Activated sintering of W-HfC composite materials

The features of consolidation of the particles during the activated sintering of tungsten powders with different values of dispersity (d _av = 2–3 and 0.8–1.0 μm) are investigated. Sintering was activated by introducing nickel additives (up to 0.5 wt %), tungsten nanoparticles (up to 30 wt %), and finely dispersed hafnium carbide (5–30 vol %) with subsequent milling in a vibrating mill. The uniaxial compaction of the samples has been performed under pressures from 50 to 1000 MPa, and sintering was performed in vacuum at 1850°C with holding for 1 h. It is shown that the additives of tungsten carbide increase the density of sintered billets and, in combination with dispersed hafnium carbide, tungsten-based composite materials with a grain size up to 2 μm can be obtained.     

Effect of scandia on tungsten oxide powder reduction process

Scandia doped tungsten powders were prepared by spray drying combined with two-step hydrogen reduction. The particle size of doped tungsten powder, powder morphology and doped tungsten matrix were characterized by scanning electron microscope, X-ray diffraction and laser diffraction particle size analyzer, respectively. The reduction behavior of Sc₂O₃ doped tungsten oxide and the effect of Sc₂O₃ on the property of tungsten powder were studied by the temperature programmed reduction. The experimental results showed that the precursor powders prepared by spray drying had spherical shape. The addition of Sc₂O₃ could decrease the reduction temperature of tungsten oxide. The scandia doped tungsten powder had sub-micrometer size in the range of 0.1 to1 μm and scandium distributed evenly in the powder. By using this kind powder, sub-microstructure cathode matrices with semispherical grains and homogenous distribution of scandium were obtained.     

Use of powders of the tungsten—Rhenium alloys for the preparation of impregnated cathode skeletons I. Densification and formation of porous structure in sintering of powders of tungsten-rhenium alloys

The effect of phase composition, fineness, and morphology of W-Re powders with different rhenium content on compaction, and formation of porous structure has been studied. Fine particle powders with a specific surface of 5–8 m²/g, mechanical mixtures of tungsten and rhenium powders, and W-Re alloy powders were used. Local nonuniform compaction is observed during sintering of powder mixtures and alloy powders. It was established that alloy powders and intermetallic powders have lower sintering activity.     

ELEVATED-TEMPERATURE COMPACTION OF METALS AND CERAMICS BY GAS PRESSURES

Abstract

Techniques have been developed for the isostatic compaction of metals, cermets, and ceramics with no appreciable variation in density throughout the compacted structure. The temperatures needed are much lower than those normally utilized for sintering, so that a fine-grain, tough structural material can be produced. The starting material may be loose powder, cold-pressed preforms, explosively impacted preforms, coated particles, spherical particles, or vibratory-packed powder.

Toxic materials are easily handed. Since the process is quite adaptable to the preparation of complex shapes by direct compaction of powders, components can be produced from the more expensive materials, such as beryllium and tungsten, with a minimum loss of material during processing.     

Porous tungsten with controlled porosity by low temperature sintering

Abstract

Porous tungsten as a high current density cathode is one of the important applications of the metal, which is mostly used in high temperature conditions due to its exceptional resistance to melting (T _m = 3410±20°C). Its porous form has been a crucial component of dispenser cathodes used in electronic valves and high power lamps. Porous tungsten skeleton forms the matrix, which is then impregnated with an electron emissive compound. Upon every emission from the surface, new material has to be fed into the surface pores via the open pore channels. Hence it may be proposed that a uniform porosity is needed for a better performance. However, a controlled porosity has not been achieved yet. Moreover, sintering of tungsten has always been difficult due to the extreme process conditions. A high sintering temperature (T _s≥2000°C) and a strong reductive atmosphere (hydrogen) have been the absolute necessity in making these parts. This study further explores an alternative sintering technique being developed. The idea is based on the reactive sintering concept. The energy output from the exothermic reactive system of tungsten oxide and aluminium has been the heat source for sintering porous tungsten. As a result, sintering temperature and time have been reduced considerably. Higher homogeneity, thus more uniform pore distribution, was observed. A better control of porosity related to the pressing and sintering conditions was achieved by the characterisation method previously developed. Microhardness has been a useful monitor of the scatter in porosity of the parts. Throughout the study, SEM was used to observe the porous structures and powder morphologies. DSC and XRD were useful to follow the microstructural evolution in the reactive system.     

The properties of tungsten processed by chemically activated sintering

Two tungsten powders have been treated with small concentrations of sintering activators to provide for enhanced low temperature sintering. The experimental study focused on the determination of the processing effects on properties such as sintered density, grain size, hardness, and strength. Variables in the plan included tungsten particle size, type of activator, amount of activator, compaction pressure, and sintering temperature. The sintered density is found to have a dominant effect on strength and hardness. The various processing variables are analyzed in terms of their effects on density. At high sintered densities, grain growth acts to degrade the strength. Additionally, the nature of the sintering activator influences the fracture strength. In this study optimal strength occurred with a 0.7 μm tungsten powder treated with 0.29 wt pct Ni, sintered at 1200 °C for one hour. The resulting density was 18.21 g/cm³, with aR _A hardness of 69 and a transverse rupture strength of 460 MPa.     

Mechanism of oxide adherence on Fe-25Cr-4Al (Y or Sc) alloys

Oxidized alloys of Fe-25Cr-4Al, some containing small additions of elements (Yttrium or Scandium) which strongly promote oxide adherence, have been subjected to extensive structural studies by scanning, replication, and transmission electron microscopy as well as other techniques. The morphological details of the Al₂O₃ scales, which develop on these alloys during high temperature oxidation, and the structural changes at the oxide-substrate interfaces are discussed with respect to oxidation kinetics and oxide adherence. The oxide scales grow into the alloys at rates consistent with the diffusion of oxygen down grain boundaries of the fine-grained scales. Yttrium and expecially scandium additions tend to increase oxidation rates by providing rapid diffusion paths in the form of yttria and scandia stringers. More importantly, the morphological data led to the conclusion that the spallation of nonadherent scale is caused by distributions of small voids at the oxide-substrate interface. Consistent with this conclusion, it is found that the oxide scale is tenaciously adherent when voids are absent, as is the case when yttrium or scandium is added to the alloy. It is maintained that these alloy additions promote oxide adherence by forming vacancy complexes with excess vacancies or by providing internal oxide boundaries for the condensation of excess vacancies, which would have otherwise condensed at the oxide-substrate interface and formed the voids responsible for oxide spallation. The role of vacancy sinks is further confirmed by showing that oxide adherence is promoted in Fe-25Cr-4Al, in the absence of yttrium of scandium, by a distribution of Al₂O₃ particles. This result indicates that particles, which have been used in engineering alloys for strengthening purposes, can also promote oxide adherence. The morphological data are also discussed with respect to other mechanisms that had been proposed for oxide adherence. J. K. TIEN, formerly Senior Research Associate, Materials Engineering and Research Laboratory, Pratt and Whitney Aircraft, Middletown, Conn.     

Phase transformations in reactive sintering of tungsten

Abstract

It has been demonstrated recently that tungsten (T _m = 3410±20°C) can be sintered by reactive sintering in a reductive atmosphere such as hydrogen. This alternative technique to the conventional sintering (T _s>2000°C) makes use of a small amount of aluminium addition which acts as a sintering aid and hence lowers the sintering temperature significantly (T _s1200°C). This study explores the phase transformations that take place during reactive sintering of tungsten in view of the mechanisms involved. DSC, SEM and TEM have been used for a fundamental understanding of this system.     

Effect of Al2O3 and B4C particle additions on microstructure of PM copper based brake linings

Abstract

This paper highlights the effect of different ceramic particles on the structure of PM copper based brake linings. The copper based brake linings using a range of ceramic additives (1–6 wt-%) were prepared by powder metallurgy (PM). The optimum conditions for the production of brake linings were determined as compaction under 400 MPa and sintering at 805°C for 20 min in an argon atmosphere. The density of copper based brake linings decreased after sintering with the increase in ceramic powder contents for both Al₂O₃ and B₄C ceramic particles. The microstructural characterisation of produced samples showed that the lower boiling point elements in the as supplied powder vaporise during sintering from the structure and this leads to an increase in the porosity amount of the final component.     

Compaction Kinetics with Liquid-Phase Sintering of W – Ni – Sn Pseudoalloys

A kinetic equation for compaction with liquid-phase sintering is found on the basis of rheological sintering theory. Kinetic coefficients of the equation are determined from physicochemical properties of a dispersed system and its components. Compaction kinetics are studied for the system W – Ni – Sn where the liquid phase is eutectic alloy Ni – Sn with the greatest nickel content. It is shown that experimental results are in good agreement with those obtained by the kinetic equation on the coordinates porosity – time. Diffusion coefficient is evaluated for tungsten in melt (67.5 mass% Ni + 32.5 mass% Sn): at 1200°C it is 0.113·10_–5 cm₂/sec for grain diameter L ₀ = 3 μm. Time dependences are also obtained for the compaction rate and viscosity of the pseudoalloys studied.     

DISSIMILAR-SURFACE CONTACTS AND THEIR INFLUENCE ON SINTERING CHARACTERISTICS OF POWDER COMPOSITE COMPACTS

Abstract

In compacts containing metal and oxide particles, metal/oxide surface contacts modify the diffusional processes. Different distributions of such dissimilar-surface contacts and their influence on the sintering characteristics of some metal-oxide compacts are considered.

By selecting appropriate volume fractions and diameters of matrix and second-phase particles, structures containing different types of dissimilar surface contact distribution have been produced. Isochronal sintering of these structures clearly demonstrates that (a) the presence of chemically inert second-phase particles always hinders densification of the composite, (b) the sintering densification (for a given volume fraction of second-phase particles) is a strong function of (d_m/d_s), where d_m and d_s are the diameters of the matrix and second-phase particles, respectively, and (c) the nature of dissimilar-surface contact distribution has a significant effect on the densification of the composite.     

Effectiveness of reducing agents during sintering of Cr-prealloyed PM steels

Abstract

Development of strong inter-particle necks requires successful removal of surface oxides, present on the powder particles, during the initial stages of sintering. In the case of water-atomised powder prealloyed with chromium, the surface oxide consists mainly of an iron oxide layer with some more stable fine particulate oxides. The formation of sufficiently strong inter-particle necks requires as a minimum full removal of the iron surface oxide layer. This can be achieved by gaseous reducing agents (e.g. H₂, CO or a mixture of both) or by carbon, typically admixed in the form of graphite. The reducing power of various sintering atmospheres (active gas content ≤10 vol.-%) and their combined effect with graphite has been investigated by a thermal analysis technique. Results indicate that a combination of a dry hydrogen-containing atmosphere and fine graphite allows successful sintering of chromium alloyed PM steels.     

多元复合稀土钨电极的制备及其焊接特性研究

本论文采用机械混合法结合二段氢气还原成功制备出稀土掺杂钨粉。还原后的稀土钨粉冷等静压压制成钨棒后,采用中频感应烧结法制备出稀土钨电极。实验结果表明,制备的稀土钨粉,由于稀土元素的存在,降低了钨粉的粒度。稀土元素以稀土氧化物的形式存在。与垂熔法相比,采用中频感应烧结后的钨棒机械加工性能优异。电极成品率提高了5%,能耗则降低了60%以上。并且制成电极后的焊接性能得到了进一步提高。     

DENSIFICATION PROCESSES IN THE TUNGSTEN CARBIDE-COBALT SYSTEM

Abstract

Densification of the tungsten carbide-cobalt system has been investigated by determining the effect of the principal sintering variables-composition, temperature and time of sintering, particle size, ball-milling-and by studying the processes that occur. Considerable shrinkage takes place during heating, before the eutectic temperature is attained. A 9% cobalt alloy sintered entirely in the solid state to give comparable density and mechanical properties to those attained by liquid-phase sintering, but the sintering time was increased by a factor of 10. Densification proceeds from nuclei created by ball-milling, which packs the porous cobalt agglomerates with tungsten-carbide particles; if the cobalt particles are only mixed with the tungsten carbide, then on sintering they flow out into the matrix leaving behind voids that do not fill. Densification is characterizedby two features: first, tungsten-carbide particles cement together with cobalt between grains to form clusters and filaments; secondly, the clusters and filaments contract. The solubility of the tungsten carbide in the cobalt is important, since densification occurs far less rapidly when copper is used as the binder phase. Shrinkage can virtually cease before the compact is fully dense, either because voids form as a result of unsatisfactory mixing, or, with a small amount of cobalt, because the periphery of the compact sinters to full density before the interior, preventing further overall densification.     

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.     

Extraction of Scandium and Zirconium from Their Oxides during the Electrolysis of Oxide–Fluoride Melts

The main features of scandium and zirconium extraction from their oxides to aluminum during the aluminothermic and electrolytic preparation of Al–Sc and Al–Zr alloys and master alloys in the KF–AlF₃, NaF–AlF₃, and KF–NaF–AlF₃ oxide–fluoride melts with Sc₂O₃ and ZrO₂ additives are studied. The influence of the melt composition and temperature, the synthesis time, the contents of oxides Sc₂O₃ and ZrO₂ in the melts, the mechanical stirring of aluminum, and the cathodic current density on the contents of scandium and zirconium in aluminum and on their extraction from the oxides is determined. The average values of scandium and zirconium extraction are 20–75 and 40–100%, respectively, depending on the synthesis parameters. The electrolytic decomposition of the oxides in the KF–AlF₃, NaF–AlF₃, and KF–NaF–AlF₃ melts results in the enhancement of scandium and zirconium extraction to aluminum. The parameters of the preparation of Al–Sc and Al–Zr alloys and master alloys with the scandium content to 10 wt % and zirconium content to 15 wt % during the electrolysis of oxide–fluoride melts are chosen as a result of the results obtained.     

添加成形剂对钢结硬质合金锻造性能的影响

通过对添加和未添加成形剂的粉末的流动性、氧化程度的测定及压制压力的调整，证明加入成形剂可减少压制和烧结时钢结硬质合金毛坯出现的缺陷，提高其锻造性能。分析论述了烧结过程中烧结坯内部所形成的孔隙，在锻造时对大晶粒WC的碎裂及钢基体塑性流动所产生的影响。     

Effect of the method of introduction of Y2O3 into NiAl-based powder alloys on their structure: I. Agitation in a ball mill

The effect of the sintering temperature (1100–1400°C) of NiAl alloy samples with oxide Y₂O₃ produced by hydrostatic pressing on their structure and phase composition and the distribution of oxide particles in a NiAl-based intermetallic matrix alloyed with ～0.5 at % Fe is considered. It is found that dispersed oxide particles in the compact material prepared from a mixture of oxide Y₂O₃ powder and a NiAl alloy (produced by calcium hydride reduction of a mixture of nickel and aluminum oxides) powder in a standard ball mill are nonuniformly distributed in the volume. The morphology of oxides changes during sintering: sintered samples contain rounded particles, which differ strongly from the clearly faceted angular particles of oxide Y₂O₃ added to a mixture (they represent conglomerates of single crystals). In the sintered samples, large aggregates of oxides are revealed along grain boundaries. Mass transfer is possible at the NiAl/Y₂O₃ interface in the system: it leads to partial substitution of aluminum and/or iron atoms for yttrium atoms in the Y₂O₃ lattice and to the formation of submicroscopic particles of (Fe,Al)₅Y₃O₁₂-type oxides.     

AN ELECTRON-MICROSCOPE AND X-RAY INVESTIGATION OF THE MILLING OF TUNGSTEN CARBIDE/COBALT MIXTURES

Abstract

It is well known that some interaction takes place between the two components when mixtures of cobalt and tungsten carbide are milled. To gain a further insight into this phenomenon, the milling process has been studied by means of the electron microscope and by the BET and X-ray methods.

During the milling of cobalt powder the number of stacking faults and the amount of the hexagonal phase both increase. On milling 80:20 tungsten carbide/cobalt mixtures increasing agglomeration of cobalt and fine tungsten carbide particles was found with rise in milling intensity. The larger tungsten carbide particles appeared to have a smooth surface. However, if the cobalt was dissolved in hydrochloric acid, the true surface of the tungsten carbide particles was revealed. This became rougher with increasing milling intensity. From this it can be concluded that, during milling, cobalt settles between the surface irregularities.

The variation in distribution of the cobalt that results from different milling conditions leads to a difference in sintering behaviour. A dilatometric study has been made of this aspect.

The original particle size of the cobalt used for hard-metal mixtures does not affect the properties of the sintered product, if the mixtures are milled very intensively.     

Dynamic deformation behavior of an oxide-dispersed tungsten heavy alloy fabricated by mechanical alloying

The objective of this study is to investigate the dynamic deformation and fracture behavior of an oxide-dispersed (OD) tungsten heavy alloy fabricated by mechanical alloying (MA). The tungsten alloy was processed by adding 0.1 wt pct Y₂O₃ powders during MA, in order to form fine oxides at triple junctions of tungsten particles or at tungsten/matrix interfaces. Dynamic torsion tests were conducted for this alloy, and the test data were compared with those of a conventional liquid-phase sintered (LPS) specimen. A refinement in tungsten particle size could be obtained after MA and multistep heat treatment without an increase in the interfacial area fraction between tungsten particles. The dynamic test results indicated that interfacial debonding between tungsten particles occurred over broad deformed areas in this alloy, suggesting the possibility of adiabatic shear-band formation. Also, oxide dispersion was effective in promoting interfacial debonding, since the fine oxides acted as initiation sites for interfacial debonding. These findings suggest that the idea of forming fine oxides would be useful for improving self-sharpening and penetration performance in tungsten heavy alloys.