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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 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.

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 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 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 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.

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.