Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.scriptamat.2020.07.005

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.actamat.2019.04.048

Publishing type：Research paper (scientific journal)  

In titanium alloys, the omega(hexagonal)-phase transformation has been categorized as either a diffusion-mediated isothermal transformation or an athermal transformation that occurs spontaneously via a diffusionless mechanism. Here we report a diffusionless isothermal omega transformation that can occur even above the omega transformation temperature. In body-centered cubic beta-titanium alloyed with beta-stabilizing elements, there are locally unstable regions having fewer beta-stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal omega transformation occurs even when the entire beta region is stable on average so that athermal omega transformation cannot occur. This anomalous, localized transformation originates from the fluctuation-driven localized softening of 2/3[111](beta) longitudinal phonon, which cannot be suppressed by the stabilization of beta phase on average. In the diffusionless isothermal and athermal omega transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of {111}(beta) pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the omega-phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent beta/omega interface. Thus, the transformation occurs at slower rates than the athermal transformation, which occurs by the widely spread phonon softening. Consequently, the nucleation probability reflecting the beta/omega interface energy is the rate-determining process in the diffusionless omega transformations.

DOI： 10.1103/PhysRevMaterials.3.043604

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.actamat.2018.12.029

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.actamat.2018.11.037

Publishing type：Research paper (scientific journal)   Kind of work：Single Work  

DOI： 10.2355/tetsutohagane.tetsu-2019-052

Publishing type：Research paper (scientific journal)  

Nanoporous gold (NPG) with sponge-like structures has been studied by atomic-scale and microsecond-resolution environmental transmission electron microscopy (ETEM) combined with ab initio energy calculations. Peculiar surface dynamics were found in the reaction environment for the oxidation of CO at room temperature, involving residual silver in the NPG leaves as well as gold and oxygen atoms, especially on {110} facets. The NPG is thus classified as a novel self-Activating catalyst. The essential structure unit for catalytic activity was identified as Au-AgO surface clusters, implying that the NPG is regarded as a nano-structured silver oxide catalyst supported on the matrix of NPG, or an inverse catalyst of a supported gold nanoparticulate (AuNP) catalyst. Hence, the catalytically active structure in the gold catalysts (supported AuNP and NPG catalysts) can now be experimentally unified in low-Temperature CO oxidation, a step forward towards elucidating the fascinating catalysis mechanism of gold.

DOI： 10.1038/s41467-018-04412-4

Publishing type：Research paper (scientific journal)  

DOI： 10.1088/1742-6596/1063/1/012047

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.msea.2017.10.069

Publishing type：Research paper (scientific journal)  

Formation of metastable phases in Zr-ion-irradiated corundum alumina (Al2O3) upon thermal annealing was examined using transmission electron microscopy. A metastable cubic spinel phase was formed in the topmost layer of the as-irradiated microstructure. During thermal annealing at temperatures ranging from 1073 to 1273 K, this spinel layer grew in extent via an unusual corundum-to-spinel phase transformation. A normal spinel-to-corundum phase transformation was observed at post-irradiation annealing temperatures greater than 1473 K. In addition, ZrO2 nanocrystals embedded in alpha-Al2O3 were observed to form at these higher temperatures. High-resolution transmission electron microscopy observations and electron diffraction experiments revealed that the structure of the ZrO2 precipitates observed in this study are consistent with a high-pressure metastable orthorhombic phase of ZrO2 known as the Ortho-I phase.

DOI： 10.1093/jmicro/dfx028

Publishing type：Research paper (scientific journal)  

A micromechanics-based method ("inverse self-consistent approximation") that can extract all the independent elastic constants of single crystals from those of polycrystals with crystallographic texture was newly developed. In the developed method, all the elastic constants in an anisotropic polycrystal were measured by resonant ultrasound spectroscopy, and the crystallographic orientations and shapes of the grains were analyzed. Then, the elastic constants of a single crystal, which reproduce those of the polycrystal, were determined on the basis of Eshelby's inclusion theory and the effective-medium approximation, taking into account the elastic interaction between the grains, which reflects their shapes and orientations. The developed method determined the elastic constants of pure Cu and pure Mg single crystals from those of polycrystals quite precisely. The differences between the elastic constants obtained using our method and the values measured using single crystals were only similar to 1%. In contrast, the application of an inverse Voigt-Reuss-Hill approximation, which cannot consider the effect of the grain shape on the elastic interaction, resulted in a relatively poor evaluation for pure single-crystalline Cu which exhibits strong elastic anisotropy. This indicates that the grain shape clearly affects the elastic interaction between the grains exhibiting high elastic anisotropy and influences the extracted single crystalline elastic constants. In terms of the actual application of the inverse self-consistent approximation, the single-crystalline elastic constants of AZ31 Mg alloy, whose single crystals cannot be prepared easily, were clarified. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2016.09.040

Publishing type：Research paper (scientific journal)  

Using molecular dynamics simulations we show that two distinct crystallization processes, depending on the temperature at which crystallization occurs, appear in a supercooled liquid. As a model for glass-forming materials, an Al2O3 model system, in which both the glass transition and crystallization from the supercooled liquid can be well reproduced, is employed. Simulations in the framework of an isothermal-isobaric ensemble indicate that the calculated time-temperature-transformation curve for the crystallization to gamma(defect spinel)-Al2O3 exhibited a typical nose shape, as experimentally observed in various glass materials. During annealing above the nose temperature, the structure of the supercooled liquid does not change before the crystallization, because of the high atomic mobility (material transport). Thus, the crystallization is governed by the abrupt crystal nucleation, which results in the formation of a stable crystal structure. In contrast, during annealing below the nose temperature, the structure of the supercooled liquid gradually changes before the crystallization, and the formed crystal structure is less stable than that formed above the nose temperature, because of the restricted material transport. Published by AIP Publishing.

DOI： 10.1063/1.4949329

Publishing type：Research paper (scientific journal)  

Changes in the elastic properties during room-temperature aging (RT aging) of metastable Ti-Nb-based alloy single crystals with low body-centered cubic (bcc)-phase stability were investigated. The elastic stiffness components of Ti-Nb-Ta-Zr alloys with different Nb concentrations were measured by resonant ultrasound spectroscopy during RT aging; the results revealed that shear moduli c' and c(44) were increased by RT aging. In the alloy with the lowest Nb concentration, i.e., with the lowest bcc phase stability, shear moduli c' and c44 were enhanced by the largest amount. The increase rates were similar to 5% for 1.1 x 10(7) s (127 days), whereas the bulk modulus was hardly changed by aging. In Ti-Nb-Ta-Zr-O alloys with different oxygen concentrations, shear moduli cl and c44 of the alloy with the lowest oxygen concentration increased most significantly. Moreover, the electrical resistivity of Ti-Nb-Ta-Zr and Ti-Nb-Ta-Zr-O alloys was increased by RT aging. Importantly, the enhancements of shear moduli and electrical resistivity were suppressed by increases in the bcc-phase stability (i.e., increase in the Nb concentration) and oxygen concentration; these factors are known to suppress omega (hexagonal) phase formation. However, transmission electron microscopy (TEM) observations revealed that only a diffuse or structure-an omega-like lattice distortion-was formed after RT aging. On the basis of alloying element effects, TEM observations, and analysis of the changes in elastic properties by using a micromechanics model, it was deduced that the enhancements of shear moduli and electrical resistivity were possibly caused by the formation of a diffuse omega structure. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2015.09.030

Publishing type：Research paper (scientific journal)  

The effects of stacking sequence and short-range ordering of solute atoms on the elastic properties of Mg-Zn-Y alloy single crystals with an 18R- or 10H-type long-period stacking ordered (LPSO) structure were studied. Instead of single crystals, the growth of which can be quite difficult, two directionally solidified (DS) Mg-Zn-Y alloy polycrystals, mainly consisting of 18R- or 10H-type LPSO structure, were prepared. X-ray pole figure analyses revealed that fiber textures, which differed in the two prepared alloys, were formed in the DS polycrystals. For the DS polycrystals, a complete set of elastic constants was measured during cooling from 300 to 7.5 or 5.5 K. By analyzing the elastic stiffness of DS polycrystals on the basis of a newly developed inverse Voigt-Reuss-Hill approximation, in which the detailed crystallographic texture could be taken into account, the elastic stiffness components of the single-crystalline LPSO phases from 300 to 7.5 or 5.5 K were determined. The elastic properties of the 18R- and 10H-LPSO phases were also evaluated by first-principles calculations based on density functional theory. Comparison of the measured elastic properties at 5.5 or 7.5 K with the first-principles calculations revealed that the elastic properties of the LPSO phase were virtually dominated by the stacking sequence of the LPSO structures and the formation energy of short-range ordered solute atom clusters, formed at the four consecutive atomic stacking layers. Importantly, the effects of the formation energy and stacking sequence were significant in the elastic moduli related to the atomic bonding between the stacking layers. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2015.06.005

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.actamat.2014.11.006

Publishing type：Research paper (scientific journal)  

The tensile and compressive deformation in porous Mg with unidirectionally oriented cylindrical pores and a unique fiber texture in which the normal direction of the {10 $(1) over bar $3} plane was preferentially oriented was studied. Porous Mg specimens with unidirectional pores and texture were prepared by unidirectional solidification in a hydrogen atmosphere using a continuous-casting technique and their quasi-static tensile deformation and quasi-static and dynamic compressions were investigated. In tensile loading parallel to the orientation direction of the pores (the "pore direction"), the porous Mg exhibited a large tensile elongation of similar to 60% strain despite the presence of, similar to 42% porosity, whereas it showed high energy absorption of similar to 30 kJ kg(-1) along the same direction. To clarify these superior mechanical properties, the underlying operative deformation modes and rotation of crystallographic orientation during loadings were analyzed by X-ray pole figures, optical microscopy and crystal plasticity finiteelement modeling. The analyses revealed that in the initial stage of both the compression and tensile loadings along the pore direction, basal slip mainly operated. Importantly, the activity of basal slip was enhanced during the tensile loading by rotation of the crystallographic orientation, which resulted in high tensile elongation. On the other hand, the activation of basal slip was initially suppressed by the crystal rotation during compression. However, the localization of basal slip originating from the elongated grains with the unique texture subsequently enhanced the activity of basal slip, which suppressed the steep increase in the flow stress. This unique localized deformation gave rise to the superior impact energy absorption. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2014.10.024

Publishing type：Research paper (scientific journal)  

Formation of curious deformation bands has been reported as one of the deformation mechanisms occurring in an Mg-based long-period stacking ordered (LPSO) phase. The origin of the deformation band is still unknown, and the possibility of the deformation kink band and/or the deformation twin has been discussed. To clarify this, the crystallographic nature of deformation bands formed in the LPSO phase was examined by scanning electron microscope-electron backscatter diffraction (SEM-EBSD) pattern analysis. The results were compared to those of the deformation kink bands formed in hcp-Zn and deformation twins formed in hcp-Mg polycrystals. The deformation bands in the LPSO phase was confirmed not to exhibit a fixed crystal orientation relationship with respect to the matrix, different from the case shown in the deformation twin. Instead, the deformation band in the LPSO phase showed three arbitrariness on its crystallographic nature: an ambiguous crystal rotation axis that varied on the [0001] zone axis from band to band; an arbitral crystal rotation angle that was not fixed and showed relatively wide distributions; and a variation in crystal rotation angle depending on the position even within a deformation band boundary itself. These features were coincident with those observed in the deformation bands formed in Zn polycrystals, suggesting that the formed deformation bands in LPSO phase crystals are predominantly deformation kink bands.

DOI： 10.1080/14786435.2014.987843

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

The self-diffusivity of oxygen in amorphous Al2O3 (a-Al2O3), a-Ta2O5, and a-Nb2O5 was investigated along with structural analysis in terms of pair distribution function (PDF). The low activation energy, similar to 1.2 eV, for diffusion in the oxides suggests a single atomic jump of oxygen ions mediated via vacancy-like defects. However, the pre-exponential factor for a-Ta2O5 and a-Nb2O5 with lower bond energy was two orders of magnitude larger than that for a-Al2O3 with higher bond energy. PDF analyses revealed that the short-range configuration in a-Ta2O5 and a-Nb2O5 was more broadly distributed than that in a-Al2O3. Due to the larger variety of atomic configurations of a-Ta2O5 and a-Nb2O5, these oxides have a higher activation entropy for diffusion than a-Al2O3. The entropy term for diffusion associated with short-range structures was shown to be a dominant factor for diffusion in amorphous oxides. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4889800

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.msea.2013.10.078

Publishing type：Research paper (scientific journal)  

The elastic properties of single-crystalline omega (hexagonal) phase of titanium are studied. Understanding the elastic properties is important for the development of biomedical titanium alloys with a low Young's modulus. However, the elastic properties of the omega phase have remained unclear because of the difficulty in preparing a large single crystal consisting of a single phase of the omega phase, even though the omega phase has been believed to exhibit a higher elastic modulus than the beta (body-centered cubic) phase. In this work, pure titanium was severely deformed by high-pressure torsion processing, to obtain polycrystalline specimens consisting exclusively of the omega phase, which is metastable at room temperature. For the omega-phase polycrystal, the complete set of elastic stiffness components was measured by RUS combined with laser Doppler interferometery. By analyzing the elastic stiffness of the omega-phase polycrystal on the basis of an inverse Voigt-Reuss-Hill approximation, the elastic stiffness components of the single-crystalline omega phase were determined. The Young's modulus of the omega phase along < 0001 > was found to be clearly higher than that along < 11 (2) over bar0 >, and the shear modulus also exhibited anisotropy. Importantly, the Young's modulus and shear modulus of the metastable omega phase were higher than those of the beta phase and also higher than those of the alpha (hexagonal close-packed) phase, which is stable at room temperature. Furthermore, analysis by a micromechanics model using the determined elastic stiffness deduced the effect of omega phase formation on the elastic properties of beta-phase titanium alloys. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2013.08.036

Publishing type：Research paper (scientific journal)  

The elastic properties of an Mg85Zn6Y9 (at.%) alloy single crystal with a long-period stacking-ordered (LPSO) structure, synchronized with periodic enrichment of Zn and Y atoms, were investigated, the properties having remained unclear because of the difficulty in growing large single crystals. Directionally solidified (DS) Mg85Zn6Y9 alloy polycrystals consisting of a single phase of the 18R-type LPSO structure were prepared using the Bridgman technique. For the DS polycrystals, a complete set of elastic constants was measured with resonant ultrasound spectroscopy combined with electromagnetic acoustic resonance, in which the texture formed by the directional solidification was taken into account. By analyzing the elastic stiffness of DS polycrystals on the basis of a newly developed inverse Voigt Reuss Hill approximation, the elastic stiffness components of the single-crystalline LPSO phase were determined. It was revealed that the Young's modulus of the LPSO phase along (00 01) in the hexagonal expression was clearly higher than that along < 1 1 (2) over bar0 >, and the Young's modulus and shear modulus were clearly higher than those of pure magnesium. These findings were validated by first-principles calculations based on density functional theory. Analyses by first-principles calculations and micromechanics modeling indicated that the long periodicity of the 18R-type stacking structure hardly enhanced the elastic modulus, whereas the Zn/Y-enriched atomic layers, containing stable short-range ordered clusters, exhibited a high elastic modulus, which contributed to the enhancement of the elastic modulus of the LPSO phase in the Mg-Zn-Y alloy. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2013.06.041

Publishing type：Research paper (scientific journal)  

The effect of the solidification condition and alloy composition on the formation of cylindrical pores oriented along the solidification direction was investigated in Ni(100-x)Al (x) (x = 20, 25, 30, and 50 at. pct) alloys that were solidified unidirectionally in hydrogen atmosphere. It was revealed that the uniformity of the pores strongly correlates with the width of the mushy zone (i.e., the region of solid-liquid coexistence) in the solidification front. In alloys with x = 25 and 50 (i.e., NiAl and Ni3Al intermetallic compounds, respectively), uniform cylindrical pores were formed, reflecting small freezing intervals, which lead to narrow mushy zones. On the other hand, irregular pores were formed in x = 20 and 30 two-phase alloys comprising Ni solid-solution and Ni3Al phases and Ni3Al and NiAl phases, respectively, that had large freezing intervals leading to wide mushy zones. This is because the large amount of primary crystals with dendritic structures prevents the growth of directional pores in the mushy zone. For the x = 20 and 30 alloys, the increase in the temperature gradient of the solidification front, which decreases the mushy zone width, clearly enhances the uniformity of the pores. Consequently, decreasing the mushy zone width results in the growth of uniform cylindrical pores.

DOI： 10.1007/s11661-013-1790-z

Publishing type：Research paper (scientific journal)  

The omega transformation and its correlation with elastic properties were investigated in cold-worked Ti-36Nb-2Ta-3Zr-xO mass% alloys with low body-centered cubic (beta) phase stability, known as gum metal. Analysis of the temperature dependence of the omega (hexagonal) phase formation using transmission electron microscopy and of the elastic properties of solution-treated and cold-worked alloys using resonant ultrasound spectroscopy revealed that in the solution-treated 0.36% and 0.51% O alloys, the high concentration of oxygen suppressed omega-phase formation from room temperature to a fairly low temperature of similar to 13 K. However, the omega phase was formed by cold working at room temperature in the 0.30% and 0.47% O alloys. Importantly, the fraction of the omega phase clearly increased upon cooling, which indicates that the formation of the omega phase is thermodynamically favorable near and below room temperature in the cold-worked 0.30% and 0.47% O alloys. This formation of the omega phase and the low stability of the beta phase related to the low electron/atom (e/a) ratio were the dominant factors determining the elastic properties near and below room temperature in the cold-worked Ti-Nb-Ta-Zr-O alloys. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2012.09.041

Publishing type：Research paper (scientific journal)  

Porous Ti-xAl-8Nb (x=45, 48, and 49at%) alloys (i.e., intermetallic TiAl-Nb compounds) with cylindrical pores oriented along a single direction were prepared via unidirectional solidification in a hydrogen and helium gas atmosphere, while making use of the differences in the hydrogen solubilities of the liquid and solid phases of the alloys. The microstructures and pore morphologies of the alloys revealed that pore formation in the alloys was closely related to their solidification processes. In the case of the alloy with x=45at%, a primary solidified β (bcc) phase with high hydrogen solubility suppressed the formation of pores during solidification. On the other hand, in the case of the alloy with x=49at%, the formation of the primary β phase was suppressed, leading to the formation of elongated pores along the solidification direction. After being heat treated, the porous TiAl-Nb alloys exhibited a fully lamellar structure composed of α2-Ti3Al and γ-TiAl phases. These porous TiAl-Nb alloys with lamellar structures and directional pores exhibited superior compressive properties parallel to the direction of the pores. © 2013 Elsevier Ltd.

DOI： 10.1016/j.matdes.2013.02.031

Publishing type：Research paper (scientific journal)  

Lotus-type porous materials exhibit some unique anisotropic mechanical and thermal properties which are very useful for a number of industrial applications. This paper evaluates several computational models for determining the compressive engineering elastic modulus and engineering yield stress of lotus-type iron in transversal and longitudinal direction in regard to pore orientation with parametric nonlinear finite element computational simulations for porosities ranging from 0 to 0.65. The considered pore topologies of evaluated computational models are either regular (indirectly reconstructed) or irregular (directly reconstructed). Comparison of computational results, experimental tests and analytical estimations shows good correlation of some evaluated computational models. The simplified porous model with pi/4 rotated aligned regular pores can be recommended for fast computational estimation of lotus-type material behaviour under mechanical loading, when some material parameters for homogenised lotus-type material modelling have to be determined. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved.

DOI： 10.1016/j.commatsci.2012.07.004

Publishing type：Research paper (scientific journal)  

The elastic anisotropy of the Ti-15Mo-5Zr-3Al (mass%) beta-Ti alloy, an ISO certified biomedical material, was investigated using its single crystal. It was revealed that the Young's modulus exhibited pronounced anisotropy. The Young's modulus was reduced to 44.4 GPa along the < 100 > direction in the Ti-15Mo-5Zr-3Al single crystal, that is comparable to that of human cortical bones. We determined the strategy that beta-Ti alloys with extremely low moduli can be developed by reducing the electron-atom (e/a) ratio in alloys, and by suppressing the formation of the omega-phase at the same time. This new knowledge must lead to the development of "single crystalline beta-Ti implant materials" as hard tissue replacements for reducing the stress shielding effect. (C) 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jmbbm.2012.05.005

Publishing type：Research paper (scientific journal)  

This paper addresses the thermal and mechanical properties of lotus-type porous copper. Due to their cellular metal characteristics in combination with strong anisotropy, lotus-type materials exhibit unique properties. As an example, directional thermal conduction enables the controlled transport of thermal energy in the pore direction without the need of strong thermal insulation. In this paper, thermal and mechanical finite element analyses are performed. The effective thermal conductivity, Young's modulus, and the 0.2%-offset yield strength are determined. Special consideration is given to the anisotropy of the material. In order to guarantee accurate discretization of the complex material geometry, calculation models are directly based on computed microtomography data. Elastic properties are compared to experimental data and good agreement is found. For the characterization of the thermal anisotropy, a second numerical approach, called the Lattice Monte Carlo method, is used along with thermal finite element analysis. In addition to the numerical methods, the analytical Maxwell, Dulynev, and Bruggeman models are applied. Good agreement for the application of two-dimensional versions of Dulynev's and Bruggeman models is observed whereas the Maxwell model significantly overestimates the material properties.

DOI： 10.1002/adem.201100205

Publishing type：Research paper (scientific journal)  

Dynamic and quasi-static compressive deformation of as-cast and normalized porous S30C and S45C carbon steels with unidirectional cylindrical pores was investigated to evaluate the effect of matrix brittleness on the appearance of a plateau stress region where deformation proceeds with almost no stress increase. Dynamic compression tests were carried out at room (298 K) and cryogenic (77 K) temperatures using the split Hopkinson pressure bar method, and quasi-static compression tests were carried out at room temperature using universal testing machine. Compression perpendicular to the orientation of the pores does not generate a plateau stress region, regardless of the matrix brittleness, which depends on the strain rate, temperature, carbon content, and heat-treatment conditions. Localized deformation and crack formation originating from a high concentration of stress around the pores during perpendicular compression promote densification in the early stage of the stress-strain curves, thereby precluding the appearance of the plateau stress region. On the other hand, a plateau stress region appears during compression parallel to the orientation of the pores. The appearance of the plateau stress region is, however, limited when rapid crack propagation and large work hardening are suppressed by the ductility of the matrix and the formation of deformation bands, which originate from the intermediate brittleness of the matrix metal and anisotropic pores. The appearance of the plateau stress region confers a high-energy absorption capacity on the as-cast porous carbon steel S45C, with an absorbed energy value of 86.8 +/- 1.6 kJ kg(-1), which is ten times higher than that of aluminum foams with isotropic pores. The energy absorption efficiency reaches 85.9 +/- 6.8%, which is almost the same as that of the aluminum foams. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2011.11.099

Publishing type：Research paper (scientific journal)  

Dynamic and quasi-static compressive deformation of as-cast and normalized porous S15CK carbon steels with cylindrical pores oriented in one direction was investigated at 298 and 77 K, using the split Hopkinson pressure bar method and a universal testing machine, combined with an acoustic emission measurement system, to clarify the formation mechanism of a plateau stress region where deformation proceeds with almost no stress increase. Dynamic and quasi-static compressions perpendicular to the orientation of the pores at 298 and 77 K do not produce a plateau stress region in the as-cast and normalized porous S15CK, because the localized crack formation and slip deformation that originate from the large concentration of stress around pores promotes densification in the early stage of the stress-strain curves. When the samples undergo dynamic compression parallel to the pore direction at 77 K, the matrix becomes brittle, and cracks are easily formed. However, the pores do not easily collapse, because they are oriented along the compressive direction. Therefore, densification occurs at a higher strain level. In addition, the formation of small cracks in the matrix decreases the work hardening rate. As a result, a plateau stress region with high stress amplitude and wide strain range appears, which is independent of the microstructure. This mechanism for the formation of the plateau stress region is completely different from that of metal foams with isotropic pores, which is based on sequential inhomogeneous deformation. As a result, energy absorption 10 times that of commercial aluminum foams is achieved. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2011.11.001

Publishing type：Research paper (scientific journal)  

Dynamic and quasi-static compressive deformation of lotus-type porous carbon steel S15CK (lotus S15CK) with cylindrical pores elongated in one direction was investigated. In the quasi-static and dynamic compressions parallel and perpendicular to the pore direction, a clear plateau stress region does not appear. However, in the dynamic parallel compression, the rate of increase in the flow stress is rather low in the strain range of similar to 5-50%. Therefore, the efficiency of absorbed energy is comparable to that of conventional aluminum foams, and the absorbed energy is ten times higher than that of conventional aluminum foams. This is because the cylindrical pores parallel to the compressive direction suppress the stress concentration around pores and the densification in early stage of compression.

Publishing type：Research paper (scientific journal)  

Amorphous Al2O3 thin films containing nanovoids were fabricated through the annealing of amorphous Al2O3 thin films that were prepared by an electron beam deposition method. The elastic properties of nanoporous amorphous Al2O3 thin films were measured by a picosecond laser ultrasound method, which clarified that the elastic stiffness drastically increased after the annealing despite nanovoid formation; this increase indicate a change in the amorphous structure during the annealing.

Publishing type：Research paper (scientific journal)  

The origin of the low Young's modulus of cold worked Ti-36Nb-2Ta-3Zr-xO mass% polycrystals with a body-centered cubic (beta-phase) structure, referred to as gum metal, was investigated with a focus on the roles of oxygen concentration, the electron-atom (e/a) ratio, and the cold working process. Analysis of the temperature dependence of the microstructures and elastic properties of single crystals at x = 0.09, 0.36, 0.51% O using transmission electron microscopy and an electromagnetic acoustic resonance method, respectively, revealed that the shear moduli c' and c(44) of the 0.36 and 0.51% O alloys softened upon cooling near room temperature (RT) and exhibited low values at RT. This was because suppression of the alpha '' martensitic transformation by oxygen addition led to retention of the low stability single beta-phase state at RT. The Hill approximation indicated that the low c' and c(44) values caused by softening gave rise to the low Young's modulus, which is common to some Ti-Nb-based alloys with an c/a ratio of similar to 4.24. Analysis of the microstructures and elastic properties of solution-treated and cold worked x = 0.06, 0.30, 0.47% O alloy polycrystals at RT revealed that the Young's modulus increased upon 90% cold working due to formation of the alpha '' martensite phase (0.09% O) and omega phase (0.09, 0.30, and 0.47% O) with a high elastic modulus in the beta-phase matrix. However, increasing the oxygen concentration suppresses the increase in Young's modulus because oxygen addition decreases the amount of alpha '' and omega phases formed while retaining the low stability beta phase. Therefore, cold working combined with oxygen addition produces a low Young's modulus compatible with high strength. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2011.07.050

Publishing type：Research paper (scientific journal)  

The effect of W on the nanovoid formation in annealed amorphous Al2O3 was studied by transmission electron microscopy and molecular dynamics simulations. A comparison of the void formation behavior in electron-beam deposited Al2O3 (without W) and resistance-heating deposited Al2O3 (with 10 at. % W) revealed that W enhances the formation and growth of nanovoids. An analysis of the pair distribution function (PDF) in both types of amorphous Al2O3 showed that the introduction of W into amorphous Al2O3 brings about a significant change in the amorphous structure. Furthermore, it was found by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) that sub-nm sized W clusters exist in as-deposited Al2O3 prepared by resistance-heating and then dissolve in the amorphous matrix with annealing. The combination of PDF analysis and HAADF-STEM observation provides evidence that the enhancement of void formation originates in the heterogeneous short-range atomic configurations induced by the addition of W. (C) 2011 American Institute of Physics. [doi:10.1063/1.3639290]

DOI： 10.1063/1.3639290

Publishing type：Research paper (scientific journal)  

The formation mechanism of a high density of nanovoids by annealing amorphous Al2O3 thin films prepared by an electron beam deposition method was investigated. Transmission electron microscopy observations revealed that nanovoids similar to 1-2 nm in size were formed by annealing amorphous Al2O3 thin films at 973 K for 1-12 h, where the amorphous state was retained. The elastic stiffness, measured by a picosecond laser ultrasound method, and the density, measured by X-ray reflectivity, increased drastically after the annealing process, despite nanovoid formation. These increases indicate a change in the amorphous structure during the annealing process. Molecular dynamics simulations indicated that an increase in stable AlO6 basic units and the change in the ring distribution lead to a drastic increase in both the elastic stiffness and the density. It is probable that a pre-annealed Al2O3 amorphous film consists of unstable low-density regions containing a low fraction of stable AlO6 units and stable high-density regions containing a high fraction of stable AlO6 units. Thus, local density growth in the unstable low-density regions during annealing leads to nanovoid formation (i.e., local volume shrinkage). (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2011.04.008

Publishing type：Research paper (scientific journal)  

The dynamic compression behavior of porous carbon steels with cylindrical pores oriented in one direction was investigated. The application of dynamic compression parallel to the orientation of the pores at 77 K resulted in the formation of a plateau stress region, in which considerable impact energy is absorbed. The amount of energy absorbed is 10 times that of commercial aluminum foams. Plateau stress regions appear when crack propagation is suppressed, suggesting a starkly different mechanism from that of metal foams. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.scriptamat.2011.01.006

Publishing type：Research paper (scientific journal)  

Porous aluminum with a porosity of 17.6% and porous copper with a porosity of 39.7% (the pores of both aluminum and copper were cylindrical and oriented in one direction) were deformed by equal channel angular extrusion using a 150 degrees die with sequential 180 degrees rotations (route C), and the mechanical strength and pore morphology after the extrusions were investigated. In the case of porous aluminum with low porosity, the pores were collapsed by the extrusions that were both parallel and perpendicular to the orientation direction of the pores. In contrast, the porosity of porous copper decreased slightly after extrusions that were parallel to the orientation direction of the pores, and the pores thus remained even after four extrusions. The yield strength after the second extrusion was 7.3 times greater than it was before the extrusion, even though there was a decrease in porosity of only 8%. On the other hand, almost all the pores of the porous copper collapsed after the fourth extrusion, when the extrusion direction was perpendicular to the orientation direction of the pores. Thus, the yield stress cannot be enhanced without being accompanied by progressive densification. 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2010.12.051

Publishing type：Research paper (scientific journal)  

The tensile deformation of lotus-type porous copper with cylindrical pores oriented in one direction was investigated. Deformation was occured homogeneously in the copper matrix for loadings parallel to the orientation direction of pores (pore direction), while deformation was localized in the matrix around pores for loadings perpendicular to the pore direction. In the case of parallel loading the decrease in cross section of tensile specimen was smaller than that of nonporous copper, because of the constant-volume law (i.e. incompressibility condition) for deformation was not applicable to the deformation of pores. In the case of perpendicular loading, the deformed regions were disconnected and constant-volume law holds only in the matrix around the pores, and thus, the cross section hardly decreases during the tensile deformation.

DOI： 10.4028/www.scientific.net/MSF.695.545

Publishing type：Research paper (scientific journal)  

Lotus-type porous aluminum with cylindrical pores oriented in one direction was deformed by Equal Channel Angular Extrusion (ECAE) through a 150 degrees die with sequential 180 degrees rotations, and the pore morphology and Vickers hardness after the extrusion were investigated. The Vickers hardness increases with increasing number of passes in the extrusions both parallel and perpendicular to the pore direction, accompanied by the decrease of porosity. The densification occurs more easily in the perpendicular extrusions than in the parallel extrusions, and the large deformation by the densification gives rise to the large increase in the Vickers hardness for the perpendicular extrusions.

DOI： 10.4028/www.scientific.net/MSF.695.263

Publishing type：Research paper (scientific journal)  

The composition and temperature dependence of the elastic properties and phase stability of quaternary Ti-Nb-Ta-Zr beta-phase alloys with a body-centered cubic structure, developed for biomedical applications, were investigated using their single crystals, in order to clarify the origin of the low Young's modulus in polycrystals. Transmission electron microscopy observations clarified that alpha '' martensitic transformation occurred in a temperature range that depended on the beta-phase stability below room temperature. Electromagnetic acoustic resonance measurements clarified that the shear moduli c' and c(44) of single crystals softened upon cooling from room temperature and became rather low near the martensitic transformation start temperature, i.e. the lower limit of beta-phase stability. An analysis by the Hill approximation indicates that low c' and c(44) caused the low Young's modulus, and thus it is probable that the softening in c' and c(44) is the origin of the low Young's modulus. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2010.09.007

Publishing type：Research paper (scientific journal)  

Porous Al 5 mass%Ti alloy was fabricated by unidirectional solidification in hydrogen atmosphere using a continuous casting technique The porous Al Ti alloy was prepared at different transfer (solidification) velocities and the effect of transfer velocity on the pore morphology was investigated It was found that the pore shape changes with increasing transfer velocity while the porosity does not change with increasing transfer velocity In the case of a low transfer velocity (0 5 mm min(-1)) elongated pores surrounded by the columnar microstructure are formed which indicates that the pores grow along the solidification direction together with the solid phase In the case of a middle transfer velocity (5 0 mm min(-1)) elongated pores surrounded by the columnar microstructure and needle or plate like Al3Ti alloys are formed In the case of a high transfer velocity (10 0 mm min(-1)) spherical pores surrounded by the equiaxed microstructure are formed because the primary crystals formed in the solidification front prevent the growth of elongated pores It is suggested that the pore morphology is closely related with the solidification rate [doi 10 2320/matertrans M2010223]

DOI： 10.2320/matertrans.M2010223

Publishing type：Research paper (scientific journal)  

The tensile deformation of anisotropic porous copper with unidirectionally oriented cylindrical pores was investigated by an acoustic emission method. In the loadings parallel and perpendicular to the orientation direction of the pores, many cracks are formed after yielding and they strongly affect the deformation. The formed cracks rapidly grow and connect with each other near the peak stress of the stress strain curve, thereby leading to final fracture. Crack formation is easier under perpendicular loading than under parallel loading, because high stress concentration and stress triaxiality occurs around the pores. As a result, the strength and elongation for perpendicular loading are much smaller than those for parallel loading. Furthermore, in the case of perpendicular loading, the localized deformation around pores drastically decreases the plastic Poisson's ratio. These results indicate that a porous copper macroscopically behaves as a semibrittle material under perpendicular loading, while the porous copper exhibits ductility under parallel loading.

DOI： 10.1557/JMR.2010.0261

Publishing type：Research paper (scientific journal)  

Al-3.7 pct Si-0.18 pct Mg foams strengthened by AlN particle dispersion were prepared by a melt foaming method, and the effect of foaming temperature on the foaming behavior was investigated. Al-3.7 pct Si-0.18 pct Mg alloy containing AlN particles was prepared by noncompressive infiltration of Al powder compacts with molten Al alloy in nitrogen atmosphere, and it was foamed at different foaming temperatures ranging from 1023 to 1173 K. The porosity of prepared foam decreases and the pore structure becomes homogeneous with increasing foaming temperature. When the foaming temperature is higher than 1123 K, homogeneous pores are formed in the prepared ingot without using oxide particles and metallic calcium granules, which are usually used for stabilizing a foaming process. This stabilization of the foaming at high temperatures is possibly caused by Al3Ti intermetallic compounds formed at high temperature and AlN particles. Compression tests for the prepared foams revealed that the absorbed energy per unit mass of prepared Al-3.7 pct Si-0.18 pct Mg foam is higher than those of aluminum foams strengthened by alloying or dispersion of reinforcements. It is remarkable that the oscillation in stress, which usually appears in strengthened aluminum foams, does not appear in the plateau stress region of the present Al-3.7 pct Si-0.18 pct Mg foam. The homogeneity in cell walls and pore morphology due to the stabilization of pore formation and growth by AlN and Al3Ti particles is a possible cause of this smooth plateau stress region.

DOI： 10.1007/s11661-010-0247-x

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

The strain rate dependence of anisotropic compression behavior in porous iron with cylindrical pores oriented in one direction was investigated. Through high strain rate (similar to 10(3) s(-1)) compression tests along the orientation direction of pores using the split Hopkinson pressure bar method, it was shown that the stress-strain curve exhibits a unique plateau-stress region where deformation proceeds with almost no stress increase. The appearance of the plateau-stress region is related to the buckling deformation of the iron matrix and provides superior energy absorption. However, for the middle (similar to 10(-1) s(-1)) and low strain rates (similar to 10(-4) s(-1)), compression along the same direction produces no such plateau region. In fact, in contrast to compression in the parallel direction, compression perpendicular to the orientation direction of pores produces no plateau-stress regions in any of the three strain rates.

DOI： 10.1557/JMR.2010.0147

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/B978-008043152-9.02241-7

Publishing type：Research paper (scientific journal)  

Dynamic and quasi-static compression tests were conducted on lotus-type porous iron with porosity of about 50% using the split Hopkinson pressure bar method and universal testing machine, respectively. In the dynamic compression parallel to the pore direction, a plateau stress region appears where deformation proceeds at nearly constant stress, while the plateau stress region does not appear in the quasi-static compression. The plateau stress region is probably caused by the buckling deformation of matrix iron which occurs only in the dynamic compression. In contrast, the compression perpendicular to the orientation direction of pores exhibits no plateau-stress regions in the both dynamic and quasi-static compression.

DOI： 10.4028/www.scientific.net/MSF.658.193

Publishing type：Research paper (scientific journal)  

The elastic and plastic deformation behaviors of lotus-type porous copper (lotus copper) with cylindrical pores oriented in one direction were investigated using two acoustic methods (resonant ultrasound spectroscopy and acoustic emission method). All the independent components of elastic stiffness were determined by resonant ultrasound spectroscopy combined with electromagnetic acoustic resonance method, which revealed that the Young's modulus exhibits the anisotropy originating from the anisotropic porous structure and anisotropic matrix texture. The porosity dependence of the anisotropic Young's modulus can be calculated by the micromechanics modeling based on effective-mean-field theory. The tensile deformation behavior of lotus copper was analyzed by acoustic emission method, which revealed that many burst acoustic emission signals are detected during the tensile deformation. This implies that many cracks are formed during the tensile deformation.

DOI： 10.4028/www.scientific.net/MSF.658.332

Publishing type：Research paper (scientific journal)  

Al foams whose matrix contains dispersed AlN particles (Al/AlN composite foams) were prepared by a melt foaming method, and the effect of foaming temperature on the pore morphology of the prepared foams was investigated. First, Al/AlN composites were prepared by non-compressive infiltration of Al powder compacts with molten Al alloy in nitrogen atmosphere. Next, the prepared composites were melted by induction heating and foamed at various temperatures using TiH2 powders as blowing agents. The porosity of prepared Al/AlN composite foams slightly decreases with increasing foaming temperature, and the pore morphology of the foam becomes homogeneous simultaneously. When the foaming temperature is 1123 K, homogeneous pores are formed in all over the ingot. This pore homogeneity is probably achieved by the stabilization of the foaming behavior due to the formation of Al3Ti particles in the melt and dispersion of AlN particles.

DOI： 10.4028/www.scientific.net/MSF.658.189

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Lotus-type porous aluminum with cylindrical pores oriented in one direction was fabricated by a casting method utilizing the decomposition of moisture in a vacuum. Hydrogen decomposed from moisture is utilized for cylindrical pores to grow during unidirectional solidification. However, pores are not formed in the case of a casting in hydrogen or argon atmosphere, because hydrogen or argon gas pressure suppresses the pore growth. The porosity of lotus aluminum does not depend on the moisture amount, which indicates that the moisture amount is almost saturated within the amount used in this study. The average pore diameter does not depend on the moisture amount, because the pore diameter depends mainly on ambient pressure and solidification rate. The distribution of pores becomes homogeneous by decreasing melting temperature, because the rate of the reaction of moisture possibly becomes low (more suitable for pore growth) by decreasing the melting temperature. [doi:10.2320/matertrans.M2009032.]

DOI： 10.2320/matertrans.M2009032

Publishing type：Research paper (scientific journal)  

Lotus-type porous Ti(52)Al(48), possessing elongated pores aligned in one direction, was fabricated by a continuous zone melting technique under pressurized hydrogen/helium atmospheres. Compression tests were carried out on specimens with various porosities and with loading directions parallel and perpendicular to the elongated pore direction. The compressive properties of lotus TiAl significantly differ from those of conventional lotus metals with ductile matrix. Deformation and initial cracks are observed around the pores, and they progress around the pores locally. Therefore, the densification of the specimen does not occur. This is due to the brittle matrix, which is sensitive to local large stress concentration. Thus, it is concluded that compressive properties of lotus metals depend not only on macroscopic pore structure but also on the degree of ductility of matrix. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2009.01.046

Publishing type：Research paper (scientific journal)  

The mechanical properties of lotus metals were experimentally investigated and the effect of the anisotropic pores on the mechanical properties was clarified. The mechanical strength in the direction parallel to the pore direction linearly decreases with increasing porosity, while that perpendicular to the pore direction drastically decreases. This is caused by the anisotropy of stress concentration. The porosity dependence of the mechanical strength obeys a power-law formula.

DOI： 10.1007/978-1-4020-9404-0_6

Publishing type：Research paper (scientific journal)  

Lotus-type porous magnesium was fabricated through thermal decomposition of MgH2 powders as a source of hydrogen. Liquid magnesium was cast into a mold in which MgH2 powders were placed and solidified unidirectionally in the mold, which achieved the growth of unidirectional elongated pores in magnesium matrix. The fabrication method is safer than a conventional method using pressurized hydrogen atmosphere, because the risk of explosion can be avoidable. The effect of the amount of MgH2 powders and the distance from the bottom of the ingot on the porous structure was investigated, which clarified that the two factors have the large influence on the pore growth. © 2009 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/165/1/012065

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.actamat.2008.02.017

Publishing type：Research paper (scientific journal)  

DOI： 10.1557/JMR.2008.0105

Publishing type：Research paper (scientific journal)  

DOI： 10.1002/9783527621408.ch8

Publishing type：Research paper (scientific journal)  

DOI： 10.2320/matertrans.MRA2007623

Publishing type：Research paper (scientific journal)  

We studied the effect of pore size distribution and loading direction on fatigue property of lotus-type porous copper with anisotropic pore structure. The fatigue strength at finite life is closely related to its pore morphology. The strength depends on applied stress direction; the fatigue life is the longest in the direction parallel to the longitudinal axis of cylindrical pores. Abnormally large pores decrease the fatigue strength of lotus copper.

Publishing type：Research paper (scientific journal)  

Ti-rich TiAl crystals are a promising candidate as light-weight and heat-resistant materials for aerospace and automobile applications.
In this study, we fabricated Ti-48.0at.%Al (Ti-rich TiAl) crystals with lotus-type aligned pores, and examined the microstructure and plastic deformation behavior. We succeeded in obtaining the gamma/alpha(2) two-phase lamellar Ti-rich TiAl crystals with elongated pores aligned parallel to the heat transfer direction by the floating zone method under the pressure of hydrogen and helium mixed gas at a crystal growth :ate of 330 mu m/s. The as-grown and annealed crystals show a well-developed lamellar structure with a strict orientation relationship between the alpha(2) phase and six gamma variants in each grain. However, these crystals exhibit no strong texture of lamellar grains. The total pore volume and the mean pore diameter are approximately 52 % and 380 pm, respectively.
The macro-yield stress strongly depends on the loading axis relative to the elongated pore. When the loading axis is parallel and perpendicular to the elongated pore direction, the yield stress obeys K=1 and 2.5, respectively, in the equation of sigma=sigma(0)(1-P)(kappa), where a is the macro-yield stress with pores, sigma(0) is the macro-yield stress without pores and P is the volume fraction of pores. The difference of K value, K=1 and 2.5, indicates the change in deformation manner, i.e. macroscopically homogeneous and locally heterogeneous plastic deformation, respectively. The plastic deformation behavior was also investigated at elevated temperatures up to 1273 K. The relatively high stress was maintained independent of the loading axis up to around 1073 K. The K value shows no strong temperature dependence because aligned pores give no change in deformation behavior at elevated temperatures.

Publishing type：Research paper (scientific journal)  

DOI： 10.1557/JMR.2007.0385

Publishing type：Research paper (scientific journal)  

DOI： 10.1143/JJAP.46.5221

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Lotus-type porous carbon steel with cylindrical pores was fabricated by the continuous zone melting method in pressurized hydrogen gas. A three-dimensional (3D) reconstruction of the microstructure of the lotus-type porous car bon steel was made by using an image-based modeling to simulate the mechanical behavior. The stress-strain behavior of the 3D models of the lotus-type porous carbon steel was predicted by using finite element method (FEM) and was compared with the experimental results. Until the strain reaches 0.05, the compressive stress of the predicted model reaches the almost same value of the experimental result. The tensile stress also reaches approximately 0.9 times value of the experimental result. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2007.01.101

Publishing type：Research paper (scientific journal)  

We studied the effect of anisotropic pore morphology on the fatigue behavior and fracture surface of lotus-type porous copper, which was fabricated through unidirectional solidification in pressurized hydrogen and argon atmospheres. The fatigue strength at finite life is closely related to the pore morphology. The fatigue strength decreases with increasing porosity, and the strength depends on applied-stress direction. The fatigue life is the longest in the direction parallel to the longitudinal axis of cylindrical pores. The fatigue strength at finite life is proportional to the ultimate tensile strength and can be expressed by a simple power-law formula. Anisotropic pores affect the fracture surface of lotus copper; crack-initiation site depends on applied-stress direction, and the anisotropic shape pores affect the direction of crack propagation and final fracture surface.

DOI： 10.1557/JMR.2007.0164

Publishing type：Research paper (scientific journal)  

The stress shielding effect often degrades the quality and quantity of bone near implants. Thus, the shape and structure of metallic biomaterials should be optimally designed. A dominant inorganic substance in bone is biological apatite (BAp) nanocrystal, which basically crystallizes in an anisotropic hexagonal lattice. The BAp c-axis is parallel to elongated collagen fibers. Because the BAp orientation of bone is a possible parameter of bone quality near implants, we used a microbeam X-ray diffractometer system with a beam spot, which had a diameter of 50 mu m phi or 100 mu m phi, to evaluate it.
Two animal models were prepared: (1) a nail model (phi: 3.0 mm, SUS316L), which was used to understand the stress shielding effect in a rabbit tibial marrow cavity; and (2) a model of a lotus-type porous implant (phi: 3.4 mm, mean pore diameter: 170 pm, SUS304L), which was used to understand the effect of the unidirectional-elongated pore direction in anisotropic bone tissue of a beagle mandible. The porous implants were implanted so that the pore direction was parallel or perpendicular to the mesiodistal axis of the mandible.
For the porous implant model, new bone formation strongly depended on the elongated pore direction and the time after implantation. For example, four weeks after implantation, new bone formed in pores of the implants, but the BAp orientation degree in the new bone was more similar to that in the original bone in the elongated pores parallel to the mesiodistal direction than that in the perpendicular pores. These differences in bone formation inside the parallel and perpendicular pores may be closely related to the anisotropy of original bone tissue such as the orientations of collagen fiber, BAp, and blood vessels. The orientation degree of the BAp also changed in the nail model. The stress shielding effect decreased the orientation degree of the BAp c-axis in the tibia along the longitudinal axis.
Thus, the optimal design of metallic biomaterials, including such characteristics as implant shape, pore size, and elongated pore direction should be based on the anisotropy of the bone microstructure.

DOI： 10.2320/jinstmet.71.432

Publishing type：Research paper (scientific journal)  

We extend Qiu and Weng's mean-field approach for predicting yield behaviors of porous metals [J. Appl. Mech.-Trans. ASME 59, 261-268, 1992] in a more rigorous mathematical form. This extended method can explicitly take account of the pore morphology and the elastic and plastic anisotropies of metal matrix. The validity of our method is first demonstrated by applying to the lotus-type porous iron that exhibits the anisotropic yield behavior experimentally. We next consider the porous materials possessing elastically and plastically anisotropies, and carry out the calculations of their macroscopic yield behaviors. The calculations revealed that the yield stress is virtually independent of the elastic anisotropy of the matrix. but strongly depends on the plastic anisotropy and also on the pore morphology. (C) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.mechmat.2006.02.008

Publishing type：Research paper (scientific journal)  

Lotus-type porous metals with low thermal conductivity are fabricated by continuous zone melting technique, which possess directional elongated pores. The porous metals have been able to be fabricated through the conventional casting method by utilizing the solubility gap between solid and liquid in pressurized gas atmosphere. However, there is a shortcoming that the pores are coarsened in the part farther from the chill plate in the ingot. In order to overcome such a shortcoming, we developed the continuous zone melting technique and successfully produced the lotus-type porous metals with even low thermal conductivity such as stainless steel and superalloys. Furthermore, from the viewpoint of mass production with low cost, we invented novel "continuous casting technique". The molten metals dissolving gas are solidified continuously by passing through the mold cooled with chiller and thus, lotus-type porous metal plate as long as one meter was produced for short time. Sufficient uniformity of the porosity and pore size was obtained in such long porous ingots. This technique is prospective method for commercial mass production.

DOI： 10.4028/www.scientific.net/MSF.539-543.187

Publishing type：Research paper (scientific journal)  

J-GLOBAL

Publishing type：Research paper (scientific journal)  

We investigated the pore morphology in lotus-type porous copper fabricated by continuous casting technique as a function of transference velocity range from 1 to 100 mm.min(-1) under hydrogen gas pressure of 1.0 MPa. Lotus-type porous copper with long cylindrical pores aligned in one direction parallel to the transference direction was fabricated, which posses a sufficient uniformity of the porosity and pore size. The pores formed at transference velocity of 1 mm.min(-1) were larger than other condition. Necks were observed in these pores, whose formation may be attributed to bubbling in the melt. The pore size decreased with increasing transference velocity, while the porosity was not varied much by transference velocity.

DOI： 10.4028/www.scientific.net/SSP.124-126.1725

Publishing type：Research paper (scientific journal)  

A porous Ti-48.0at.%Al (Ti-rich TiAl) crystal, in which lotus-type long cylindrical pores were aligned and (gamma/alpha(2)) two-phase lamellar structure was simultaneously developed, was fabricated by floating zone method under the pressure of hydrogen and helium mixed gas. Plastic deformation behavior and microstructure of the Ti-rich TiAl crystal with lotus-type aligned pores were investigated by focusing on the elongated pore direction. The as-grown and annealed crystals show a well-developed lamellar structure and no texture accompanied by 52% porosity and a mean pore diameter of 380 mu m. Yield stress strongly depends on the loading direction against the elongated pore. When loading directions are parallel and perpendicular to the pore direction, yield stresses obey K=1 and 2.5, respectively, in equation of sigma=sigma(0)(1-P)(K), where a is the yield stress with pores, sigma(0) is the yield stress without pores and p is porosity. This reflects macroscopically homogeneous and locally heterogeneous plastic deformation between pores, respectively.

DOI： 10.4028/www.scientific.net/MSF.561-565.383

Publishing type：Research paper (scientific journal)  

We studied the fatigue strength of lotus-type porous magnesium with cylindrical pores aligned unidirectionally, which was fabricated through unidirectional solidification in pressurized hydrogen atmospheres. The fatigue strength shows anisotropy; the fatigue strength in the direction parallel to the longitudinal axis of pores is higher than that in the perpendicular direction. Not only anisotropic pores but also fiber texture grown along the pore direction contributes to the anisotropy in the fatigue strength.

DOI： 10.4028/www.scientific.net/MSF.561-565.1681

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Lotus-type porous NiAl and Ni3Al intermetallic compounds, possessing cylindrical pores aligned in the direction parallel to the solidification direction, were fabricated by using a unidirectional solidification technique in a pressurized hydrogen atmosphere of 2.5MPa. The porosity of lotus NiAl is 24.2%, and the porosity of lotus Ni3Al is 3.2%; the porosity of the porous NiAl is larger than that of Ni3Al. This is because the solubility gap of hydrogen between liquid and solid phases of NiAl is larger than that of N(i)3Al.

DOI： 10.4028/www.scientific.net/SSP.124-126.1721

Publishing type：Research paper (scientific journal)  

Lotus-type porous copper with long cylindrical pores aligned in one direction parallel to the solidification direction was fabricated by unidirectional solidification of the melt in a mixed gas of hydrogen and argon. Compression tests were performed in the direction parallel to the cylindrical pores in order to investigate the relationship between the specimen size and compressive yield strength. The compressive yield strengths and the standard deviation decrease with an increase in specimen size. Increments of the standard deviation are caused by the standard deviation of porosity occurred by inhomogeneous pore diameter and irregular pore arrangement.

DOI： 10.2320/matertrans.47.2208

Publishing type：Research paper (scientific journal)  

DOI： 10.2320/matertrans.47.2116

Publishing type：Research paper (scientific journal)  

Effects of specimen thickness on the tensile strength of lotus-type porous copper were investigated. The ultimate tensile strength in the direction perpendicular to the longitudinal axis of pores hardly depends on the thickness of a specimen when the width of the specimen is large enough compared with the pore diameter, while the ultimate tensile strength increases with an increase in the thickness when the width is not large enough compared with the pore diameter. The 0.2% offset strength in the direction and strain at the peak stress depend on the thickness of specimens; the 0.2% offset strength decreases with an increase in the thickness, and the strain at peak stress increases with the increase in the thickness.

DOI： 10.2320/matertrans.47.2203

Publishing type：Research paper (scientific journal)  

The geometrical effective thermal conductivity of different porous materials is investigated based on two different approaches: the finite element method as a representative for numerical approximation methods and the mean-field method as a representative for analytical methods. Different pore geometries, i.e., cylindrical and spherical, and different arrangements of these pores, namely in different periodic ways and random distribution are considered. It is found that the relative conductivity is practically independent of the specific pore arrangement. Only the morphology (spherical or cylindrical) of the structure influences the conductivity. (c) 2006 Elsevier B.V All rights reserved.

DOI： 10.1016/j.matlet.2006.01.067

Publishing type：Research paper (scientific journal)  

We studied the influence of agitation by ultrasonic vibration on pore formation and growth during the unidirectional solidification of watercarbon dioxide solution. The agitation of liquid affects the pore formation and growth, and the morphology of pores formed during solidification depends on the magnitude of agitation; the agitation ofliquid during unidirectional solidification shortens the length of cylindrical pores in lotus metals. This is because the agitation decreases the concentration of carbon dioxide near the solid-liquid interface.

DOI： 10.2320/matertrans.47.2183

Publishing type：Research paper (scientific journal)  

The effective-mean-field (EMF) theory, consisting of Mori-Tanaka's mean-field theory and Bruggeman's effective medium approximation. was extended in order to calculate the coefficients of thermal expansion (CTE) of composite materials. The effective elastic constants and CTE of lotus-type porous metals, possessing cylindrical pores aligned unidirectionally, were evaluated with the EMF theory. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.scriptamat.2005.10.049

Publishing type：Research paper (scientific journal)  

Lotus-type porous magnesium with cylindrical pores aligned unidirectionally was fabricated by the unidirectional solidification of molten magnesium under pressurized hydrogen atmosphere. The apparent attenuation coefficient of the free vibration of lotus-type porous magnesium was measured by hammering-vibration-damping tests, which revealed that the apparent attenuation coefficient increases with increase in porosity, i.e., the damping capacity of lotus magnesium is higher than that of non-porous magnesium. The mechanism of high damping capacity was analyzed by using the Fourier transform technique, which revealed that various vibration modes of high frequency are excited by a hammering. The excited vibrations of high frequency enhance the damping capacity of lotus-type porous magnesium. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2005.10.061

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Tension and compression fatigue property was investigated for lotus-type porous copper possessing cylindrical pores aligned in one direction. The cyclic stress was applied in the direction parallel and perpendicular to the longitudinal axis of the pores. It was found that the fatigue strength at finite life of lotus-type porous copper is lower than that of nonporous copper, and the strength in the direction parallel to the longitudinal axis of the pores is higher than that in the perpendicular direction.

DOI： 10.4028/www.scientific.net/MSF.510-511.966

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.1897069

Publishing type：Research paper (scientific journal)  

Anisotropic yield behavior of lotus-type porous iron fabricated using the continuous-zone-melting method in a pressurized nitrogen and hydrogen atmosphere has been investigated. The 0.2% offset strength (compressive yield stress) in the loading direction parallel to the longitudinal axis of pores decreases linearly with increasing porosity, while the perpendicular strength decreases steeply. The strength versus porosity curves can be expressed using a well-known power law formula. In addition, the 0.2% offset strength of lotus iron prepared in a nitrogen and hydrogen atmosphere is found to be larger than that of lotus iron prepared in a hydrogen and helium atmosphere, which is attributed to the solid solution hardening by the solute nitrogen. Furthermore, we compare the experimental results to calculations obtained by means of the extended Qiu-Weng's mean-field method, and the comparison suggests that local stresses deviated from the average stress are dominant in the macroscopic yield behavior of lotus metals.

DOI： 10.1557/JMR.2005.0009

Publishing type：Research paper (scientific journal)  

We studied the plastic behavior of lotus-type porous iron with unidirectional long cylindrical pores. Lotus-type porous iron with different porosities was fabricated by the continuous zone melting method in a pressurized hydrogen and helium atmosphere. To calculate the stress-strain curves for lotus iron, we applied a modified Qiu-Weng's micromechanical mean-field theory that has recently been proposed by the present authors [J. Mater. Res., in press], and compared the results with those of compression tests. We experimentally found that the deformation resistance and work hardening rate depend on the sample porosity and loading direction. They decrease with an increase in porosity, and their values in the loading along the direction perpendicular to the longitudinal axis of pores are smaller than those in the parallel-direction loading. Our micromechanical calculations reproduce well the stress-strain curves experimentally obtained and express the experimental trends successfully.

DOI： 10.4028/www.scientific.net/MSF.486-487.489

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.1776316

Publishing type：Research paper (scientific journal)  

We studied the elastic properties of lotus-type porous iron experimentally and theoretically. First we determined the elastic constants of lotus iron fabricated by the continuous zone-melting method by using the acoustic resonance techniques. All the elastic moduli are found to follow the well-known power-law formula. Next, we extended the effective-mean-field (EMF) theory so as to consider effects of the pore orientation on the effective elastic constants. The model calculations proved that the extended EMF theory is capable of calculating satisfactorily the elastic properties of lotus metals. (C) 2004 Acta Materialia, Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2004.07.030

Publishing type：Research paper (scientific journal)  

The effective-mean-field method, an evaluation method for macroscopic elastic constants of composites, was discussed. It was found that the method was based on Mori-Tanaka's mean-field (MTMF) and the effective-medium approximation methods were replaced by the average fields of a virtually homogenized composite. It was observed that the effective elastic constants of the lotus-type porous copper were calculated. Analysis shows that for porous samples the method reproduced power-law behavior with regard to porosity.

DOI： 10.1063/1.1769587

Publishing type：Research paper (scientific journal)  

We studied temperature dependence of anisotropic elastic constants of lotus-type porous copper that consists of unidirectionally aligned pores and the matrix of oriented crystallites. All the independent elastic constants were measured from room temperature to 400 degreesC using an electromagnetic acoustic resonance method. All the elastic constants decrease with increase of temperature. The porosity and oriented crystallites have the influence on the temperature dependence of the elastic constants. Micromechanics theory allows calculation of the temperature dependence of the elastic constants of lotus copper from those of monocrystal copper and shows agreement with the measurements. It is concluded that micromechanics model is then valid to predict the temperature dependence of the elastic constants of the lotus-type porous metals. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.matlet.2003.11.013

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

We measured the anisotropic elastic constants of a lotus-type porous magnesium, which possesses cylindrical pores aligned unidirectionally. We used the combination of resonant ultrasound spectroscopy and electromagnetic acoustic resonance methods to determine a complete set of the elastic constants. The acoustic measurements revealed that lotus magnesium shows hexagonal elastic symmetry with the c-axis along the longitudinal direction of the pores. Young's modulus E-parallel to parallel to the pore direction decreases linearly with porosity, while the normal Young's modulus E-perpendicular to drops rapidly.

DOI： 10.2320/jinstmet1952.67.9_417

Publishing type：Research paper (scientific journal)  

We have measured the elastic constants of duplex stainless steel, JIS-SCS14A (MM), aged isothermally at 400 degreesC up to 10,000 h, using resonant ultrasound spectroscopy method. The elastic constant c(11) (= lambda + 2mu) increases monotonically with the aging time, but C-44 (= mu) remains virtually unchanged. Using a micromechanics model, we have deduced the elastic constants of alpha-phase on the assumption that those of gamma-phase are unchanged with the aging. The increase of the alpha-phase elastic constants reflects the spinodal decomposition of alpha-phase. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S1359-6462(02)00388-3

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

We studied the elastic constants of unidirectional (lotus-structured) porous copper, regarding it as a composite material showing hexagonal elastic symmetry with the c-axis along the longitudinal direction of the pores. We used the combination of resonance ultrasound spectroscopy and electromagnetic acoustic resonance methods to determine five independent elastic constants c(ij) of the composite. The resulting Young's modulus E-// decreases linearly with porosity, while E-L drops rapidly and then slowly: they are given by E-// = 73.1x(1-p)(1.14) and E-perpendicular to == 96.6x(1-p)(2.26) GPa for porosity p, respectively.

Publishing type：Research paper (scientific journal)  

We have studied the anisotropic elastic constants of crept copper with the oblate ellipsoidal pores and lotus-type porous copper with long straight pores aligned unidirectionally. We used the combination of resonant ultrasound spectroscopy and electromagnetic acoustic resonance methods to determine all the independent elastic constants with high accuracy. The elastic anisotropy depends on porosity, pore shape, pore orientation, and matrix's texture. We have calculated the elastic constants by a micromechanics theory, assuming that the pores are ellipsoidal inclusions of zero elasticity. We have obtained good agreement between micromechanics calculations and measurements.

DOI： 10.2320/jinstmet1952.66.11_1073

Publishing type：Research paper (scientific journal)  

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Formation of curious deformation bands has been reported as one of the deformation mechanisms occurring in a Mg-based long period stacking ordered (LPSO) phase. The origin of the deformation band is still unknown, and the possibility of the deformation kink band and/or the deformation twin has been discussed. To clarify this, the crystallographic nature of deformation bands formed in the LPSO phase was examined by scanning electron microscope-electron backscatter diffraction (SEM-EBSD) pattern analysis. The deformation band in the LPSO phase was found to show three arbitrariness on its crystallographic nature: an ambiguous crystal rotation axis that varied on the [0001] zone axis from band to band; an arbitral crystal rotation angle that was not fixed and showed relatively wide distributions; and a variation in crystal rotation angle depending on the position even within a deformation band boundary itself. These features were coincident with those observed in the deformation bands formed in Zn polycrystals, suggesting that the formed deformation bands in LPSO phase crystals are mainly deformation kink bands.

DOI： 10.1299/jsmemm.2014._OS0101-1_

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.2320/materia.51.95

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.2320/materia.47.196

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Lotus-type porous magnesium was fabricated through the use of hydrogen thermally decomposed from MgH(2) powders during unidirectional solidification in argon gas atmosphere. Liquid magnesium was cast into a mold in which MgH(2) powders were placed, and it was unidirectionally solidified, which achieved the growth of pores elongated along the solidification direction. The fabrication method is simpler than the conventional method where lotus metals are produced in pressurized hydrogen atmosphere. The porosity of lotus magnesium fabricated with MgH(2) powders increases with increasing amount of MgH(2) powders. The porosity and pore size change with advancing solidification (pore growth), and the rate of the change depends on the amount of the MgH(2) powders.

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.2320/materia.46.836

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.2320/materia.46.70

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Presentation type：Oral presentation (invited, special)  

Venue：Rome, Italia  

Presentation type：Oral presentation (invited, special)  

Venue：PACIFICO YOKOHAMA  

Presentation type：Oral presentation (invited, special)  

Venue：京都テルサ(オンライン)  

Presentation type：Oral presentation (invited, special)  

Venue：オンライン開催  

Presentation type：Oral presentation (general)  

Venue：名古屋⼤学 東⼭キャンパス(オンライン)  

Presentation type：Public lecture, seminar, tutorial, course, or other speech  

Venue：大阪市立大学(オンライン)