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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.

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.

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.

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 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.

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 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 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.

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.

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.

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.

We studied fatigue crack initiation and propagation in lotus-type porous copper with cylindrical pores aligned in one direction. For fatigue loadings in the direction parallel to the longitudinal axis of pores, stress field in the matrix is homogeneous. Therefore, slip bands are formed all over the specimen surface. On the other hand, for the perpendicular loadings, slip bands are formed only around pores in which stress highly concentrates. Since the localized slip bands form fatigue crack, fatigue fracture occurs even when the total plastic strain range is small. Stress field in the matrix of lotus copper affects the direction of crack propagation. For the parallel loadings, a crack propagates along a straight line as well as nonporous copper. On the other hand, for the perpendicular loading, a crack propagates along a path in which stress highly concentrates. Since stress highly concentrates around anomalously large pores, fatigue cracks are preferentially formed around the large pores and cracks propagate by crossing the large pores.

We studied the effects of anisotropic pores and fiber texture on the fatigue strength and fracture surface of lotus-type porous magnesium fabricated through unidirectional solidification in pressurized hydrogen and argon atmospheres. 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. The fatigue strength at finite life of lotus magnesium is closely related to the ultimate tensile strength; the fatigue strength is proportional to the ultimate tensile strength for both loadings parallel and perpendicular to the pore direction. The fracture surface of lotus magnesium is not flat, which originates from porous structure. For parallel loading, fiber texture in lotus magnesium also contributes to the irregular surface, i.e., a combination of texture and pore structure affects fracture surfaces.

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.

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 carbon 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.

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.

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 Ni3Al.

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.

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.

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.

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.

Lotus-type porous iron and stainless steel (SUS304L) whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized hydrogen or nitrogen gas atmosphere. Pores are formed as a result of precipitation from the supersaturated gases when the liquid metal dissolved with gas atoms is solidified. The ultimate tensile and yield strengths of the porous iron produced in nitrogen atmosphere are about two times higher than in a hydrogen atmosphere. Such superior strength is attributed to solid-solution hardening due to solute nitrogen atoms in iron matrix.

Lotus-type porous materials whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification from the melt dissolving gas. The pores were evolved by supersaturation of gas atoms when the liquid was solidified. Although such porous metals with high thermal conductivity were produced by casting process, the process could not produce porous metals with low thermal conductivity, which possess uniform pore size and porosity. In order to obtain uniform pore size and porosity in metals with low thermal conductivity, we invented a new “continuous zone melting technique”. Using this technique various metals, alloys, intermetallic compounds and semiconductors were fabricated. Mechanical properties of tensile strength on lotus-type porous metals are described; lotus-type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength. The lotus materials can be applied to various functional materials. Some examples are also shown in this paper.

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.

Lotus-type porous magnesium with a large number of unidirectional cylindrical pores was fabricated by unidirectional solidification of melt dissolving hydrogen in a pressurized hydrogen atmosphere. The vibration-damping capacity of the lotus-type porous magnesium plate which has many open pores was measured in this work. The attenuation coefficients of the free vibration of lotus-type porous magnesium were measured by hammering-vibration-damping test, which revealed that the attenuation coefficients increase with increase in porosity; the damping capacity of lotus magnesium is higher than that of non-porous magnesium. The mechanism for high damping capacity was analyzed on the basis of the Fourier transform technique, which indicates that various vibration modes of high frequency are observed. The excited vibrations of high frequency enhance the damping capacity of lotus-type porous magnesium.

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.

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.

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.

The stress shielding effect often degrades the quality and quantity of bones near implants. Thus, the shape and structure of metallic biomaterials should be optimally designed. A dominant inorganic substance in bones is a 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 bones 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 or 100 mm� , to evaluate the BAp orientation of bones. Two animal models were prepared: (1) a nail model (� : 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 (� : 3.4 mm, mean pore diameter: 170 mm, SUS304L), which was used to understand the effect of the unidirectional-elongated pore direction in an 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 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 tissues such as the orientations of collagen fiber, BAp, and blood vessels. The orientation degree of 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, optimal design of metallic biomaterials such as implant shape, pore size, elongated pore direction, etc., should be based on the anisotropy of the bone microstructure. [doi:10.2320/matertrans.47.2233]

Ni3Al with and without boron was melted and unidirectionally solidified in pressurized hydrogen atmosphere by using a continuous zone melting method. For Ni3Al without boron, cylindrical pores elongated in the direction parallel to the solidification direction, are formed in Ni3Al matrix. On the other hand, no pore is formed in Ni3Al with 0.23 mol% B solidified under the same condition. This result suggests that the solubility gap of hydrogen between liquid and solid Ni3Al with boron is extremaly small.

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.

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.

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.

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 °C 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.

We have measured the elastic constants of duplex stainless steel, JIS-SCS14A (CF8M), aged isothermally at 400 C up to 10,000 h, using resonant ultrasound spectroscopy method. The elastic constant c11 (¼ k þ 2l) increases monotonically with the aging time, but c44 (¼ l) remains virtually unchanged. Using a micromechanics model, we have deduced the elastic constants of a-phase on the assumption that those of c-phase are unchanged with the aging. The increase of the a-phase elastic constants reflects the spinodal decomposition of a-phase. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.