Fabrication of porous FeAl-based intermetallics via thermal explosion

Porous FeAl-based intermetallics were fabricated by thermal explosion (TE) from Fe and Al powders. The effects of sintering temperature on phase constitution, pore structure and oxidation resistance of porous Fe-Al intermetallics were systematically investigated. Porous Fe-Al materials with high open porosity (65%) are synthesized via a low-energy consumption method of TE at a temperature of 636 °C and FeAl intermetallic is evolved as dominant phase in sintered materials at 1000 °C. The porous materials are composed of interconnected skeleton, large pores among skeleton and small pores in the interior of skeleton. The interstitial pores in green powder compacts are the important source of large pores of porous Fe-Al intermetallics, and the in-situ pores from the melting and flowing of aluminum powders are also significant to the formation of large pores. Small pores are from the precipitation of Fe-Al intermetallics particles. In addition, the porous specimens exhibit high resistance to oxidation at 650 °C in air.     

The preparation of TiAl-based intermetallics from elemental powders through a two-step pressureless sintering process

Pressureless sintering is not considered feasible for the preparation of TiAl-based intermetallics from elemental powders due to the Kirkendall effect. When the compacts are sintered without pressure, swelling and surface cracking occur in the billets. Porous billets are obtained, which cannot be further used. Nevertheless, pressureless sintering is still worth investigating because of lower cost and easier operation. In this study, a two-step pressureless sintering process was developed to prepare the TiAl-based intermetallic preforms. The first step was to eliminate the aluminum particles in the billets through a solid-state sintering procedure to prevent the aluminum from melting and to suppress the formation of the Kirkendall pores by sintering with a constrained mold. The second step was to make the billets homogenized and densified by a high-temperature sintering procedure. Results show that the billets sintered with constrained molds are dense and crack free after the first step. Then, the billets are densified in the second step. Therefore, the application of constrained molds makes pressureless sintering feasible in the preparation of the TiAl-based intermetallics from the elemental powders. The microstructure is fully transformed from the aluminum and titanium compacts to the TiAl-based intermetallics after this two-step pressureless sintering process.     

Microstructure Evolution and Pore Formation Mechanism of Porous TiAl_3 Intermetallics via Reactive Sintering

Porous TiAl_3 intermetallics were fabricated through vacuum reactive sintering from Ti–75 Al at.% elemental powder mixture. The phase compositions, expansion behaviors, pore characteristics and microstructure evolution of TiAl_3 intermetallics were investigated, and the pore formation mechanism was also proposed. It was found that the actual temperature of compacts showed an acute climb from 668 to 1244 °C in 166 s, while the furnace temperature maintained the linear growth of 5 °C/min, which indicated that an obvious thermal explosion(TE) reaction occurred during sintering,and only single-phase TiAl_3 intermetallic was synthesized in TE products. The open porosity increased from 22.2(green compact) to 32.8% after reactive diffusion sintering at 600 °C and rised to 58.7% after TE, then decreased to 51.2% after high-temperature homogenization at 1100 °C. Therefore, TE reaction is the dominated pore formation mechanism of porous TiAl_3 intermetallics. The pore evolution in porous TiAl_3 intermetallics occurred by the following mechanisms:certain intergranular pores remained among powder particles of green compact, then low-temperature sintering resulted in a further increase in porosity due to the Kirkendall effect. Moreover, TE reaction gave rise to a dramatic volume expansion because of the rapid increase in temperature, and high-temperature sintering caused densification and a slight shrinkage.     

放电等离子合成Ti3AlC2-TiB2复合材料的相形成研究

采用放电等离子烧结工艺,以Ti,Al,B4C,TiC为原料制备Ti3AlC2／TiB2复合材料。通过X射线衍射分析了从600℃到1300℃Ti3AlC2／TiB2系统反应过程的相形成规律。用扫描电镜观察了不同温度下试样的显微组织演变。结果表明,在900℃之前,主要的反应是Ti和Al反应生成Ti—Al金属间化合物,900℃之后,Ti—Al金属间化合物与TiC逐渐生成Ti3AlC2和TiB2相,形成致密Ti3AlC2／TiB2复合材料。     

Pore constitution and tortuosity factor of reactively synthesized porous TiAl intermetallic compound

The pore constitution and tortuosity factor of porous TiAl intermetallic were studied on the basis of the variation behavior of pore structure parameters and the discrete particle model. The pore formation mechanism of porous TiAl is mainly ascribed to three aspects: the clearance space in green compact, the diffusive pores in the reaction process and the phase transition pores, resulting in the open porosities of 5.6%, 42.9% and 1.3%, respectively. According to the Hagen?Poiseuille equation, the tortuosity factor of porous TiAl is determined in the range of 1.3?2.2. Based on the discrete particle model and the variation rule of the tortuosity factor, the tortuosity factor depends mainly on the parameters of fabrication constant, particle shape factor, clearance distance and powder particle size. The quantitative relationships among them have been established, which can be used as the basis for adjusting the pore structure of porous intermetallics.     

Highly porous open cellular TiAl-based intermetallics fabricated by thermal explosion with space holder process

High porosity TiAl-based intermetallics were prepared through thermal explosion (TE) from Ti–50Al at.% powders with NaCl as soluble template. The results showed that the space holder particles of NaCl were removed completely in green compacts, and porous Ti–Al materials were synthesized via a low-energy consumption method of TE at a temperature of 600 °C. TiAl was evolved as dominant phase in sintered materials at 1100 °C. With adding 80 vol.% NaCl to Ti–50Al at.% powders, the open porosity was significantly elevated up to 84%. Moreover, the porous materials exhibited a bimodal pore size distribution: large pores (200–500 μm) replicating NaCl particles and small pores (<50 μm) embedded in pore walls. The interconnected small and large pores make open cellular porous TiAl materials, which prescribe them promising for a wide range of applications in separation, heat insulation and catalysis.     

Porous FeAl intermetallics fabricated by elemental powder reactive synthesis

Fabrication of porous FeAl intermetallics has been realized through the Fe and Al elemental powder reactive synthesis. The swelling behavior, synthesis process and microstructure of the porous FeAl intermetallics fabricated by reactive synthesis have been systematically studied. The pore structural parameters as a function of the sintering temperature have also been systematically investigated. It has been confirmed that the pore evolution in the porous FeAl intermetallics is attributed to the following four steps: the inter-particle pores formed during the pressing procedure, the Kirkendall pores formed during the solid-state sintering, pore formed through the liquid Al reaction, and the phase transformation during the high temperature sintering.     

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.     

Application of compression lubricant as final porosity controller in the sintering of tungsten powders

Sintering behavior of two tungsten powders (1.2 μm and 6 μm) was studied for preparing infiltrable porous skeleton. Both powders were compressed by mechanical press (MP) and cold isostatic press (CIP) with and without stearic acid respectively as compaction lubricant. Results showed that presence of solid lubricant powder in addition of its essential effect on soundness of parts, depending on its size and distribution, could mainly affect sintered microstructure. Stearic acid as compaction lubricant in addition of decreasing friction between particles during the compaction, has acted as spacing particles between primary powder particles. In the cases that lubricant particles are much bigger than tungsten particles a big pore remained after evaporation of lubricant. During the sintering, big pores became bigger due to coarsening mechanism and formed an interconnected network of pores and on the other hand small pores shrank or even disappeared due to densification. By exact controlling of the size of tungsten powder and lubricant powder, infiltrable tungsten skeletons with 80% of theoretical density were produced successfully at low sintering temperatures such as 1500 °C.     

孔结构对Ni3Al金属间化合物多孔材料抗腐蚀性能的影响（英文）

采用元素粉末反应合成的方法并用尿素作为造孔剂制备Ni3Al金属间化合物多孔材料。用电化学和浸泡实验表征Ni3Al金属间化合物多孔材料在6 mol/L KOH溶液中的抗腐蚀行为。系统研究孔结构对材料抗腐蚀性能的影响。研究结果表明:孔隙率较大的Ni3Al金属间化合物多孔材料较孔隙率较小的样品腐蚀更严重,这是由于孔隙率较大的样品具有复杂的联通孔结构以及较大的比表面积。然而,材料的腐蚀速率与比表面积并不成正比,这是因为随着孔隙率的增大,材料的孔径大小、孔径分布以及孔隙形状都随之变化。不同孔隙率大小的Ni3Al金属间化合物多孔材料在碱溶液中均表现出较好的抗腐蚀性能。     

液体中脉冲放电制备(Ti,Al)C陶瓷涂层的研究

用粉末模压成型工艺和无压烧结方法制备了Ti-Al混合烧结工具电极,采用所制备的电极在煤油中对45钢表面进行脉冲放电沉积(Ti,Al)C陶瓷涂层。探讨了粉体配比对工具电极致密度和相组成以及对液相放电所制备的涂层组织与表面形貌的影响。结果表明,随着Al含量的提高,工具电极密度和相对密度先下降后升高,当Al含量为60%时,电极密度达到最小;XRD结果显示制备出涂层的物相主要为(Ti,Al)C,而制备出的涂层表面形貌明显不同,随着电极中Al元素的升高,熔滴状的熔池尺寸越来越大,宏观上越来越粗糙,截面涂层厚度约为20μm。     

Fe-Al、Ti-Al和Ni-Al系金属间化合物多孔材料的研究进展

总结Fe-Al、Ti-Al、Ni-Al 3大系金属间化合物的物相结构和基本特性,论述Fe-Al、Ti-Al和Ni-Al 3大类金属间化合物多孔材料的制备方法、孔结构表征以及耐腐蚀性能,并指出孔结构参数的可控性研究、复合材料的制备和焊接性能的提高是金属间化合物多孔材料未来的研究重点.     

Influence of aluminum oxides on abrasive wear resistance of Fe–50 at.% Al intermetallic sinters

This paper presents the influences of processing parameters on the tribological properties of Fe–50 at.% Al intermetallics obtained by sintering in the presence of liquid phase elemental Fe and Al powders. The wear resistance of FeAl under dry sliding against C45QT steel under ambient conditions was studied in a pin-on-disc contact configuration. Although processing parameters such as sintering temperature and homogenization time had an effect on the wear resistance of FeAl intermetallics, the particle size of Al₂O₃ oxides derived from oxidized aluminum and iron powders and formed during sintering had the greatest influence.     

Formation of porous metallic glass compacts by electro-discharge sintering

A single pulse of 0.1–0.9 kJ/0.45 g atomized amorphous Cu₅₄Zr₂₂Ti₁₈Ni₆ powders in size range of 90–150 μm was applied to fabricate porous metallic glass compacts using electro-discharge sintering (EDS) with 3 and 4 mm in diameter. The structural and thermal analysis of the samples indicated that formation of the porous metallic glass compacts occurs only when low electrical input energy was induced on the amorphous powders. Furthermore, the critical input energy inducing crystallization of the amorphous phase during EDS is strongly dependent on the size of the sample.     

Fabrication and compressive properties of porous TiAl-Mn intermetallics by powder metallurgical route

We investigated the effects of process parameters on the microstructure and compressive properties of reactive-sintered porous TiAl-based intermetallics. Different extrusion ratios were selected in order to change the as-extruded microstructure and thus obtain various types of porous structures (unidirectional and spherical pores) after reactive sintering. In addition, different environmental conditions, such as vacuum and air, were used during the reactive sintering stage. It was revealed that the compressive strength is influenced by the unidirectionalty of the pores. High unidirectionalty led to higher yield strength at room temparature and at 1073 K. The compressive properties of the porous TiAl intermetallics were also influenced by the environment where reactive sintering is carried out.     

Stress–strain behavior of porous Ni–Ti shape memory intermetallics synthesized from powder sintering

The microstructure and stress–strain behavior of porous Ni–Ti intermetallics fabricated by powder sintering of Ni, Ti and/or TiH₂ are investigated. The pores are small and well distributed in the present porous Ni–Ti alloys and the phase constituent and stress–strain behavior of porous Ni–Ti alloys are significantly influenced by the sintering conditions (sintering temperature and sintering time) and TiH₂ additions. With increasing sintering temperature, the pseudoelasticity (PE) increases while the hysteretic width decreases. A second compression also leads to an increase of PE, elastic modulus and related deformation resistance. Further, the stress–strain behavior of porous Ni–Ti alloys is different in some way from that of bulk Ni–Ti alloys and other porous materials.     

反应合成Ti_3Al/TiC+Al_2O_3复合材料烧结过程热力学分析

将高能球磨后的Ti-Al粉末和TiC,Al2O3粉末混合进行热压烧结,在烧结的过程中反应生成金属间化合物为增强相的复合材料。通过对粉料的X射线衍射分析、热分析(DSC)和烧结体的成分分析表明,最终的金属间化合物只有Ti3Al而没有其它金属间化合物相。通过热力学计算,分析了反应烧结过程并发现在低温由固相间原子扩散控制生成TiAl3,TiAl,Ti3Al的渐进过程,和在高温下金属间化合物的合成机理,而且增强相和基体界面间处于稳定状态。     

Characterization of porous Ti-bioglass composite produced by mechanical milling and space holder sintering

In this study, porous titanium-10 wt% bioglass(BG) composites were fabricated by the process of combining mechanical alloying with space holder sintering. The pore morphology and phase constituents of the milled powders and porous compacts were characterized by scanning electron microscopy(SEM), X-ray diffractometry(XRD), and Fourier transform infrared spectroscopy(FT-IR). The mechanical properties were determined by running compression test. The porosity of the sintered samples shows a downward trend with the increase of milling time. As the porosity increases, both the compressive strength and elastic modulus decrease. The results illustrate that the fabricated porous compacts with high porosity and suitable mechanical properties have the potential application in bone tissue engineering.     

Fabrication of homogeneous tungsten porous matrix using spherical tungsten powders prepared by thermal plasma spheroidization process

In this work, spherical and dense tungsten particles with average size of 13 μm were synthesized by thermal plasma spheroidization process, and were further used to fabricate porous tungsten matrix with homogeneous pore distribution and open pore channel. The influences of sintering temperatures, dwelling time and additive on the microstructure and microhardness evolution of porous products were investigated, and the experimental results show that spherical and dense particles could keep their initial shape and favor the reservation of packed pores with narrow pore size distribution, which exhibits superiority in fabrication of tungsten matrix with uniform pore distribution compared with irregular tungsten powders. Specially, the porosity of porous tungsten matrix could be finely tuned from 25% to 30%, which has obvious effect on microhardness of obtained porous skeleton. The sintering kinetic analysis indicates that grain boundary diffusion is the primary mass transport mechanism during the fabricating porous tungsten matrix process. Furthermore, WCu composites fabricated by spherical powders exhibit higher thermal conductivity than that of irregular powders, which reveals the superiority of spherical tungsten powder.     

Fabrication of a porous material with a porosity gradient by a pulsed electric current sintering process

A porous structure with a porosity gradient can be applied to the preparation of continuous FGM, where liquid or chemical vapor of the second phase is infiltrated into the graded pores. It also has applications in skeletal implant materials and ultrafiltration media. An attempt was made to fabricate a porous material with a porosity gradient by means of a pulsed electric current sintering (PECS) process. The present work describes not only the measured value of the temperature difference between the upper and lower part of the specimen, which brings about a gradual change in pore distribution, but also the sintering characteristics of the porous structure obtained by the pressureless PECS process.