Anisotropic compressive properties of porous copper produced by unidirectional solidification

Porous copper whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a mixture gas of hydrogen and argon. The compressive yield strength of the porous copper with the cylindrical pores orientated parallel to the compression direction decreases linearly with increasing porosity. For the porous copper whose pore axes are perpendicular to the compressive direction, the compressive yield strength slightly decreases in the porosity range up to 30% and then decreases significantly with increasing porosity. The compressive stress–strain curves depend on the compressive direction with respect to the pore direction, which are due to the stress concentration around the pores and the buckling of the copper between the pores. From two different types of stress–strain curve, the energy absorption capacity of the porous copper with the pores parallel to the compressive direction is higher than that perpendicular to the compressive direction at a given porosity.     

Highly porous hydroxyapatite scaffolds with elongated pores using stretched polymeric sponges as novel template

This study reports a simple way of improving the compressive strength of highly porous hydroxyapatite (HA) scaffolds by adopting elongated polymeric sponges as a novel template. In this method, as-received polymeric sponges with isotropic pores were stretched uniaxially to 50% elongation at 200 °C for 2 h, and then coated with a HA slurry. The HA-coated sponges were heat-treated at 800 °C for 3 h to remove the polymeric sponges and at 1250 °C for 3 h to sinter the HA walls. The fabricated samples showed a highly anisotropic pore structure with elongated pores parallel to the direction of the elongation of the polymeric sponge. This simple method allowed a highly porous scaffold to have a high compressive strength of 3.8 ± 0.1 MPa at a porosity of 76% when tested parallel to the direction of pore elongation.     

The compressive behaviour of porous copper made by the GASAR process

Recently, porous metals and ceramics have been made by melting the solid in a hydrogen atmosphere and then cooling through the eutectic point; the technique is known as the GASAR process. The size, shape, orientation and volume fraction of the pores can be controlled by the direction and rate of cooling and the pressure of the system. Here we describe the uniaxial compressive behaviour of GASAR copper with cylindrical pores oriented in the direction of loading. The elastic modulus and yield strength of the porous materials increase linearly with increasing relative density. Initial plastic deformation was found to be due to plastic yielding of the solid rather than buckling of the cells walls. The characteristic densification strain decreased linearly with increasing relative density.     

Fabrication of lotus-type porous carbon steel via continuous zone melting and its mechanical properties

Lotus-type porous carbon steel (lotus carbon steel) AISI1018 rods with long cylindrical pores aligned in one direction were fabricated using the continuous zone melting technique under nitrogen gas pressure of 2.5 MPa. The porosity decreased with increasing transference velocities of 40–160 μm s⁻¹. Tensile tests of the fabricated lotus-type carbon steel rods were performed. The elongation of lotus carbon steel increased after normalizing at 1200 K. The tensile strength and the Young's modulus decreased with increasing porosity. In contrast, the yield strength of lotus carbon steel did not decrease, even with a porosity of 20%, compared with that of non-porous carbon steel. This superior characteristic is attributed to solid-solution strengthening by solute nitrogen.     

Thermal and chemical properties of TiO2-SiO2 porous glass-ceramics

The thermal and chemical stability of porous glass-ceramics of the TiO₂-SiO₂ system have been investigated. Porous glass-ceramics containing both anatase and rutile had thermal expansion coefficients of 40 to 55 x 10^–7 K^–1 in the range 0 to 700° C. They remained porous up to 1000° C, while the pores of high-silica porous glass of Vycor type collapsed completely at that temperature. The high thermal stability of the porous glass-ceramics may be attributed to a high viscosity due to dispersed crystallites of anatase and rutile in the skeleton. Most of the anatase in the skeleton was transformed to rutile by heat treatment at 900° C, but some of it remained untransformed even after 6 h. Surface -OH groups identified by IR spectroscopy were removed by dehydration polycondensation with heating up to 900° C. The porous glass-ceramics were quite durable to alkali solution compared with high-silica porous glass. The excellent durability of these porous glass-ceramics was attributed to the large amount of TiO₂ contained in the skeletal structure.     

Equal-channel angular extrusion process of lotus-type porous copper

Deformation behavior of lotus-type porous copper with long cylindrical pores aligned in one direction through equal-channel angular extrusion (ECAE) process was investigated using dies with channel angles 90° and 150°. The lotus-copper rod was densified by the uni-axial compression in the entry channel, and the pores were thinned and elongated by shearing at the corner of the 90°-die. The inclination angle of the elongated thinned pores can be explained by a geometric deformation model considering the densification. After the second path of the ECAE the pores were either further thinned and elongated or opened again according to the pass route. Extrusion through the 150°-die changed the pore morphologies little. The Vickers hardness increased through the ECAE process. From these results we found that it may be possible to improve the mechanical properties of porous metals and to control the pore morphology by means of the ECAE process.     

Structures and electrical properties of zinc oxide films prepared by low pressure sputtering system

Polycrystalline zinc oxide films with and without admixed aluminum and copper were prepared on amorphous substrates by using a co-sputtering process. The crystallographic orientation and electrical properties varied with both sputtering gas pressure and contents of admixed aluminum or copper. In a high sputtering gas pressure a normal orientation with c-axis perpendicular to the film surface was predominant, whereas, in a low sputtering gas pressure a parallel orientation with c-axis parallel to the film surface was predominant. The admixed aluminum inhibited growth of the normal orientation, but the admixed copper enhanced a growth of the parallel one.     

Fabrication of bimodal porous alumina ceramics

An alumina foam with two kinds of pores was prepared by combining the sponge method and the pore-former method. The large pores were provided by the sponge method, and their sizes were in the range of 1-2 mm. The small pores were produced by the pore-former method with size in the micrometer range. The large pores offered a high porosity while the small pores offered a large surface area. The strength of samples sintered at different temperatures was measured, and the effect of sintering temperature on foam strength was analyzed by discussing porosity and grain bonding area. The sample sintered at 1550 °C has a compressive strength of 1.3 MPa and a porosity of 86%.     

Yield behavior of porous nuclear fuel (UO2)

Uranium dioxide (UO₂) is one of the most common nuclear fuels. During burn-up, the fuel undergoes substantial microstructural changes including the formation of pressurized pores, thus becoming a porous material. These pores reduce the elastic modulus and alter the yield behavior of the material. In this work, a finite-element-based homogenization technique has been used to map the yield surface of UO₂ with pressurized pores. Two scenarios are considered; in the first, the fuel matrix is a ductile material with a Von-mises type behavior, while in the second, the matrix is quasi brittle, which is simulated using the concrete damaged plasticity (CDP) model available in ABAQUS. For both of the scenarios, it is found that the yield strength decreases with an increase in porosity for a given internal pore pressure. For a given porosity, the yield surface shifts towards the negative hydrostatic axis in the Haigh-Westergard stress space with an increase in pore pressure. When the matrix is quasi brittle, the decrease in tensile hydrostatic strength is less than the increase in compressive hydrostatic strength, whereas in the case of a ductile matrix, the changes in the hydrostatic strengths are same. Furthermore, the shape of the yield surface changes from one deviatoric plane to another in both scenarios. Analytical equations, which are functions of pore pressure and porosity, are developed to describe the yield surface of porous UO₂ while accounting for the changes in shape of the yield surface from one deviatoric plane to another. These yield functions can be used to predict the failure of porous UO₂ fuel.     

Microstructure and mechanical properties of Mg -2Gd -3Y -0.6Zr alloy upon conventional and hydrostatic extrusion

The Mg-12Gd-3Y-0.6Zr (wt. %) alloy was subjected to conventional and hydrostatic extrusion in two subsequent steps. The best combination of mechanical properties (strength and ductility) was achieved by RT hydrostatic extrusion following conventional extrusion at 430 °C, with the ultimate tensile strength (UTS), tensile yield strength (TYS) and elongation being 485 MPa, 413 MPa and 5.2% at room temperature. The texture results of extruded rods indicate that the c-axis of most grains was aligned preferentially perpendicular to the extrusion direction, forming a typical extrusion Mg fiber texture.     

Fabrication of Lotus‐type Porous Metals and their Physical Properties

Lotus‐type porous metals whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized gas atmosphere. The pores are formed as a result of precipitation of supersaturated gas when liquid metal is solidified. The lotus‐type porous metals with homogeneous size and porosity of the evolved pores produced by a mould casting technique are limited to the metals with high thermal conductivity. On the other hand, the pores with inhomogeneous pore size and porosity are evolved for metals and alloys with low thermal conductivity such as stainless steel. In order to obtain uniform pore size and porosity, a new “continuous zone melting technique” was developed to fabricate long rod‐ and plate‐shape porous metals and alloys even with low thermal conductivity. Mechanical properties of tensile and compressive strength of lotus‐type porous metals and alloys are described together with internal friction, elasticity, thermal conductivity and sound absorption characteristics. All the physical properties exhibit significant anisotropy. Lotus‐type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength.     

Effect of initial orientation on the microstructure and mechanical properties of textured AZ31 Mg alloy during torsion and annealing

We studied the effect of crystallographic orientation and temperature on the microstructure and mechanical properties of extruded AZ31 magnesium alloy bar by torsion and subsequent annealing. The results show that the orientation between torsion axis (TA) and extrusion direction (ED) has a significant impact on the microstructure evolution. With TA parallel to ED, profuse extension twins appeared. After annealing, zones close to the surface were completely recrystallized and refined, while the center still had some extension twins left. With TA perpendicular to ED, extension twins were inhibited. It is speculated that, except extension twins, contraction twins is also acting as a major deformation mode. Upon annealing, this specimen was completely recrystallized, even at the center, which is considered as a result of more preferred nucleation sites for static recrystallization from contraction twins. As demonstrated, the temperature has little impact on the microstructure development when twisted at room temperature or liquid nitrogen temperature. Moreover, the compression tests show that the compression ductility and yield strength were improved simultaneously for both samples when compressed on the direction either along ED or perpendicular to ED, due to the combined effects of grain refinement and texture weakening.     

规则多孔铜热膨胀性能的研究

在氢气或氢气和氩气的混合高压气氛中,采用定向凝固技术制备了规则多孔铜材料。测试了不同气孔率的规则多孔铜在平行和垂直气孔方向上的热膨胀系数;研究了气孔、气孔方向和气孔率对其热膨胀系数的影响规律,并对其规律做了理论预测。结果表明,规则多孔铜的热膨胀系数随着温度升高先急剧增大到一定值后趋于平稳;温度在40~130℃,气孔中存在闭孔时,规则多孔铜的CTE值随气孔率的增大而缓慢增大,且比纯铜时略大;当气孔主要以通孔形式存在时,气孔率与孔径的比值越大,规则多孔铜的CTE值越低。温度>130℃时,规则多孔铜的热膨胀系数与纯铜的几乎相同,气孔的存在对铜的膨胀无明显影响。     

Fabrication, properties and application of porous metals with directional pores

This paper reviews the recent development of fabrication methods, various properties of porous metals with directional pores and its applications. This porous metals are fabricated by unidirectional solidification in pressurized gas atmosphere such as hydrogen, nitrogen and oxygen. The pores are evolved from insoluble gas when the melt metal dissolving the gas is solidified. The nucleation and growth mechanism of the directional pores in metals are discussed in comparison with a model experiment of carbon dioxide pores in ice. Three fabrication techniques, mold casting, continuous zone melting and continuous casting techniques, are introduced. The latter two techniques can control the solidification velocity and the last one possesses a merit for mass production. The porosity and pore size are able to be controlled by solidification velocity and ambient gas pressure, while the pore direction can be controlled by solidification direction. Not only metals and alloys but also intermetallic compounds, semiconductors and ceramics can be produced by this method. Anisotropy in the mechanical and physical properties is resulted from anisotropic pore morphology. The experimental results on the anisotropy in the elastic property and electrical conductivity are consistent with those calculated with an effective-mean-field theory. The anisotropic behaviors of tensile, compressive and fatigue strength are explained in terms of the dependence of stress concentration on the pore orientation. This porous metals exhibit good sound absorption and vibration-damping properties. Several possible applications are in progress for heat sink, golf putter, biomaterials and so on.     

包覆成孔剂法制备高性能多孔氮化硅陶瓷工艺

为了制备高强度且分布均匀的氮化硅陶瓷,采用包覆成孔剂法改进普通添加成孔剂的方法,常压烧结氮化硅多孔陶瓷,采用阿基米德法、三点弯曲法分别测试材料的孔隙率及抗弯强度,用扫描电镜和光学放大镜对氮化硅多孔陶瓷显微结构和表观结构进行研究.结果表明,添加包覆过的成孔剂强度比添加未包覆的成孔剂强度高,孔隙率为50%时,强度增加近一倍.强度的提高归因于特殊的微观结构,即气孔的均匀分布和孔与孔之间相间隔分布.     

The microstructure,tensile, and shear deformation behavior of an AZ31 magnesium alloy after extrusion and equal channel angular pressing

The shear punch testing (SPT) technique and the uniaxial tension tests were employed to evaluate the mechanical properties of the equal channel angularly pressed (ECAPed) AZ31 magnesium alloy. After extruding, the material was ECAPed for 1, 2, and 4 passes using route B_C. The grain structure of the material was refined from 20.2 to 1.6 μm after 4 passes of ECAP at 200 °C. The 4 pass ECAPed alloy showed lower yield stress and higher ductility as compared to the as-extruded condition, indicating that texture softening has overcome the strengthening effects of grain refinement. The same trends in strength and ductility were also observed in shear punch testing. Similar shear strength and ductility values of the samples taken perpendicular to the extrusion direction (ED) and normal direction (ND) after 4 passes of ECAP indicated that {0 0 0 2} basal planes were inclined (∼45°) to the extrusion axis. The shear punch testing technique was found to be a useful method for verifying directional mechanical properties of the miniature samples of the ECAPed magnesium alloys.     

Electrical and flexural anisotropy in freeze tape cast stainless steel porous substrates

Engineered metal foams have strong potential in applications such as fuel cell electrodes, sensors, variable springs, filtering media, and compositionally graded composite structures. In this study, the variation in mechanical and electrical properties was characterized in engineered porous metal foams with aligned porosity to ascertain the degree of anisotropy that can be induced in these substrates. Porous substrates were prepared by freeze tape casting of powdered ferritic stainless steel. After directional solidification of the slurry, the solvent was sublimed from ~ 1 mm thick tapes, yielding porous green metal compacts which were sintered in a protective atmosphere. The resulting disks exhibit long range ordered acicular pores with substantial anisotropy in the mechanical and electrical properties that is related to the cross sectional pore morphology and connectivity. DC conductivity testing revealed up to 61% variation depending on direction of measurement relative to the alignment of pores. Also, an 89% variation in flexural rigidity in relation to pore orientation was observed in identical disks.     

Low-temperature processing of porous SiC ceramics

Porous silicon carbide ceramics were fabricated from SiC, polysiloxane, and polymer microbead (as a pore former) at a temperature as low as 800 °C by a simple pressing and heat-treatment process. The effects of polysiloxane and template contents on the porosity and strength of the ceramics were investigated. During heat treatment, the polysiloxane transformed to an amorphous SiOC phase, which acted as the bonding material between SiC particles, and the polymer microbeads decomposed into gases and left pores. The porosity of porous SiC ceramics could be controlled within a range of 26–56 % with the present set of processing variables. The porous SiC ceramics showed a maximal porosity of 56 % when 10 μm SiC particles and 16 % polysiloxane were used with 20 % polymer microbeads. Flexural strength generally increased with increasing polysiloxane content and decreased with increasing polymer microbead content. Typical flexural strength of the porous SiC ceramics was 53 MPa at 42 % porosity.     

Effects of porosity and pore distribution on static strength properties of porous zirconia

Bulk porosity, along with size and spatial distribution of pores, play key roles in strength of porous ceramics, as reported in a study on porous alumina. Hence, a fracture mechanics procedure was proposed to evaluate their strength by presuming that behavior of pore distribution is equivalent to that of crack distribution, and each pore is surrounded by virtual crack. In contrast to alumina, zirconia has distinct spherical‐shaped pores. Moreover, its strength properties vary with stabilizing additives. In this research, strength properties of yttria‐stabilized zirconia ceramics were studied to verify applicability of the procedure proposed for simulating strength of porous ceramics. The effect of pore characteristics on static strength properties was determined experimentally and confirmed by Monte‐Carlo simulations. It was revealed that simulated strength coincided with experimental results within a narrow scatter band, ie, factor of 2^1/2. Therefore, the proposed procedure was found to be appropriate for estimating strength of porous zirconia.     

Modeling plastic deformation and fracture of porous materials

Strain hardening of a porous material was numerically modeled. The corresponding stress-strain (σ-ε) curves, the ultimate strength, and the strain at break were calculated for iron with a relative porosity in the interval from 0 to 30%. Anomalous behavior of these characteristics is observed at a porosity corresponding to the percolation transition from isolated pores to the “infinite” pore cluster. The proposed model adequately describes the available experimental data.