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The elastic properties and room-temperature aging behaviors of body-centered cubic Mg–40Li and Mg–40Li–5Al (at.%) alloy single crystals, developed for biodegradable orthopedic implants, were investigated by measuring their single-crystalline elastic constants. It was found that the Young's moduli of Mg–40Li and Mg–40Li–5Al single crystals strongly depend on the crystallographic direction, and the Young's modulus along the 〈100〉 direction is approximately 21 GPa, which is almost the same as that of natural bones. More importantly, we found that the Young's moduli along 100 > in the Mg–40Li and Mg–40Li–5Al alloys changed during aging near room temperature, which may have been caused by the precipitation or dissolution of the θ -phase MgAlLi2 or hcp α-phase.

The elastic properties of a single-crystalline Ti3SiC2 MAX phase with a nanolayered crystal structure, comprising Ti-Si and two distinct Ti-C bonding layers, that had remained unclear because of the difficulty in growing large single crystals, were studied. Rather than unavailable large single crystals, polycrystalline samples with a crystallographic texture were prepared. By analyzing the polycrystalline elastic constants on the basis of an inverse Voigt–Reuss–Hill approximation, the elastic properties of a single crystal Ti3SiC2 with a hexagonal symmetry were determined. This revealed that the single-crystalline Young's modulus was almost isotropic despite its highly anisotropic layered structure. The shear modulus for (0001) 〈 11 2 ¯ 0 〉 was higher than that for {11 2 ¯ 0}[0001] in contrast to the basal slip-dominated plastic deformation reflecting the layered structure. Furthermore, first-principles calculations revealed that heterogeneous interlayer elastic deformation caused by the stabilization of Ti-Si bonding is the origin of the elastic isotropy in a Ti3SiC2 MAX phase.

The electrodeposition of tungsten at moderate temperatures (4 and WCl5 in 1-ethyl-3-methylimidazolium chloride (EMIC) and EMIC–AlCl3 ionic liquids at 80 °C–120 °C. W-rich films with a thickness of approximately 1 μm were obtained from the Lewis acidic EMIC–AlCl3–WCl5 bath, whereas the other baths did not yield any deposits. The films obtained from the EMIC–AlCl3–WCl5 bath at 80 °C had higher W contents of 54 at.% than those obtained at 120 °C. X-ray absorption near-edge structure spectra of the W-rich films indicated that W existed in an oxidized state. The findings of this study can be used as a guide to explore optimal electrolytes and electrolytic conditions for the electrodeposition of metallic W at moderate temperatures.

We have investigated the effects of solute oxygen on the kinetics of diffusionless isothermal omega (DI-ω) transformation in β-titanium vanadium alloys. This transformation constitutes a third category of ω transformation beside the athermal and isothermal modes. Thermal analysis, hardness and internal friction measurements were conducted after quenching oxygen-containing and near-oxygen-free alloys with ~ 21 at%V from the β-stable temperature. At this level of vanadium concentration, the athermal ω transformation is not expected. It is found that the DI-ω transformation more rapidly progresses in the low oxygen alloy and the relaxation strength of the elementary process of {111}β collapse is significantly reduced.

Nanocomposite microstructures dominating the anisotropic elastic properties in carbon fibers were studied, to construct a micromechanics model that can explain the anisotropic elastic properties of carbon fibers. Aluminum-based composites containing five types of carbon fibers were prepared, and their anisotropic elastic properties were measured using resonant ultrasound spectroscopy combined with electromagnetic acoustic resonance. Then, all the independent elastic stiffness components of the carbon fibers were extracted from those of the composites using a composite model based on Eshelby's inclusion theory, Mori–Tanaka mean-field theory, and effective-medium approximation. Moreover, we newly developed a nanocomposite microstructure model that can fully reproduce all the anisotropic constants of carbon fibers exhibiting a wide variety of Young's moduli. In the developed model, the microstructures of carbon fibers were approximated as nanocomposites comprised of an amorphous carbon matrix and graphite-crystal inclusions that are aggregations of graphite nanocrystallites. Based on this nanocomposite microstructure model, the shape of the graphite-crystal inclusions and the elastic properties of the amorphous carbon matrix were analyzed, considering the volume fraction and orientation of the graphite nanocrystallites, as determined using X-ray diffraction. The analysis revealed that the shapes of the graphite-crystal inclusions are flat ellipsoids elongated along the fiber axis, and the aspect ratio of the graphite-crystal inclusions dominantly affects the anisotropy in the Young's modulus. This indicates that the graphite nanocrystallites are connected along the in-plane directions of the graphitic layers, and not the shape of nanocrystallites but their two-dimensional connectivity dominates the anisotropic elastic modulus in carbon fibers.

Diffusionless isothermal omega ($\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$) transformation, which was recently defined in Phys. Rev. Materials 3, 043604 (2019), is classified into a third category of the $\ensuremath{\omega}$ transformation modes, other than the well-known isothermal and athermal modes. This work reveals the characteristic features of the $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformation in the $\ensuremath{\beta}$-titanium vanadium alloy system, specifically, focusing on variations in the microstructure and mechanical properties with the proceeding of the $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformation. After quenching alloys of Ti-15at%V, Ti-21at%V, and Ti-27at%V down to below room temperature from the $\ensuremath{\beta}$-stable temperature, in addition to the occurrence of the athermal $\ensuremath{\omega}$ transformation for Ti-15at%V, all of the alloys gradually undergo the $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformation upon aging at a temperature as low as 373 K, leading to a moderate increase in the hardness. The degree of the hardness increase in these alloys can be successfully explained by a local instability concept based on quenched-in thermal concentration fluctuations. It is also shown that internal friction (Ti-21at%V) diminishes after the low-temperature aging, which indicates the annihilation of such unstable regions showing a dynamic collapse of ${{111}}_{\ensuremath{\beta}}$ pairs to form a transient $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ phase. Comparison of inelastic x-ray scattering and ultrasound measurements can see a trail of the $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformation remaining even in the Ti-27at%V alloy, which has no obvious athermal omega transformation temperature. Based on the results, the difference between athermal $\ensuremath{\omega}$ and $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformations is finally discussed.

Elastic-modulus mismatch and the resultant stress partitioning in two-phase Mg–Zn–Y alloys comprised of α -Mg and long-period stacking ordered (LPSO) phases were studied. Two-phase polycrystals containing anisotropically oriented 18R- or 14H-type LPSO phase and single-phase polycrystals consisting of α -Mg or 18R-type LPSO phase were prepared by extrusion and directional solidification processes and their complete sets of anisotropic elastic properties were measured using resonant ultrasound spectroscopy. Elastic properties of the single and two-phase alloys were analyzed using Eshelby's inclusion theory, effective-medium approximation, and inverse Voigt-Reuss-Hill approximation, in which the crystallographic textures and microstructures formed by the preparation processes were taken into account. The analyses revealed that the elastic properties of 18R-LPSO phase were not unique and they depended on the solute Zn and Y atom concentrations. Additionally, the elastic modulus of 18R-LPSO phase embedded in the two-phase alloy was lower than that of the alloy consisting of single-phase 18R-LPSO phase. The analysis using first-principles calculations based on density functional theory indicated that the low elastic modulus was caused by low density and low stability of short-range ordered solute atom clusters embedded in the LPSO phase of the two-phase alloy. Because of low elastic modulus in the LPSO phase, the elastic mismatch and resultant elastic interaction between the α -Mg and LPSO phases were very small. As a result, the formation of LPSO phase had little effect on the stress partitioning to the LPSO phase, which was independent of the LPSO-phase morphology.

Tane M., Nishiyama H., Umeda A., et al. Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations. Physical Review Materials 3, 043604 (2019); https://doi.org/10.1103/PhysRevMaterials.3.043604., In titanium alloys, the ω(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 ω transformation that can occur even above the ω transformation temperature. In body-centered cubic β-titanium alloyed with β-stabilizing elements, there are locally unstable regions having fewer β-stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal ω transformation occurs even when the entire β region is stable on average so that athermal ω transformation cannot occur. This anomalous, localized transformation originates from the fluctuation-driven localized softening of 2/3[111]β longitudinal phonon, which cannot be suppressed by the stabilization of β phase on average. In the diffusionless isothermal and athermal ω transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of {111}β pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the ω-phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent β/ω 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 β/ω interface energy is the rate-determining process in the diffusionless ω transformations.

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 c 44 were enhanced by the largest amount. The increase rates were ∼5% for 1.1 × 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 c ′ and c 44 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 ω (hexagonal) phase formation. However, transmission electron microscopy (TEM) observations revealed that only a diffuse ω structure—an ω -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 ω structure.

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., Nanoporous gold (NPG) has gained significant attention, but its catalytically active structure has not yet been clarified. Here, the authors identify the catalytically active and dynamic structure in NPG by combining atomic-scale and microsecond-resolution environmental transmission electron microscopy with ab initio calculations.

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.

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 {1 0 1 ¯ 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 ∼60% strain despite the presence of ∼42% porosity, whereas it showed high energy absorption of ∼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 finite-element 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.

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 α-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.

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 [0 0 0 1] zone axis f...

Dynamic and quasi-static compressions of porous copper and iron with cylindrical unidirectional pores were investigated at 298 and 77 K, with a focus on the effect of the interaction between the cylindrical pores and the deformation twins on the appearance of a plateau stress region in the stress–strain curves where the deformation proceeds at almost constant stress. The dynamic and quasi-static compressions were examined using the split Hopkinson pressure bar method and a universal testing machine, respectively. When porous copper undergoes dynamic (298 and 77 K) and quasi-static (298 K) compressions parallel to its cylindrical pores, the flow stresses increase monotonically with an increase in the strain. This is due to a homogeneous slip deformation originating from the high mobilities of the dislocations. However, when porous iron undergoes dynamic compression parallel to its pores at 298 K, macroscopically localized deformation bands analogous to bucking deformation occur, owing to the interaction between the oriented cylindrical pores and the deformation twins. This interaction acts as a trigger for the deformation band formation. As a result, the work hardening rate decreases drastically. Thus, in the case of dynamic parallel compression of porous iron at 298 K, a plateau stress region with a high stress (~300 MPa) and a wide strain range (of up to ~45%) appears. Because of this, the absorption of high-impact energy six times that can be absorbed by commercial aluminum foam with isotropic pores is achieved.

The elastic properties of single-crystalline ω (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 ω phase have remained unclear because of the difficulty in preparing a large single crystal consisting of a single phase of the ω phase, even though the ω phase has been believed to exhibit a higher elastic modulus than the β (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 ω phase, which is metastable at room temperature. For the ω-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 ω-phase polycrystal on the basis of an inverse Voigt–Reuss–Hill approximation, the elastic stiffness components of the single-crystalline ω phase were determined. The Young’s modulus of the ω phase along 〈0001〉 was found to be clearly higher than that along 〈 1 1 2 ¯ 0 〉 , and the shear modulus also exhibited anisotropy. Importantly, the Young’s modulus and shear modulus of the metastable ω phase were higher than those of the β phase and also higher than those of the α (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 ω phase formation on the elastic properties of β-phase titanium alloys.

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 〈0 0 0 1〉 in the hexagonal expression was clearly higher than that along 〈 1 1 2 ¯ 0 〉 , 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.

Porous Ti–xAl–8Nb (x = 45, 48, and 49 at%) 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 = 45 at%, 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 = 49 at%, 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.

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.

The ω transformation and its correlation with elastic properties were investigated in cold-worked Ti–36Nb–2Ta–3Zr–xO mass% alloys with low body-centered cubic (β) phase stability, known as gum metal. Analysis of the temperature dependence of the ω (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 ω-phase formation from room temperature to a fairly low temperature of ∼13 K. However, the ω phase was formed by cold working at room temperature in the 0.30% and 0.47% O alloys. Importantly, the fraction of the ω phase clearly increased upon cooling, which indicates that the formation of the ω phase is thermodynamically favorable near and below room temperature in the cold-worked 0.30% and 0.47% O alloys. This formation of the ω phase and the low stability of the β 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.

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 γ(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.

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 π/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.

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.

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.

The origin of the low Young’s modulus of cold worked Ti–36Nb–2Ta–3Zr–xO mass% polycrystals with a body-centered cubic (β-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 c44 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 α″ martensitic transformation by oxygen addition led to retention of the low stability single β-phase state at RT. The Hill approximation indicated that the low c′ and c44 values caused by softening gave rise to the low Young’s modulus, which is common to some Ti–Nb-based alloys with an e/a ratio of ∼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 α″ martensite phase (0.09% O) and ω phase (0.09, 0.30, and 0.47% O) with a high elastic modulus in the β-phase matrix. However, increasing the oxygen concentration suppresses the increase in Young’s modulus because oxygen addition decreases the amount of α″ and ω phases formed while retaining the low stability β phase. Therefore, cold working combined with oxygen addition produces a low Young’s modulus compatible with high strength.

Lotus-type porous aluminum with cylindrical pores oriented in one direction was deformed by Equal Channel Angular Extrusion (ECAE) through a 150° die with sequential 180° 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.

The formation mechanism of a high density of nanovoids by annealing amorphous Al 2 O 3 thin films prepared by an electron beam deposition method was investigated. Transmission electron microscopy observations revealed that nanovoids ∼1–2 nm in size were formed by annealing amorphous Al 2 O 3 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 AlO 6 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 Al 2 O 3 amorphous film consists of unstable low-density regions containing a low fraction of stable AlO 6 units and stable high-density regions containing a high fraction of stable AlO 6 units. Thus, local density growth in the unstable low-density regions during annealing leads to nanovoid formation (i.e., local volume shrinkage).

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.

The composition and temperature dependence of the elastic properties and phase stability of quaternary Ti–Nb–Ta–Zr β-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 α ″ martensitic transformation occurred in a temperature range that depended on the β-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 β-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.

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.

Numerical calculations for porous magnesium with cylindrical oriented pores showed significant influences of active deformation modes in magnesium (Mg) on distinctive "two-stage deformation behavior" during compression. Calculated results also imply that larger grain aspect ratio results in better energy absorption efficiency.

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.

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.

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.

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.

The strain rate dependence of anisotropic compression behavior in porous iron with cylindrical pores oriented in one direction was investigated. Through high strain rate (˜103 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 (˜10−1 s−1) and low strain rates (˜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.

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 Al 3 Ti alloys are formed. In the case of a high transfer velocity (10.0mm·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.

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.

The cause of a low Young’s modulus was investigated in quaternary β-type Ti–Nb–Ta–Zr alloys, as the modulus is decreased to prevent bone absorption and degradation of bone quality when these alloys are implanted into human bones. This investigation was carried out using the alloys′ single crystals. Acoustic measurements and analysis by the Hill approximation revealed that a low Young’s modulus in a polycrystalline form is caused by the low shear modulus c′, related to the low β-phase stability, low c44, and relatively low bulk modulus B compared with those of binary Ti-based alloys. Furthermore, it was found that the single crystals had strong orientation dependence on Young’s modulus, where that in the 〈1 0 0〉-direction E100 is the lowest of all crystallographic orientations. For quaternary Ti–29Nb–13Ta–4.6Zr alloy (mass%), E100 is only ∼35 GPa, which is similar to Young’s modulus of human cortical bones as a result of the low B and c′. These results indicate that decreases in c′, c44 and B are essential for decreasing Young’s modulus of novel β-type Ti alloys which are expected to be developed in the near future.

Porous magnesium with directional cylindrical pores (or “lotus-type” porous magnesium) was fabricated through the use of hydrogen decomposed from MgH2 powders during unidirectional solidification. Liquid magnesium was cast into a mold in which MgH2 powders were placed and was unidirectionally solidified, which achieved growth of pores elongated along the direction of solidification. The effect of the amount of the MgH2 powders on the pore structure (porosity, diameter, and number density of pores) and the change in the pore structure along the pore growth direction were clarified. The porosity and number density of pores increase with increasing amount of MgH2 powder, and the average diameter of pores decreases with increasing amount of MgH2 powder. The pore structure changes with the growth of pores along the solidification direction.