Masakazu Tane

Anisotropic elastic constants of lotus-type porous copper: measurements and micromechanics modeling

Abstract We studied the elastic constants of a lotus-type porous copper, regarding it as a composite material showing hexagonal elastic symmetry with the c-axis along the longitudinal direction of the pores. We used the combination of resonance ultrasound spectroscopy and electromagnetic acoustic resonance methods to determine the elastic constants of the composite. The resulting Young’s modulus E∥ decreases linearly and c33 does slowly with porosity, while E⊥ and c11 drop rapidly and then slowly. Micromechanics calculations considering the elastic anisotropy of the copper matrix can reproduce the measured anisotropic elastic constants. This indicates that the elastic properties of various types of porous metals can be predicted and designed with the present approach using micromechanics modeling.

Anisotropic yield behavior of lotus-type porous iron : Measurements and micromechanical mean-field analysis

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.

Diffusion of oxygen in amorphous Al2O3, Ta2O5, and Nb2O5

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

Anisotropic electrical conductivity of lotus-type porous nickel

We studied the anisotropic electrical conductivity of lotus-type porous nickel, with cylindrical pores aligned unidirectionally. We measured the electrical conductivities in the directions parallel and perpendicular to the longitudinal axis of the pore with four-probe method. The electrical conductivity of lotus nickel shows the anisotropy that reflects the anisotropic porous structure and can be summarized with a well-known power-law formula (Archie’s law); the conductivity in the direction parallel to the pore decreases linearly with increase in porosity, and that in the perpendicular direction decreases steeply with porosity increase, in agreement with Archie’s power-law formula. Furthermore, we constructed the effective-mean-field (EMF) theory to predict the effective electrical conductivity of composites. The electrical conductivity of lotus nickel, evaluated by this theory, is consistent with measurement data, and this EMF theory can fully simulate Archie’s power-law formula.

Effective-mean-field approach for macroscopic elastic constantsof composites

We propose an evaluation method for macroscopic elastic constants of composites, which is called the effective-mean-field method. The method is based on Mori–Tanaka’s mean-field (MTMF), and the effective-medium approximation methods, in which complex elastic fields disturbed by many inclusions are replaced by the average fields of a virtually homogenized composite, and the MTMF formulae are utilized in the entire fraction range of inclusion. It is demonstrated for porous samples that the proposed method can reproduce power-law behavior with regard to porosity.

Elastic constants of lotus-type porous magnesium: Comparison with effective-mean-field theory

Lotus-type porous (LTP) metals possess a markedly anisotropic porous structure, in which long straight pores are homogeneously aligned unidirectionally. In this paper, we first describe a procedure for the determination of reliable elastic constants of LTP metals with high porosity, and next apply the technique to LTP magnesium, and finally compare the measurement results with those from the effective-mean-field (EMF) theory that has been recently proposed by Tane and Ichitsubo [Appl. Phys. Lett. 85, 197 (2004)]. Young’s modulus in the direction parallel to the longitudinal axis of pores decreases virtually linearly with porosity p, i.e., E‖≈45.3(1−p)1.33, whereas the normal Young’s modulus E⊥, falls rapidly, i.e., E⊥≈45.3(1−p)2.64. The Mori-Tanaka mean-field theory yields values that are close to the measured elastic constants in a wide porosity range, but cannot explain this power-law behavior. In contrast, the EMF theory gives more accurate values in the high-porosity range, and can reproduce the power...

Fabrication of Lotus-type Porous NiTi Shape Memory Alloys using the Continuous Zone Melting Method and Tensile Property

Lotus-type porous NiTi alloys with unidirectionally elongated cylindrical pores were fabricated using the continuous zone melting method in a high-pressure mixture of hydrogen and helium gases. The lotus-type porous NiTi alloy with elongated pores was successfully fabricated under a hydrogen partial pressure less than 1.75 MPa. However, under a hydrogen partial pressure greater than 2.0 MPa, the pores formed during solidification rapidly combined and finally did not grow homogenously. Since the increase in the pore volume due to pore combination reduced the area of the solidliquid interface where supersaturated hydrogen was released from the solid, hydrogen was not provided in sufficient amounts to the pores. The stress-strain curves of porous and nonporous NiTi with tensile direction parallel to the pore orientation exhibit similar tendency while the tensile strength decreases with increase of the porosity.

Elastic property of aged duplex stainless steel

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.

Tensile deformation of anisotropic porous copper with directional 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 Poissons 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.

Crystallographic nature of deformation bands shown in Zn and Mg-based long-period stacking ordered (LPSO) 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 from band to band; an arbitral crystal rotation angle that was not fixed and showed relatively wide distributions; and a variation in crystal rotation angle depending on the position even within a deformation band boundary itself. These features were coincident with those observed in the deformation bands formed in Zn polycrystals, suggesting that the formed deformation bands in LPSO phase crystals are predominantly deformation kink bands.