The effect of low melting point phases on the elevated temperature microstructural stability of hot pressed beryllium

The factors governing the growth of BeO particles and beryllium grains in hot-pressed beryllium block have been studied. The size of BeO particles was found to increase as the aluminum, magnesium and silicon contents were raised. Higher processing temperatures during manufacture also increased the size of the oxide. Silicon doping of the beryllium powder before pressing was found to be a particularly effective way of increasing BeO particle size. The pressure-temperature cycle employed during hot pressing was also found to affect the oxide morphology. Cycles that produced full density at the lowest temperature minimized the extent of oxide agglomeration. Diffusion of aluminum and a combination of aluminum, silicon and iron into high purity beryllium at 1366 K produced a grain boundary liquid phase which resulted in an accelerated grain growth rate and BeO agglomeration.     

Factors affecting the tensile strength,elongation and impact resistance of low oxide,hot isostatically pressed beryllium block

The tensile properties nd impact strength of low BeO (0.5 pct), hot isostatically pressed, high purity beryllium have been measured. The effect of annealing and silicon doping have been studied. Both tensile elongation and impact values were increased as the BeO volume fraction was reduced. Silicon doping produced a slight increase in elongation at the expense of strength and a lowering of the grain coarsening temperature.     

Factors affecting the tensile strength, elongation and impact resistance of low oxide, hot isostatically pressed beryllium block

The tensile properties nd impact strength of low BeO (0.5 pct), hot isostatically pressed, high purity beryllium have been measured. The effect of annealing and silicon doping have been studied. Both tensile elongation and impact values were increased as the BeO volume fraction was reduced. Silicon doping produced a slight increase in elongation at the expense of strength and a lowering of the grain coarsening temperature. formerly of Kawecki Berylco Inc.     

The effect of oxide dispersions on the recrystallization of beryllium

The recrystallization behavior of beryllium containing fine BeO dispersions has been studied by following the changes of hardness during annealing and the modifications of the microstructure by optical and transmission electron microscopy. It was found that for equal concentrations of BeO the recrystallization temperature was dependent upon the size of the dispersoid, being higher the finer the median particle size of the oxide. In an alloy with a median size of 600Å a dislocation substructure was maintained up to very near the melting point of beryllium.     

Creep mechanisms in beryllium

Observations of beryllium samples which have been creep tested between 922 K and 1422 K indicate that creep behavior is controlled by the relative strengths of the grain boundaries and the matrix. Since creep deformation can occur predominantly by grain boundary sliding or entirely by deformation within the grains, the creep strength was found to be controlled by the weaker of the two features. Low melting phases containing aluminum and silicon which formed along the grain boundaries acted as stress concentrations which favored localized grain boundary deformation, and recrystallization. Creep resistance was found to drop markedly when the BeO content was reduced substantially below 1 pct.     

惯性器件用铍材微屈服强度的研究

介绍了惯性器件用铍材微屈服强度（ＭＹＳ）的研究概况、测试方法,讨论了晶粒尺寸、杂质（主要是ＢｅＯ）、热处理与微合金化等对铍材ＭＹＳ的影响。结果表明,采用粒径细小低氧化铍的冲击研磨粉末和等静压固结工艺是获得高ＭＹＳ铍材的重要途径。     

Creep mechanisms in beryllium

Observations of beryllium samples which have been creep tested between 922 K and 1422 K indicate that creep behavior is controlled by the relative strengths of the grain boundaries and the matrix. Since creep deformation can occur predominantly by grain boundary sliding or entirely by deformation within the grains, the creep strength was found to be controlled by the weaker of the two features. Low melting phases containing aluminum and silicon which formed along the grain boundaries acted as stress concentrations which favored localized grain boundary deformation, and recrystallization. Creep resistance was found to drop markedly when the BeO content was reduced substantially below 1 pct.     

Recovery and recrystallization in Cold-Rolled Al-SiCw composites

Recrystallization behavior has been studied in 50, 70, and 90 pct cold-rolled silicon carbide whisker-reinforced aluminum composites containing fine aluminum-oxide particles. The distribution of aluminum-oxide particles in the composites was not uniform. Macrohardness measurement, transmission electron microscopy (TEM), and light microscopy were used in the investigation. It was found that in regions with low aluminum oxide content, the introduction of SiC whiskers resulted in recovery reactions during and after cold rolling and in an increase in the growth rate of subgrains during annealing. These recovery reactions were enhanced by an increase in the degree of deformation. The number density of nuclei varied between areas with different contents of aluminum-oxide particles. The impingement of nuclei and recovery reactions limited the growth of nuclei, resulting in a process where recrystallization (growth of nuclei by high-angle grain boundary migration) and extended recovery (growth of subgrains) took place simultaneously. The relative amounts of recrystallization and extended recovery that occur simultaneously affect the recrystallization kinetics as well as the grain size distribution and texture after recrystallization.     

Observations on the evolution of potassium bubbles in tungsten ingots during sintering

The present article describes the evolution of potassium bubbles during sintering of tungsten ingots pressed from doped powder. In the manufacture of incandescent lamp filaments, tungsten powder is produced by reduction of blue tungstic oxide which is doped with potassium disilicate and aluminum chloride. The reduced tungsten particles contain submicron pores. Analytical transmission electron microscopy (TEM) identifed two types of pores in reduced tungsten powder. First, there are pores which contain particles which consist of potassium, aluminum, and silicon, and second, there are pores which contain aluminum and silicon alone. On sintering at 2100 °C or 2300 °C, potassium aluminosilicate particles migrate together with grain boundaries to necks which form between tungsten particles. Sintering at 2100 °C or 2300 °C reduces the potassium, aluminum, and silicon concentrations of the particles. Atomic absorption spectroscopy (AAS) also measured reductions in the bulk potassium, aluminum, and silicon concentrations. In the present study, analytical TEM and Auger electron spectroscopy (AES) were used to describe the decomposition of dopant particles and the evolution of elemental potassium bubbles in sintered ingots.     

针铁矿法处理铁渣的工艺研究

介绍了针铁矿法从硫酸法生产工业氧化铍过程中产生的铁渣中回收工业BeO的原理、工艺条件及技术指标。     

从高氟硫酸铍溶液中萃取铍工艺研究

高氟铍矿石中的成分复杂,其中氟、磷、硅、铝、铁等杂质在传统法工艺中对氧化铍的回收率及质量影响很大;萃取法生产氧化铍过程中,P204对铍浸出液中的阳离子进行交换,浸出液中的阴离子氟、硅、磷等基本不被萃取;利用阳离子在P204中的萃取顺序,将铁还原成二价铁,并在还原气氛中减少铝铁与铍同萃机会。同时高氟铍矿石中的氟与铝的络合作用可抑制铝的萃取,萃入有机相中的铝铁经洗涤后基本可除去。洗后的有机相经反萃、水解沉淀、煅烧得到合格的工业氧化铍。     

针铁矿法处理铁渣的工艺研究

介绍了针铁矿法从硫酸法生产工业氧化铍过程中产生的铁渣中回收工业BeO的原理、工艺条件及技术指标。     

THE INFLUENCE OF SILICON AND OTHER ELEMENTS IN CONTROLLING INTERPARTICLE FRICTION AND SINTERING OF BERYLLIUM POWDER

Abstract

The effect of small additions of activating elements such as silicon on the consolidation behaviour of beryllium powder has been investigated. Evidence is given that compacts of activated powder have more uniform high density than those produced from non-activated material. Studies carried out on prepared beryllium discs show that silicon modifies the micro-structure of the surface layer of beryllium oxide and, in consequence, affects its sliding behaviour and bonding characteristics.

From these results a model is proposed to account for the observations made on both sintered and hot-pressed beryllium which leads to the conclusion that, in addition to interparticle bonding, some measure of metal particle rearrangement is necessary for maximum densification. Activating elements may, in modifying the surface characteristics of the individual powder particles, assist in achieving an improved balance between particle sliding on the one hand and interparticle bonding on the other. In taking into account the bulk consolidation characteristics as well as the micromechanics of the process, the model also explains the observed influence of particle-size distribution on porosity in the compact.

The extent to which friction and sliding can influence compaction has been demonstrated by using a system of coloured Plasticine balls to simulate individual powder particles. Analysis of the behaviour of the Plasticine compacts substantiates the proposed model of the hot pressing of beryllium powder.     

Production of Extra Low Oxide Beryllium Powder by the Rotating Electrode Process

The Rotating Electrode Process (REP) is a method for producing high quality spherical metal powders. REP powder is made by melting the end of a cylindrical bar, which is rotated at high speed about its longitudinal axis, using an arc source for melting. As the end of the bar is melted, metal droplets are ejected by centrifugal force and solidify during flight through a helium atmosphere. The advantage of REP for producing powders of reactive metals is that molten metal is never in contact with other metals or refractories. This factor reduces the likelihood of contamination and makes possible the production of powder that is close to the input material in cleanliness and composition. The plasma rotating electrode process (PREP) is an extension of REP in which the tungsten cathode heat source is replaced by a plasma torch. Spherical Be powders can be produced using either REP or PREP, although extra low oxide powders having BeO contents less than 300 PPM are produced via PREP. Mechanical properties of low oxide PREP beryllium powder consolidated by hot isostatically pressing, while lower than those of commerical low oxide grades, still compare favorably. Yield strengths are in the range of 140 to 204 MPa (20 to 30 ksi) and tensile strengths 173 to 259 MPa (25 to 37 ksi) depending on HIP conditions. Microyield strength is between 34 and 40 MPa (5 to 6 ksi), which is exceptionally high considering the coarse grain si2e and low strength of the material.     

Correlation of microyield behavior with silicon in X-520 and HIP-50 beryllium

Significant differences were observed in the measured values of the microyield strength (MYS) for the X-520 and HIP-50 grades of beryllium. The MYS values for as-received and heat treated HIP-50 samples were in the range of 17 to 27 kpsi (117 to 186 MPa); those for similarly heat treated X-520 samples, however, ranged only from about 8 to 9 kpsi (55 to 62 MPa). Differences were also noted in the value of the strain exponent, which was measured as the slope of the line generated by plotting the stressvs residual plastic strain data on logarithmic coordinates. Strain exponent values of 0.20 to 0.34 were observed for the HIP-50 data compared to the range of 0.38 to 0.48 observed for the X-520 data. Notable differences were also observed between the X-520 and HIP-50 microstructures which appeared to explain the observed differences in microyield behavior. Second phase particles in both the beryllium grades were mainly located at the grain boundaries. However, the average size of the particles in HIP-50 was typically one-half of those in X-520, and their number density was also much higher. The particles were mainly composed of the oxides of Be in HIP-50; however, the X-520 particles almost always contained Si, Fe, and O. X-ray and STEM results showed that the particles in X-520 were predominantly BeO containing possible reacted regions of the Be₂SiO₄ phase. These results indicated that to produce high MYS Be it is important to keep the Si content low. Use should be made of fine impact attritioned powder, and hot isostatic pressing should be used as the densification procedure. The data indicated that Fe content (to the extent present in X-520 Be) was not very significant.     

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.     

Beryllia—Production of Ceramic-Grade Powders

The high thermal conductivity and specific heat of high purity beryllium oxide ceramics at low temperatures make them useful for applications in electronics, automotive and aircraft igniters, and in microwave devices. Production, properties, and health aspects of beryllium oxide (BeO) are described.     

Formation and Sintering Behavior of Nanocrystalline Fe-Ni Powder by Mechanical Alloying

Thepotentialapplicationofnanostructuredma terialsusedasnovelstructuralorfunctionalengi neeringmaterialslargelydependsontheconsolida tionofpowdersbywhichthebulknanostructuredsolidsaremade .Theretentionofthemetastablemi crostructureintheconsolidationprocessismandato ryforpreservingthesuperiormechanical,electricalorcatalyticpropertiesofthematerial.Severalau thorsshowedthatthepressure assistedsinteringisadequateforbothreachingfulldensityandprevent inggraingrowth ,besidesthenanostructuredmateri als…     

Processing and Mechanical Properties of Cast Al (Mg,Mn)-Al2O3 (MnO2) Composites Containing Nanoparticles and Larger Particles

The composites reinforced with nanoparticles result in improved strength and ductility while those containing coarser particles of micron size have limited ductility. The present study investigates the outcome of mechanical properties in a composite reinforced simultaneously with coarse and fine particles. High energy milling of manganese dioxide particles with excess of aluminum powder ensures that nanoparticles generated, either of MnO₂ or alumina, are mostly separate and surrounded by aluminum particles. The milled powder when added to aluminum alloy melt, the excess aluminum particles will melt leaving behind separate oxide nanoparticles without significant agglomeration. Different amounts of milled powder mix have been stirred into molten aluminum alloy where nanoparticles of MnO₂ react with melt to form alumina. The resulting slurry is cast into composites, which also contains coarser (nearly micron size) alumina particles formed by internal oxidation of the melt during processing. The microstructure of the composites shows good distribution of both the size categories of particles without significant clustering. The oxide particles are primarily γ-alumina in a matrix of aluminum-magnesium-manganese alloy containing some iron picked up from the stirrer. These composites fail during tensile test by ductile fracture due to debonding of coarser particles. The presence of nanoparticles along with coarser particles in a composite improves both strength and ductility considerably, presumably due to delay in debonding of coarser particles to higher stress because of reduced mismatch in extension caused by increased strain hardening in presence of nanoparticles in the matrix. The composites containing only coarser oxide particles show limited strength and ductility attributed to early debonding of particles at a relatively lower stress due to larger mismatch in extension between matrix and larger particles. Higher addition of powder mix beyond a limit, however, results in deterioration of mechanical properties, possibly due to clustering of nanoparticles. The present work, however, did not optimize the relative amounts of the different sized particles for achieving maximum ductility.     

A MECHANISM FOR THE PRESSURE-SINTERING (HOT PRESSING) OF BERYLLIUM

Abstract

Previous work on the vacuum hot pressing of beryllium powders is reviewed to illustrate the dominant role of powder chemistry in consolidation behaviour. New electron-microscope evidence is advanced to show that oxides are present on the surface of beryllium powder particles but, in contrast, are not found predominantly at the grain boundaries of hot-pressed compacts.

Finally, a model is proposed incorporating the macro and micro features of consolidation, which involves sintering of the compact followed by recrystallization.