium chloride (LiCl) and 55 mass% potassium chloride (KCl) was employed as the electrolyte. Mg–9 mass%Al–1 mass% Zn (AZ91) alloy was used as cathode material while graphite selected as anode. Experimental results showed that the electrolysis current linearly depended on the applied working voltage. Deposition of lithium occurred on the cathode surface. After the electrolysis experiments, Mg–12 mass% Li–9 mass% Al–1 mass% Zn alloy sheet can be obtained. At working voltage of 4.2 V and one hour electrolysis, the hexagonal-closed-pack AZ91D sheet (1.5-mm thickness) was fully converted to body-centered-cubic Mg–Li–Al–Zn alloy. The formation of Mg–Li phase during electrolysis was studied by inductively coupled plasma-atomic emission spectrometer (ICP), X-ray diffraction, and optical microscopy. It is diffusion rather than deposition rate (electrolysis current) that controlled the depth of Mg–Li phase formed in the cathode sample. The Mg–Li–Al–Zn alloy used as master alloys could be melted and casted in air without ignition. However, the content of lithium in the as-cast block was reduced to 3.77 mass%, properly due to oxidation of lithium metal during melting.
  • Change in Ultrasonic Parameters and Dislocation Structures during Fatigue Process of Aluminum Alloy under High Stress Amplitude

    pp. 1360-1367

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    DOI:10.2320/matertrans.46.1360

    Aluminum alloy (A2024-T3) specimens were used for a fatigue testing by subjecting them to a stress amplitude of 150 MPa. At different numbers of the fatigue cycles, specimens were removed from the fatigue tester, and then subjected to the ultrasonic measurement, the dislocation density measurement, the hardness testing, and so on. From the Fourier spectrum of the bottom echo, the peak intensity (PI), the peak frequency (PF) and the average gradient of the transfer function (AGTF) were obtained. The dislocation density was obtained by the X-ray diffraction analysis. AGTF, the dislocation density and the hardness decreased at the initial stage, and then gradually increased with the increasing number of the fatigue cycles. PI showed a tendency to increase as fatigue cycles increased, but no change occurred in PF. The change of ultrasonic parameters in the fatigue process was quantitatively discussed according to Granato and Lücke dislocation-string model. Then the in-process ultrasonic measurement was carried out in the fatigue testing of the aluminum alloy by using the water bag to obtain ultrasonic parameters. PI and AGTF rapidly increased at the initial stage, and then gradually increased with increasing number of the fatigue cycles. These results suggest that the dislocation density steeply increased at the initial stage of the fatigue process, and then they gradually increased.
  • Production Conditions of Acicular Magnetic Metal Nanoparticles for Magnetic Recording

    pp. 1368-1377

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    DOI:10.2320/matertrans.46.1368

    Hydrogen reduction conditions of α-Fe2O3 particles and surface passivation conditions of metal nanoparticles obtained through a hydrogen reduction process of acicular α-Fe2O3 particles and a slow oxidation process in a low oxygen concentration have been studied in order to determine optimum production conditions of acicular magnetic metal nanoparticles utilized for magnetic recording.
    Acicular magnetic metal nanoparticles were produced by using the surface-treated acicular α-FeOOH nanoparticles containing 12.6 at%Co–9.4 at%Si–7.0 at%Al under the condition of 823 K in temperature of dehydration of α-FeOOH particles by heating, 5×10−6 m3(STP)·s−1 in flow rate, 50 mol%H2–50 mol%Ar in composition of a reducing gas, and 823 K in reduction temperature. The resulting reduced metal particles are composed of either a single crystal or polycrystals. The inside of the particles was a reduced dense metal phase in which the crystal growth with neither defect nor pore was sufficiently proceeded.
    The surface oxide layer of metal particles which is formed by the slow oxidation results in a spinel structure of Fe3O4. The metallic core was entirely coated by the surface oxide layer, and it was observed that the dense crystal having no defect; pore and so forth inside the oxide layer was sufficiently grown. It is assumed that a passivation layer possessing a higher corrosion-resistant property can be obtained with a thicker surface oxide layer, since the oxide layer on the surface of the metal nanoparticle becomes thicker with higher temperature of slow oxidation.
    Acicular magnetic metal nanoparticles having the excellent property of corrosion resistance were able to be produced under the condition of 338 K in oxidation temperature, 5×10−6 m3(STP)·s−1 in flow rate and 0.5 mol%O2–99.5 mol%N2 in composition of oxidizing gas.
  • Modelling Columnar Dendritic Growth into an Undercooled Metallic Melt in the Presence of Convection

    pp. 1378-1387

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    DOI:10.2320/matertrans.46.1378

    A front-tracking technique on a fixed Cartesian grid, based on the kinetics of dendritic growth, is used to model the progress of an undercooled columnar dendritic front in non-equilibrium 2D solidification controlled by conduction and thermal natural convection. The effect of the alloy latent heat of fusion is included in this single-domain model through a careful definition of source terms in the energy conservation equation to account for both the advance of solidification front and subsequent thickening of the mushy zone within a control volume. The model is compared with the enthalpy approach showing its superiority in the detection of the undercooled liquid zone and, thus, in potentially modelling of columnar/equiaxed grain structures. It is used to predict the influence of both alloy composition and convective heat transfer coefficient on the size of the undercooled liquid zone in front of columnar dendrite tips during solidification of Al–Cu in a square mould. The predictions obtained confirm that natural convection in the melt reduces local temperature gradients and thus widens the undercooled liquid zone ahead of a curve joining columnar dendrite tips, increasing the potential for growth of equiaxed grains.
  • Synergistic Effects of Heavy Ion and Helium Irradiation on Microstructural and Dimensional Change in β-SiC

    pp. 1388-1392

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    DOI:10.2320/matertrans.46.1388

    The influences of helium on microstructural development and dimensional stability in high purity β-SiC after Si2+-ion irradiation with and without He+-ion injection at high temperature were studied. The microstructural observations of β-SiC irradiated up to 10 dpa at irradiation temperatures of 1073, 1273, and 1673 K were performed by transmission electron microscopy, respectively. ‘Black spot’ defects and dislocation loops were observed densely in all irradiated β-SiC. Small cavities were formed at grain interior of β-SiC above 1273 K. Helium increased the number density of cavities, but helium dose not effect on the cavity size. Swelling in β-SiC irradiated up to 3 dpa at 1273 K was measured by precision-surface profilometry. The influences of damage rate (dpa/s) and helium on the swelling were studied. The swelling values were saturated above 1 dpa after single-ion and dual-ion irradiation under higher dpa/s condition (1.0×10−3 dpa/s). In lower dpa/s case (5.0×10−5 dpa/s), the swelling was also saturated after single-ion irradiation, but the saturated swelling value was approximately half of the higher dpa/s case. On the contrary, the swelling value of β-SiC irradiated with dual-ion under the lower dpa/s condition increased at 3 dpa without saturation. Small cavities observed in this specimen, which were formed on {111} family planes, may cause the enhanced swelling at 3 dpa.
  • Crystal Structures of La–Mg–Nix (x=3–4) System Hydrogen Storage Alloys

    pp. 1393-1401

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    DOI:10.2320/matertrans.46.1393

    The new alloys of the La–Mg–Ni (Ni⁄(La+Mg)=3–4) system absorb and desorb hydrogen at room temperature, and their hydrogen storage capacities are greater than those of conventional AB5-type alloys. We investigated the crystal structures of the La0.7Mg0.3Ni2.5C00.5 (alloy T1) and the La0.75Mg0.25Ni3.0C00.5 (alloy T2) using ICP, SEM-EDX and XRD. We found that alloy T1 consisted of Ce2Ni7-type La3Mg(Ni, Co)14 and PuNi3-type La2Mg(Ni, Co)9 phases, and alloy T2 consisted of Ce2Ni7-type La3Mg(Ni, Co)14 and Pr5Co19-type La4Mg(Ni, Co)19 phases. These alloy systems had layered structures and showed polytypism that originated from differences in the stacking patterns of the units, which were composed of several [CaCu5]-type layers and a single [MgZn2]-type layer along the c-axis. The crystal structure of La3Mg(Ni, Co)14 was of a hexagonal 2H-Ce2Ni7-type with a=0.5052(1) nm, and c=2.4245(3) nm. La2Mg(Ni, Co)9 had a trigonal 3R-PuNi3-type structure with a=0.5062(1) nm, and c=2.4500(2) nm. La4Mg(Ni, Co)19 had a hexagonal 2H-Pr5Co19-type structure with a=0.5042(2) nm and c=3.2232(5) nm. In all these structures, the La–La distance of the [CaCu5] layer was 0.38–0.40 nm and that of the [MgZn2] layer was 0.32 nm. We also found that Mg occupied the La site in the [MgZn2] layer. Selective occupation by Mg of the La site in the [MgZn2] layer makes the alloy stable against repeated reaction cycles with hydrogen. The alloy system that forms this material group can be described by the general formula Lan+1MgNi5n+4, where n=0,1,2,3,4,….
  • Magnetic Properties and Microstructure of High-Density Sintered Iron Formed by Warm Compaction Using Die Wall Lubrication

    pp. 1402-1406

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    DOI:10.2320/matertrans.46.1402

    Sintered iron is a useful soft magnetic material. However, its magnetic flux density is smaller than that of wrought magnetic iron, due to the low density of the sintered material. In order to increase the sintered density, we developed a new warm compaction technique using die wall lubrication (WC-DWL) with lithium stearate. With this method, a very high-density sintered body can be fabricated. Pure iron powder compacted at 1176 MPa and sintered at 1523 K showed the following properties: sintered density = 7.76 Mg·m−3, maximum permeability (μm)=5300, magnetic flux density; B160=1.16 T, B240=1.28 T, B400=1.40 T, B2k=1.60 T, and coercivity (bHc)=110 A·m−1. Some of the sintered iron showed an anisotropic dimensional change and anisometric microstructure, which were linked to abnormal grain growth of up to several mm in length. This anomaly was more pronounced the higher the green density of the specimens.
  • Effect of Mn or Co on Kinetics of Eutectoid Decomposition in Sintered α-Fe2Si5 Alloys

    pp. 1407-1412

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    DOI:10.2320/matertrans.46.1407

    The evolution of the eutectoid decomposition, α-Fe2Si5 → β-FeSi2 + Si, in sintered α-Fe2Si5 alloys with added Mn or Co has been traced by measuring the electrical resistivity under isothermal conditions. The time-temperature-transformation (TTT) diagram was obtained for each alloy, and presents a typical C shape in the temperature range of 873–1148 K. The addition of Mn or Co decreases the overall transformation rate, prolonging both the induction and transformation periods. The kinetics of the eutectoid reaction is discussed in the theoretical framework of Johnson–Mehl–Avrami (JMA) theory. It was found that the transformation mechanism of the eutectoid reaction is not affected by the addition of Mn or Co, and the effective activation energy of the Mn- or Co-added alloy is the same as that of the non-doped alloy. The added Mn or Co causes the decrease in the pre-exponential constant K0, which decreases the overall transformation rate.
  • Characteristics of Sputtered Al-Sc and Al-Nd Alloy Thin Films for use in Liquid Crystal Display

    pp. 1413-1416

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    DOI:10.2320/matertrans.46.1413

    The interconnections among thin film transistor liquid crystal displays (TFT-LCDs) are laid on the films, made by the spotted coating method. This work studies the effect of heat-treatment, at temperatures from 523 K to 673 K, on the microstructure and physical properties of the deposited alloy thin films. The experimental results imply that, following heat treatment, the roughness of the aluminum-scandium (Al-Sc) alloy film is less than that of the aluminum-neodymium (Al-Nd) alloy, and the hillocks of the Al-Sc alloy film are lower. Accordingly, components are predicted to fail less often. The resistivity of the Al-Sc thin film is less than that of the Al-Nd thin film before it is thermally treated. Heat treatment reduces the resistivity of both, even though the reduced resistivity of Al-Nd exceeds the resistivity of the Al-Sc alloy film. Restated, the resistivity of the thermally stable Al-Sc thin film exceeds that of the Al-Nd thin film. Experimental results imply that the Al-Sc alloy is the more effective for making the driving circuit wires in the TFT-LCDs.
  • Erosion and Corrosion Behaviors of ADI Deposited TiN/TiAlN Coatings by Cathodic Arc Evaporation

    pp. 1417-1424

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    DOI:10.2320/matertrans.46.1417

    Austempered ductile iron (ADI) is a heat-treated ductile iron with acicular ferrite and high-carbon austenite as the matrix of the microstructure. The purpose of this research is to investigate the influence of different hard coatings (PVD-TiN and PVD-TiAlN) on the erosion and corrosion properties of ADI. Also, the coating structure and property were analyzed by using XRD, Rockwell C tester and SEM. The results showed that TiN and TiAlN films identified by XRD could be well deposited on the ADI substrate by the PVD method of cathodic arc evaporation (CAE). Adhesion between coating-layer and ADI substrate was better than that between coating-layer and DI substrate. It was found that the initial source of coated layer peeled for the both substrates occurred at the graphitic sites. The depositing rate of TiAlN was rapider than that of TiN, but it also resulted in the thicker coating thickness and rougher surface for the specimen coated with TiAlN film. After slurry erosion test, the result revealed that erosion resistance of coated specimens was better than that of uncoated specimen. In 3.5 mass% NaCl aqueous for polarization curve test, corrosion current of TiAlN film was smaller than that of TiN film. In 10 vol% HCl solution for immersion test, both TiN and TiAlN coatings could raise the corrosion resistance for ADI material.
  • Martensitic Transformation of Ni2.18Mn0.82Ga Single Crystal Observed by Synchrotron Radiation White X-Ray Diffraction

    pp. 1425-1432

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    DOI:10.2320/matertrans.46.1425

    A synchrotron radiation white X-ray diffraction of a Heusler-type Ni2.18Mn0.82Ga single crystal was observed by changing the temperature from 400 to 103 K. To investigate precisely the process of the martensitic transformation, changes in Laue spots during the transformation process were observed. The following phenomena were discovered: The single crystal at 300 K before increasing temperature shows a tetragonal structure, the a- or b-axis of which is parallel to the direction of crystal growth. It shows a cubic Heusler structure at 400 K after heating. One of the cubic axes is parallel to the direction of crystal growth. With decreasing temperature, the crystal transforms into many small variants with tetragonal structures. One of the variants evolves so that the a- or b-axis is parallel to the direction of the crystal growth, and the orientation of the other variants can be represented by its rotation about the cubic ⟨111⟩ axis. The rotation angle is at most a few degrees, which changes gradually with decreasing temperature. At 103 K the transformation is almost complete and many tetragonal variants are rearranged into one tetragonal structure, the a- or b-axis of which is parallel to the direction of crystal growth.
  • Growth Kinetic Model for Isothermal Omega Phase Particles in Low-Cost Beta Titanium Ti–6.8Mo–4.5Fe–1.5Al Alloy

    pp. 1433-1435

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    DOI:10.2320/matertrans.46.1433

    The growth kinetics of the isothermal omega phase in Ti–6.8Mo–4.5Fe–1.5Al can be monitored by the continuous measurement of the electrical resistivity as shown in our previous publication in this Journal. The rate law for the growth of these isothermal omega particles radius riso is obtained assuming the resistivity to be proportional to the surface of the interface between the beta matrix and the quasi spherical isothermal omega particles. The rate law r3isot is observed at the beginning of the growth process, that is before the interaction of the outward diffusion fields from the particles. A mathematical model based on a diffusional transfer of the beta stabilizers from the ωiso particles through the interface between the ωiso particles into the beta matrix is developed. The growth rate slows down when the outward diffusional fields from the particles interact. Ostwald ripening is not observed.
  • Characteristics and Sintering Behavior of Oxide Coated Iron Nanopowder Synthesized by Plasma Arc Discharge Process

    pp. 1436-1439

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    DOI:10.2320/matertrans.46.1436

    This paper investigates the effect of the surface oxide layer of Fe nanopowder synthesized by plasma arc discharge (PAD) process on the densification process in hydrogen atmosphere. The densification process was divided into two steps: rapid densification at low temperature and densification retarding at high temperature. The reduction process of the oxide layer was quantitatively analyzed, and it was observed that the reduction process dominantly affected the initial densification process at lower temperature. The volume shrinkage due to the reduction of the oxide layer corresponded to 75% of the initial volume shrinkage at low sintering temperature. The volume shrinkage of the reduced oxide layer and the densification rate were discussed in terms of microstructural evolution.
  • Characterization of the Gas Tungsten Arc Welded Cu54Ni6Zr22Ti18 Bulk Metallic Glass Weld

    pp. 1440-1442

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    DOI:10.2320/matertrans.46.1440

    Gas tungsten arc welding is well-established joining process for the structural materials. Due to the localized heat input, the non-uniform temperature distribution in weld is inevitable. In this regard, the thermal instability of the bulk metallic glass [BMG] should be considered for the weldability. Microstructure and phase evolutions of the Cu54Ni6Zr22Ti18 BMG weld were investigated and the fusion welded BMG weld is defined and the solid-state crystallization of the heat affected zone made the weld brittle. Furthermore, the thermal stress induced crack was propagated along the crystallized heat affected zone.

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