Interrelationship between the properties of powders,the conditions of pressing and sintering,and the structure of porous niobium

Conclusions With an increase in niobium powder particle size the pressability increases while the formability decreases. There is a linear relationship between the normalized radius of the pores in high-porosity niobium powder specimens and the normalized volume of the pores. This is an indication of geometric similarity of the disposition and form of the pores in specimens pressed from niobium powders with different degrees of dispersion.The product of the coefficient of side pressure and the coefficient of friction on the die wall is 0.053 ± 0.10. This means that with reasonable values of the coefficient of friction of dispersed niobium powder on the wall of the steel die is 0.14.Translated from Poroshkovaya Metallurgiya, No. 3(327), pp. 30–33, March, 1990.     

Ni-Cr-Co-W-Mo-Al-Ti合金制备多孔材料的组织与性能研究

用粉末烧结法制备了孔结构为球形中空孔和线型中空孔的镍基多孔高温合金材料.对试样进行显微组织观察和力学性能测试.结果表明:制备的多孔高温合金材料的孔隙分布均匀,孔径大小一致.通过高温烧结,多孔合金骨架处的金属颗粒之间形成了烧结颈,发生了烧结结合.生成孔的孔隙度随造孔剂(尿素)的添加量增加而增加,当造孔剂的质量分数为40%时,可得到孔隙度为81.62%的球形多孔材料.多孔材料具有优良的能量吸收性能,其压缩性能随孔隙度和孔径的增加而下降.     

Effect of heat treatment of the densification behavior of porous bodies from fine tungsten powders during sintering

Conclusions It has been established that the fall in the activity of a tungsten powder produced by reduction at 950°C and annealed, before pressing, at a temperature of 1700°C or higher is due not to growth of its particles during the annealing but to a decrease in the internal porosity of the particles and their agglomeration and spheroidization. With single-fraction as-reduced and annealed powders their is a correlation between the maximum size of pore channel constructions in a compact and the letter's sintering behavior. Prior heat treatment of tungsten powders produced by reduction at an end temperature of 700°C exerts a nonmonotonic influence on their sintering activity. Shorttime (=30 sec) sintering of compacts from such tungsten powders has been found to result in the formation of a new porous structure, with enlarged maximum pore channel constrictions, responsible for their sintering behavior. A correlation has been discovered between the volume shrinkage of compacts and the maximum size of constrictions in the enlarged capillaries.Translated from Poroshkovaya Metallurgiya, No. 11(227), pp. 25–31, November, 1981.     

Swelling during sintering of titanium alloys based on titanium hydride powder

Abstract

Compacts were prepared by pressing titanium and titanium hydride powders mixed with nickel powder and sintering under vacuum. Severe swelling was observed only for compacts based on TiH₂ powder. Pressure changes in the vacuum furnace, dilatometry results and mass loss data all indicate that dehydrogenation of TiH₂ powder compacts occurs at lower temperature than any significant sintering. Swelling appears to have been caused by a contaminant in the TiH₂ powder rather than hydrogen. The onset of severe swelling during heating was associated with the formation of liquid phase as the solidus was crossed. However, some swelling appears to take place under solid state sintering conditions. Various results indicate that the mechanism of swelling is high gas pressure within closed pores. Large pores appear to form by breakage of ligaments between small pores followed by opening of the pore. It appears that the use of (uncontaminated) TiH₂ powder in place of Ti powder would allow the benefit of lower green porosity to be retained during sintering to achieve low sintered porosity.     

A theoretical study of grain growth in porous solids during sintering

The processes of grain boundary migration, pore drag and pore/boundary separation are described on the basis of the phenomenological equations for boundary migration and surface diffusion. Cylindrical pores on triple grain junctions are assumed to represent the open porosity during intermediate-stage sintering. It is found that cylindrical pores can hardly detach from migrating boundaries. Three-dimensional closed pores, however, which predominate during final stage sintering, can separate from migrating grain junctions. The separation process is modelled numerically and the conditions for separation are formulated. Analytical approximations for the pore mobility are shown to describe the numerical results well. They serve to establish effective mobilities of grain boundaries bearing pores in various configurations. Classical theories of grain coarsening are modified by using these effective mobilities. Mechanical constitutive models of sintering contain the grain size as an internal variable. The present analysis leads to an evolution equation for the average grain size, which depends on the volume fraction of the pores and on their configuration.     

Variation of the porous structure and leakage currents of anodes of niobium condensers in electrolytic oxidation

Conclusions Shaping of the anodes results in a large reduction of the total volume and dimensions of the pores. The magnitude of this reduction is determined unambiguously by the dispersion of the initial powder and oxidation voltage. Shaping results in the formation of a certain number of closed pores.Formation of closed porosity is accompanied by a large reduction of the leakage currents. This is explained by a reduction of the number of impurity atoms concentrated mainly on the surface of sintered anodes. Therefore, the high-dispersion powders which contain, per unit mass, approximately the same amount of impurities as the standard powders can be used to produce anodes with permissible leakage currents at low sintering temperatures.Translated from Poroshkovaya Metallurgiya, No. 3(339), pp. 56–59, March, 1991.     

Relationship between the specific charge of nobium anodes and parameters of their porous structure

Conclusions The specific surface of the anodes after shaping is determined unambiguously by the total volume of the pores and their mean size.The experiments confirm the direct proportional relationship in the specific surface of the pores of the high-porosity niobium anodes and their specific charge. A model of the porous structure of the anodes was proposed. At the given porosity of the sintered anodes, the model can be used to calculate the dependence of the specific surface of the pores after shaping on their radius prior to shaping at different oxidation voltages, and estimate the optimum size of the pores prior t shaping and the maximum specific surface after shaping. The validity of the model was confirmed by means of experiments.Translated from Poroshkovaya Metallurgiya, No. 5(341), pp. 61–64, May, 1991.     

Correlation between the particle size,pore size,and porous structure of sintered tungsten

Conclusions It has been established that the maximum size of pore channel constrictions D₁ is close to the mean size of pore sections in microsections of porous skeletons (-=22–44%) from tungsten powders of 1- to 5-m mean particle size. A rapid method of assessment of an integral fineness characteristic of a tungsten powder is proposed consisting in the determination of the pore size D₁ in a compact (-_c=25–45%), followed by the calculation of the mean size of agglomerated particles with Kozeny's formula. The densification of compacts from tungsten powders of 4-m particle size at sintering temperatures of about 0.6 T_melt is a result of decrease in the number of pores and increase in the equivalent size of agglomerated particles. In this process the mean pore section size determined by the metallographic method remains unchanged during sintering, which corresponds to a statistical model of a porous solid in the rheological theory of sintering.Translated from Poroshkovaya Metallurgiya, No. 12(312), pp. 24–31, December, 1988.     

Microstructural changes in Mo steels during sintering and effect on electrical conductivity

Abstract

An electrical conductivity measurement method was used for studying the sintering mechanism and microstructural changes of low alloyed PM Mo steels in a temperature range between 600-1300°C. The influences of alloying method (elemental or prealloyed), Mo content (1·5 and 3·5 wt-%), and sintering temperature were investigated. The results show that the effects of, for example, formation of Mo carbide(s), ferrite-austenite phase transformation, as well as liquid phase formation during heating of the steel compacts can be detected by the technique cited. Mo dissolution during sintering of compacts from mixed powders results in a decrease of the conductivity with increasing sintering temperature while compacts from Fe-Mo prealloyed powders exhibit the standard behaviour of higher conductivity after sintering at higher temperature. Moreover, the relationship between Mo dissolution, formation of sintered contacts, and mechanical properties was demonstrated to assess the viability of the conductivity measurement method for studying the sintering behaviour of PM materials and its influence on physical and mechanical properties. An approach was also demonstrated for relating the conductivity to the microstructural parameters, e.g. total porosity and contiguity between solid phase, that would be useful for predicting relative changes in mechanical properties dependent on porosity and pore morphology.     

Pore structure of sintered niobium after reaction with hydrogen and nitrogen

Abstract

Compact structures obtained by vacuum sintering of niobium powder are widely used in electronic engineering as metal plates in oxide semiconductor capacitors, which realise electrical capacity in an Nb anodic, Nb₂O₅ solid state MnO₂ electrolyte metal dielectric semiconductor Nb–MDS system. This paper presents the method for stabilising porosity in these structures by using both Nb powder hydrogenation and nitriding of niobium hydride sintered pellets for their production. The effect obtained is explained in terms of the sintered powder porous structure.     

The influence of gas atmospheres on the first-stage sintering of high-purity niobium powders

Niobium and tantalum surfaces easily absorb oxygen. With decreasing particle size the content of oxygen increases. The role of this surface oxygen and oxygen in the sintering atmospheres on the first-stage sintering is not well established. Therefore the sintering behavior of high-purity niobium powders was studied by annealing cylindrical powder compacts (particle size <63 μm) in the temperature range from 1000°C to 1600°C in ultra-high vacuum and under low oxygen partial pressures, as well as in inert gas atrnospheres with low oxygen contents. The specific surface of the samples was determined by metallographic methods, adsorption, and capacitance measurements. Low oxygen partial pressures (10^-3 Pa) lead to a slight enhancement of the surface diffusion which is controlling first-stage sintering. High heating rates (0T > 3000 min^-1) to temperatures above the melting point of Nb₂O₅ (Tm = 1495 °C) enhances the neck growth due to the formation of a liquid oxide phase on the surface of the powder particles. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME     

THE PROPERTIES OF POROUS NICKEL PRODUCED BY PRESSING AND SINTERING

Abstract

The effects of compacting pressure and of sintering temperature and time on the properties of porous sintered nickel compacts have been studied, using three carbonyl and two reduced nickel powders. For all five powders, the density of the green compacts and the porosity of the sintered compacts were linearly related to the log compacting pressure. Similar relationships with pressure were observed for strength and electrical conductivity.

Photomicrographs of sections through the sintered compacts made from the reduced nickel powders show that there are pores in two different size ranges, originating from the porosity between the original powder particles and the pores within the particles. It is concluded that sintered compacts from all five powders containing 40–50% porosity have adequate strength and conductivity for use in fuel-cell electrodes.     

Some aspects on porous properties of iron oxides containing foreign oxides reduced by hydrogen

The objective of this study is to prepare suitable iron ores for industrial reduction processes. Therefore, the properties of porous, green and indurated hematite compacts as well as iron ore were investigated by quantitatively measuring pore volume, BET surface area and pore size distribution. Furthermore, the influence of foreign oxides on porous properties was investigated. Based on these data, the mechanism of sintering process of the compacts was discussed. The results obtained here suggest that the porosity of the compacts after almost 100 pct reduction does not depend upon whether the compacts were indurated or not. Therefore, it would be concluded that indurating the compact is not necessary for pulverized iron ores containing foreign oxides when reduced in the process such as a fluidized bed system.     

Microstructural modelling of electrical conductivity and mechanical properties of sintered ferrous materials

Abstract

The role of microstructure on mechanical properties of sintered ferrous materials was studied using a method based on electrical conductivity measurement. The method was accompanied by quantitative fractography to evaluate the dewaxing and sintering process in iron compacts. The effects of manufacturing parameters, such as compacting pressure in the range of 150–800 MPa, sintering temperature from 400 to 1300°C, sintering time up to 8 h, and lubrication mode were investigated. Several mathematical models were checked to obtain the best one for prediction of electrical conductivity changes as a function of manufacturing parameters. The mechanical properties of the sintered compacts were also evaluated to establish a relationship between conductivity, total porosity, pore morphology, and mechanical behaviour. The results show that the electrical conductivity/resistivity of sintered materials is closely related to its microstructure, so that measuring these properties can replace destructive test methods for prediction of mechanical strength of sintered materials with homogeneous matrix microstructure. The application of the method is shown for sintered Fe, Fe–0·8%C, and Fe–1·5%Mo–0·7%C compacts.     

On the swelling of silver powder during sintering

Silver powder compacts may suffer an overall expansion during sintering. A simultaneous analysis of dimensional changes, mass loss and pore closing in the course of sintering showed that volume change results from the competition between classical sintering phenomena inducing densification and material creeping under stresses caused by internal gas pressure, which induces swelling. The internal gas includes air trapped during pressing and oxygen released from particle surface inside the pores that closed during pressing or sintering. This swelling phenomenon is particularly strong with powders with large specific surface area and compacts with high green density. It is less effective during fast sintering, as observed during direct induction heating experiments. Electrical conductivity is strongly affected by swelling. Powder compacts may exhibit a lower conductivity after sintering when interparticle bonding does not balance the density decrease occurring during sintering.     

NaCl造孔剂添加量对多孔NiTi合金孔结构和力学性能的影响

选用Na Cl作为造孔剂,采用压制+烧结法制备孔结构和弹性模量可控的多孔Ni Ti形状记忆合金,采用SEM,XRD和形状回复率检测等测试手段研究造孔剂添加量对Ni Ti形状记忆合金的孔结构和力学性能的影响。结果表明:随Na Cl添加量增加,多孔体孔隙率从39%上升到72%,孔径大于50μm的孔隙数量明显增加;多孔体主要由Ni Ti奥氏体相(B2)和马氏体相(B19′)组成,并存在少量Ni Ti2,Ni3Ti和Ni4Ti3等相;合金的弹性模量随造孔剂的添加从30%时的10.8 GPa下降到70%时的1.5 GPa;当添加量为50%时,多孔体孔隙分布均匀,大于50μm的孔隙占45%,弹性模量为4.8 GPa,形状回复率达到最高值83%,最适合多孔植入体的要求。     

Recrystallization during the sintering of niobium and titanium carbide powders

Summary During the sintering of loosely poured niobium and titanium carbide powders in their homogeneity regions at temperatures above 1600°C, collective recrystallization takes place. The observed changes in the rate constants of grain growth and in the energy of activation for collective recrystallization are related to changes in the electronic structures of the carbides in their homogeneity regions.Translated from Poroshkovaya Metallurgiya, No. 7 (79), pp. 30–35, July, 1969.     

Effect of hydrostatic pressure on deformation,damage evolution,and fracture of iron with various initial porosities

This study evaluated the effects of superimposed hydrostatic pressure (138 to 1104 MPa) on densification and plastic flow behavior of porous iron (0.3 to 11.1 % porosity). Pressurization alone caused densification of the porous iron with the effect being most pronounced when the porosity was greater than 3.7% and the pressure above 276 MPa. For the porosities studied, densification as a result of pressurization increased with hydrostatic pressure and initial porosity. The 0.3% porosity iron was the only one whose density did not increase with pressurization or deformation under pressure. The effect of hydrostatic pressure on the flow stress of porous iron was small when densification resulting from pressurization was not a factor. The ductility was found to increase linearly with pressure and the effect of pressure on fracture strain increased with the initial porosity of the iron. Evaluation of the effect of hydrostatic pressure on development of porosity and growth during tensile deformation was limited to hydrostatic pressures of 138 and 276 MPa and iron compacts with initial porosities of 0.3, 1.5, and 3.7% because of the pressurization effects. It appeared that the porosity at fracture was similar in these compacts at both pressures but it was much larger than that observed at 0.1 MPa. The greater ductility of the iron compacts tested under hydrostatic pressure results from a decrease in the growth of pores with deformation and from a greater damage tolerance prior to fracture. As observed for porosity, the average maximum pore diameters at fracture for the compacts tested under pressure were similar but larger than those observed at 0.1 MPa. It appears that a general model of ductile fracture for porous materials cannot be based solely on a critical degree of dilation or on maximum pore extension as a fracture criterion.     

粉末烧结法制备开孔泡沫铝压缩性能的研究

采用粉末烧结工艺制备开孔泡沫铝并研究了其压缩性能，不同形态的尿素和氯化钠颗粒作为造孔剂使泡沫铝的孔隙度控制在70％。结果表明：粉末烧结法制备的泡沫铝呵以容易地控制孔隙度及孔径的大小，并且孔结构很好地保持了造孔剂的形状。不同的孔结构对泡沫铝的压缩性能具有显著影响，球形孔结构得到了最佳的压缩效果。