The limits of solid state foaming

Ultralight metal foams can be produced by gas expansion in either the molten or solid state by the release of H₂ during the decomposition of TiH₂ particles. An alternative approach uses a powder metallurgical route to deliberately trap a low solubility gas within interparticle spaces during consolidation. This is subsequently used to plastically expand the voided solid during a post-consolidation heat treatment. Porosities of about 40% have been reported. However this is only about half that of melt-foamed materials and there is much interest in designing processes that increase it. Micromechanical models for the plastic expansion process are developed and used to identify the practical porosity limits in this entrapped gas expansion approach. It is shown that the porosity is limited by the reduction in pore pressure as voids expand, and ultimately by the loss of gas accompanying void coalescence. Increasing the initial pore pressure is shown to also lead to the formation of face sheet delaminations in stiffened, porous core sandwich panels. Its dependence on the process methodology is considered. Achievable porosities during solid state foaming are shown to be limited to less than 50%; much less than that of metals foamed in the liquid state. A simple extension of the analysis to semi-solid state expansion shows that much higher porosities could be achievable under these conditions because void coalescence can be avoided.     

开孔隙对粉末触媒合成金刚石的影响

通过实验设计，对比研究了有、无表面开孔隙的ＮｉＭｎ粉末触媒在六面顶压机上合成金刚石效果的差异。结果表明：ＮｉＭｎ粉末触媒表面存在开孔隙，能提高合成腔体内触媒和石墨的接触面积和单位体积中金刚石的形核率，从而有利于高单产金刚石的合成；而粉末触媒表面的开孔隙存在与否，对合成金刚石的晶形无明显影响。     

Shock-Induced Microstructures in Explosively Fabricated Superconductors

Scanning and transmission electron microscopy (including electron diffraction) have been applied to examine the consolidation, bonding, and characterization of orthorhombic, superconducting YBa₂Cu₃O₇ powder particles in explosive (shock-wave) fabricated aluminum matrix monoliths. The shock wave front (at pressures of approximately 4 GPa) roughly creates an order of magnitude greater density of microtwins and other defects in the orthorhombic structure, which is unaltered by the shock-wave compression. Some evidence for interparticle melting and rapid solidification at large interparticle voids is apparent in small, scattered amorphous regions in the dynamically consolidated superconducting powder regimes. Observations of large d.c. current carrying capacities and current densities estimated to be > 10³ A/cm² in zero applied magnetic field at 77K in practical electrical configurations suggest prospects for very high cirtical current densities resulting from strong flux pinning effects in the YBa₂Cu₃O₇ superconductor at shockwave induced defects.     

Shock consolidation: microstructurally-based analysis and computational modeling

The most important microstructural processes involved in shock consolidation are identified and discussed; the energy dissipated by a shock wave as it traverses a powder is assessed. The basic microstructural phenomena are illustrated for a metal (nickel-based superalloy), an intermetallic compound (rapidly solidified Ti₃Al), and ceramics (silicon carbide). Interparticle melting, vorticity, voids, and particle fracture are observed and the plastic deformation patterns are identified. Various energy dissipation processes are estimated: plastic deformation, interparticle friction, microkinetic energy, and defect generation. An analytical expression is developed for the energy requirement to shock consolidate a powder as a function of strength, size, porosity, and temperature, based on a prescribed interparticle melting layer. This formulation enables the prediction of pressures required to shock consolidate materials; results of calculations for the superalloy and silicon carbide as a function of particle size and porosity are represented. The fracture of ceramic particles under shock compression is discussed. Tensile stresses are generated during compaction that may lead to fracture. It is shown that the activation of flaws occurs at tensile reflected pulses that are a decreasing fraction of the compressive pulse, as the powder strength increases. These analytical results are compared to numerical solutions obtained by modeling the compaction of a discrete set of particles with an Eulerian finite element program. These results confirm the increasing difficulty encountered in shock consolidating harder materials, and point out three possible solutions: (a) reduction of initial particle size; (b) reduction of shock energy; (c) post-shock thermal treatment. Two possible and potentially fruitful approaches are to shock densify (collapse voids with minimum bonding) powders and to apply post-shock thermal treatments, and to shock consolidate nanosized powders. The latter method requires high shock energy and careful minimization of the shock reflections.     

Improving the uniformity in mechanical properties of a sintered Ti compact using a trace amount of internal lubricant

Large sintered powder compacts are likely to be associated with variability in mechanical properties; an improvement of the uniformity of the mechanical properties of sintered powder compacts is important for powder metallurgy. In this work 0.3–1 wt.% stearic acid (SA) or magnesium stearate (MgSt) was added to a 40 mm diameter Ti powder compacts with height to depth (H/D) ratio of unity to give a more uniform green density. Tensile test pieces were cut from selected positions in each sintered compact to obtain the distribution of mechanical properties. Results revealed that variations in mechanical properties are due to the pore morphology with respect to size, aspect ratio and preferred orientation. A trace amount of lubricant significantly improves the uniformity in mechanical properties by optimizing the porosity distribution and minimizing the pore size and aspect ratio of pores after sintering. Such an effect was achieved by reducing the initial green density inhomogeneity and the stress induced by the mismatch of sintering shrinkage. However a relatively high 1 wt.% SA addition with a large particle size created burnt-off pores in the top and bottom zones. MgSt is not recommended since it significantly increases the oxygen content. An addition of 0.6 wt.% SA is the best choice due to the even pore distribution, small pore size and acceptable level of oxygen pick up.     

退火热处理对纳米钨粉体性能的影响

采用工业化生产技术制备纳米钨粉体,对样品进行不同温度热处理;采用SEM、XRD、EDS和全自动比表面和孔隙度分析仪对样品的形貌、物相、含量和比表面、孔结构进行表征.通过计算品格变化率判断热处理后样品的晶格变化,并测量其粒径和孔容大小来解释晶粒的长大机理和孔结构的变化.结果表明:经热处理纳米钨粉体的晶格由收缩变为膨胀,且随热处理温度升高,比表面积和孔径均减小,粒径增大.当热处理温度达1200℃时,晶粒长大迅速,其长大机理根据晶界迁移解释.     

Reaction behavior and pore formation mechanism of TiAl–Nb porous alloys prepared by elemental powder metallurgy

Ti–48Al–6Nb (at.%) porous alloys are fabricated by elemental powder metallurgy to study the pore formation and propagation mechanism. Reactive diffusion, pore formation process, and pore characteristics of the porous TiAl–Nb alloys are investigated at different temperatures. It is found that the porous alloys exhibit a uniform, maze-like network skeleton, viz., a typical α₂-TiAl₃/γ-TiAl fully lamellar microstructure. The reactive diffusivities between Ti and Al powders are dominant during the Ti–Al–Nb powder sintering. Gas release during sintering also plays an important role in the pore propagation and the compact expanding process. In addition, a pore-formation model is proposed to interpret the growth mechanism of pores and skeletons.     

银粉特性对银膜致密性的影响

研究了颗粒尺寸、比表面积、形状及孔隙等银粉特性对膜层致密性的影响。通过3种不同粉末对膜层初始外观及高温结行为的分析，解释团聚的颗粒体和空隙的收缩导致膜层开裂，归纳出结晶完整，孔隙较少的银粉有利于促进致密化和减少颗粒长大。     

Strength and ductility of powder consolidated ultrafine-grain tantalum

Bulk ultra-fine grain tantalum was successfully fabricated from pure tantalum powder particles with an average particle size of 50 μm by equal channel angular extrusion at 900° C and 1200° C using three different processing routes. The effects of extrusion route and temperature on the consolidation performance are evaluated through microstructural analysis and room temperature mechanical testing. The consolidated tantalum has a “wood grain” like structure with strong interparticle bonds and a grain size <100 nm. The lamellar structure is helpful in resisting crack formation and fracture. The bulk tantalum consolidates are much stronger than annealed wrought tantalum and have significant ductility. Tensile tests for consolidated tantalum at room temperature show a yield strength of 340–376 MPa, an ultimate strength of up to 618 MPa, and strains to failure of ~19–25%. These properties are comparable to wrought Ta. The present study demonstrates that equal channel angular extrusion is a feasible method for fabricating bulk nanostructured refractory alloys from precursor powder, with a combination of high strength and good ductility.     

采用TiH2粉末和造孔剂NH4HCO3烧结多孔钛（英文）

采用 TiH2和造孔剂 NH4HCO3混合粉末,利用粉末烧结法制备了多孔钛。研究表明,NH4HCO3的添加对于烧结的多孔钛的相组成几乎没有影响。多孔钛的孔隙度、平均孔隙尺寸、压缩强度和压缩弹性模量随 NH4HCO3添加量的增加而急剧下降。多孔钛可用添加不同量的造孔剂来调整孔隙结构和力学性能。独特的孔隙结构和适宜的力学性能满足多孔植入材料的基本要求。表明烧结的多孔钛是有前景的多孔植入候选材料。     

制多孔不锈钢过滤片的试验研究

通过以水雾化0Cr18Ni9不锈钢粉为原料,以钢模压制或等静压制成形,在真空中或纯氢中烧结制取多孔不锈钢过滤片的试验,提出了制品的气泡试验最大孔径与水雾化不锈钢粉末颗粒平均尺寸关系的经验公式,并针对不锈钢粉末粒级、成形压力和添加剂对多孔不锈钢过滤片的孔隙度、气泡试验最大孔径、相对透气系数和拉伸性能的影响进行了研究。研制的多孔不锈钢过滤环片已批量生产,取得了明显的经济效益。     

粉末冶金法制备泡沫铝时增粘过程的基础研究

在研究粉末冶金法制备泡沫铝过程中,在铝硅合金粉末中加钙增粘,提高了泡沫铝熔体的稳定性与孔结构的均匀性.论述了加钙增粘的机理及不同加钙量对泡沫铝孔隙率、孔结构的影响.经对发泡过程的动态观测,证明熔体粘度对泡沫体稳定性的影响是至关重要的.试验结果表明:在采取严格的气体保护措施条件下,加钙量在2%～2.5%时,可获得较低密度且孔径均匀的泡沫铝材料.     

两种3D打印用雾化Ti-6Al-4V粉末的对比研究

本研究分别利用水冷铜坩埚真空感应熔炼气雾化(VIGA-CC)和等离子旋转电极(PREP)两种技术制备出球形Ti-6Al-4V合金粉末,作者利用SEM、同步辐射CT扫描-三维重建和氩气含量测试等分析手段对不同粒径的Ti-6Al-4V合金粉末的孔洞缺陷和氩气含量、硬度值进行了表征。实验结果表明, VIGA-CC粉末粒度分布宽,细粉收得率较多,粉末粒度分布在40~180 μm之间, PREP粉末的粒度分布较窄,主要集中在110~180 μm之间；金属粉末内部的孔隙率、气体含量和孔尺寸随着粉末粒度的增大而增大,且同一粒径范围内VIGA-CC粉末的气孔概率多于PREP粉末；随着粉末粒径减小,粉末截面组织逐渐细化,其硬度值逐渐升高,整体上VIGA-CC粉末硬度值高于PREP粉末。     

Molecularly-mediated assembly of gold nanoparticles

The understanding of the interparticle interactions and reactivities is essential to the exploitation of the unique optical, electronic, and chemical or biological properties of gold nanoparticles in many areas of nanotechnology. This paper describes findings of a comparison of optical properties of molecularlymediated assemblies of gold nanoparticles where the interparticle interactions and spatial properties are defined by molecular mediators and templates. The changes of the surface plasmon resonance band of the assemblies of gold nanoparticles mediated by several different types of molecular mediators, including multidendate methylthiosilane thioether, homocysteine, 1-(4-methyl)-piperazinyl fullerene, and indolenine cyanine dye, are compared. In the assembly process, the size of individual Au nanoparticles and the volume fraction of gold nanoparticles in the solution should remain largely constant, whereas the interparticle distance changes, which leads to changes in the interparticle dielectric medium constant or refractive index. The changes in the SP band in relation to the change in effective refractive index are discussed, along with their implications to assembly sizes, interparticle interactions, and potential applications in designing electrical and optical sensors.     

K417G合金焊接修复用高温合金粉末设计与工艺

航空发动机涡轮导向叶片在服役过程中形成的裂纹可以采用钎焊粉末冶金修复技术进行修复,针对低压涡轮导向叶片材料和结构特点,设计了K417G合金修复所需的不同成分的粉末填充材料,测试了不同粉末粒度配比的振实密度值,采用金相法、压汞法评估了不同烧结条件下粉末烧结体的孔隙率、平均孔径、中值孔径、最可几孔径等烧结体性能值,优选了粉末填充材料,在此基础上开展了钎焊粉末冶金修复工艺研究,分析测试了钎焊粉末冶金修复接头冶金组织. 结果表明,接头成形良好,焊料润湿铺展充分,接头致密饱满,无明显焊接缺陷.     

Microstructure formation of porous silicon carbide ceramics during β-α phase transformation

The effect of particle size and initial powder phase on the microstructure of porous SiC ceramics was investigated. When β-SiC powder was used as a starting material, porous SiC ceramics were formed by rapid grain growth of α-SiC during the β-α phase transformation. As the particle size of the initial powders decreased, β-α SiC phase transformation and rapid grain growth of α-SiC occurred earlier. Grain and pore size of SiC porous ceramics can be controlled by the addition of α-SiC seed. Grain and pore size decreased as the concentration of α-SiC seed increased. The bending strength of porous SiC ceramics did not depend on pore size.     

粉末烧结和燃烧合成法制备多孔TiNiFe合金(英文)

采用粉末烧结法和燃烧合成法制备了多孔Ti50Ni48Fe2(at%)形状记忆合金。前者通过添加不同含量的造孔剂(NH4HCO3)来调节控制多孔合金的孔隙特性如孔隙形貌、孔隙度、平均孔隙尺寸;而后者通过生坯的孔隙度来控制合金的上述孔隙特性。通过两种方法制备的多孔TiNiFe合金的孔隙特性满足多孔植入材料的孔隙要求,是理想的多孔植入候选材料。     

Cross-scale characteristics of backfill material using NMR and fractal theory

This paper analyzed the pore structure, quantified the pore fractal dimension, calculated the grading index (GI) of mixed aggregate, and studied the relationship among GI, pore structure, and strength to describe the cross-scale characteristics of backfill, which is made from stone powder and cemented tailing. A series of experiments were conducted on stone powder cement tailings backfill (SPCTB). The GI formulas for mixed aggregates, containing stone powder and tailings, were derived based on the Füller theory. The nuclear magnetic resonance (NMR) fractal dimensions of backfills were derived using fractal geometry principles. Compared to the mesopore and macropore fractal dimensions, the correlation between micropore fractal dimension and macro-properties in terms of NMR porosity, pore structure complexity, uniaxial compression strength (UCS), and GI is the most significant. Macropore fractal dimension is generally correlated with UCS and GI and the other properties such as the shape of mixed aggregates also have an impact on fractal dimension. However, mesopore fractal dimension has no obvious relationship with macro-properties. Finally, the relationship between GI and UCS was studied, which contributed to improving backfill’s strength and optimizing gradation.     

Effect of Preparation Technology Parameters on the Restriction Performances of Unitary Titanium Porous Restrictor

Unitary titanium porous restrictor was prepared by a sintering welding technique. The results indicated that the restriction performance of unitary Ti porous restrictor was not only dependent on the morphology and particle size of the original powder, but also influenced by the molding pressure, sintering temperature and sintering pressure. The restrictor obtained from titanium powder with large particle size or spherical morphology possesses big porosity, large pore size and high permeability but low restr...