Koichi Kitazono

Application of mean-field approximation to elastic-plastic behavior for closed-cell metal foams

Abstract Elastic–plastic properties of closed-cell metal foams were derived using continuum micromechanical model (the equivalent inclusion method and the mean-field approximation). Assuming a hydrostatic internal stress in the cell, the effect of the gas pressure was incorporated into the calculation. The model was developed to the foams with isotropic and anisotropic geometries. The analytical results were compared with the available experimental data and the unit-cell model. Except for foams with high porosity, the micromechanical model agreed well with the experimental elastic–plastic behavior of both isotropic and anisotropic foams. On the other hand, the unit-cell model failed to estimate the mechanical properties of anisotropic foams such as lotus-structured porous metals.

Internal stress superplasticity in the NiAl–Mo eutectic alloy

Abstract Internal stress superplasticity (ISS) in a NiAl–Mo based eutectic alloy was investigated by conducting thermal cycling creep tests. The alloy was annealed at high temperatures to spheroidize the refractory metal phase. Under thermal cycling creep conditions, the alloy exhibited characteristics of ISS, that is, at low stresses, the thermal cycling creep rates were much higher than the isothermal creep rates and the corresponding stress exponent was close to 1. These results were compared quantitatively with the predictions of a theoretical model of ISS. The experimental thermal cycling creep rates agreed with the predicted values within an error less than one order of magnitude. Superplastic elongation of >200% without fracture was attained during a thermal cycling tensile creep test.

Internal stress superplasticity in anisotropic polycrystalline materials

A theoretical model of internal stress superplasticity is developed in a single-phase polycrystalline material with an anisotropic thermal expansion. Quasi-steady-state creep equation during a thermal cycle is derived quantitatively based on continuum micromechanics. The model assumes that the generated mismatch strain is accommodated simultaneously by the plastic flow of the material. The linear creep deformation, which corresponds to internal stress superplasticity, is obtained at low applied stress region, and the creep rate depends on the crystallographic texture of the material. The validity of the model is experimentally verified using polycrystalline zinc which is a typical metal having large anisotropy in thermal expansion. The calculated strain rates using the texture information and the isothermal creep equation agree quantitatively well with the experimental results. The apparent activation energy of thermal cycling creep reveals 1/n (n: stress exponent of isothermal creep) of that of isothermal creep, which is one of the characteristics of internal stress superplasticity. Except for the factors attributable to the material geometry, the thermal cycling creep equation in the polycrystalline material is identical to that in a metal matrix composite.

Formation of LuFe 2 O 4 phase from an undercooled LuFeO 3 melt in reduced oxygen partial pressure

The formation of metastable phases from an undercooled LuFeO 3 melt was investigated under reduced P o 2 since the iron ion has the tendency to change its valence state from Fe 3+ to Fe 2+ in an ambient atmosphere with low P o 2 . The nucleation and the post-recalescence temperatures of the phases were decreased with decreasing process P o 2 . Phase equilibrium was established in the Lu–Fe–O system at 1473 K by varying the oxygen partial pressure from 10 5 to 10 −1 Pa. A possible ternary metastable phase diagram depending on the oxygen composition in the bulk sample was also constructed. The formation of the LuFe 2 O 4 phase where the Fe 3+ and Fe 2+ ratio is 1:1 clearly indicated that the formation of metastable phases is related to the presence of Fe 2+ ions. Thermogravimetric analysis revealed that the increase in sample mass with decreasing process P o 2 , down to 10 −1 Pa, is relatively dependent on the amount of Fe 2+ ions.

Processing of Closed-Cell Magnesium Foams

Lightweight metallic foams are an attractive material having excellent energy absorption and acoustic damping. The density of magnesium is the smallest among structural metallic materials, and is about two third of the density of aluminum. It is, however, difficult to produce magnesium foams by conventional process because of their chemical activity. This paper provides a novel manufacturing process of magnesium foams. Accumulative diffusion-bonding process can produce a magnesium matrix composite (preform) containing titanium hydride (TiH2) particles as a blowing agent. Foaming tests of three magnesium alloys, AZ31, AZ91 and ZA146, revealed that low solidus temperature is effective to produce fine cell morphology. Chemical composition is significantly important to optimize the cell morphology of magnesium foams.

Magnesium Foam Produced from Bulk AZ31 Magnesium Alloy Sheets

Using commercial AZ31 magnesium alloy sheets, we produced a foamable preform sheet containing titanium hydride (TiH2) powder through diffusion-bonding and hot-rolling of four cycles. Heating the preform sheets in Ar atmosphere, we obtained closed-cell magnesium alloy foams with various porosities. The foamed specimen at 883 K showed the maximum porosities of 77%.

Closed-Cell Metal Foams Manufactured from Bulk Metal and Alloy Sheets through ARB Process

Aluminum foams having extremely low densities offer a large potential for lightweight structural materials. New manufacturing process without expensive aluminum alloy powder has been developed using conventional bulk aluminum alloy sheets. Preform plate containing blowing agent particles is first manufactured through accumulative roll-bonding (ARB) process. By heating the preform plate, closed-cell aluminum foams having various porosity and cell morphology are produced. It was revealed that ARB processing condition is significantly important to produce suitable aluminum foam with high porosity and uniform pore distribution. Present manufacturing process also possesses a potential to apply to many other metal and alloy foams.

Effect of Porosity on Tensile and Compressive Deformations of Superplastic Zn-22Al Alloy Foam

Closed-cell Zn-22Al superplastic alloy foams were manufactured through the melt foaming process. The Zn-22Al foams were produced with varying porosity of 51-71%. The tensile and compressive properties of the Zn-22Al foams were investigated at 523 K. The compressive specimen has m-value of 0.55 in the low strain rate region. This is because of the superplastic deformation induced by the fine microstructure of the cell wall. Though the superplastic elongation was not obtained in high temperature tensile test, the elongation was higher than that of conventional aluminum foams.

Energy Absorption Capability of Zn-22Al Superplastic Alloy Foam Manufactured through Melt Foaming Process

Superplastic Zn-22Al alloy foams were manufactured through the melt foaming process using titanium hydride powder as a foaming agent. The compressive properties of the Zn-22Al foams were investigated under quasi-static and dynamic loading conditions. Experimental results show that the flow stress and the energy absorption of the Zn-22Al foam significantly increased with increasing the strain rate. At high strain rate region, the energy absorption of the Zn-22Al foams is also larger than that of conventional aluminum foams. These behaviors are due to superplastic deformation of cell walls.

Energy Absorption of Superplastic Zn-22Al Alloy Foam Manufactured through Melt Foaming Process

Closed-cell superplastic Zn-22Al alloy foams were manufactured through the melt foaming process using sodium hydrogen carbonate powder as a foaming agent. Foaming tests were carried out under different foaming temperatures, times and additive amounts of foaming agent. The porosity of Zn-22Al alloy foams were between 30 and 70%. The cell wall consisted of fine equiaxial crystal grains after solution treatment. The compressive properties of the Zn-22Al alloy foams were investigated at room temperature and high temperature. Zn-22Al alloy foams exhibited high strain rate sensitivity, which was caused by superplastic deformation of the cell wall material.