Microstructures of explosively consolidated rapidly solidified aluminium and Al-Li alloy powders

The microstructures and the characteristics of water-atomized, nitrogen gas-atomized Al powders and ultrasonic argon gas-atomized Al-Li alloy powder were investigated by means of metallography, SEM, Auger electron spectroscopy and X-ray diffraction techniques. Rapidly solidified powders were explosively consolidated into different sized cylinders under various explosive parameters. The explosively consolidated compacts have been tested and analysed for density microhardness, retention of rapidly solidified microstructures, interparticle bonding, fractography and lattice distortion. It is shown that the explosive consolidation technique is an effective method for compacting rapidly solidified powders. The characteristics of surface layers play a very important role in determining the effectiveness of the joints sintered, and the Al-Li alloy explosive compacts present an abnormal softening appearance compared to the original powder.     

Readily superplastic forging at high strain rates in an aluminium-based alloy produced from nanocrystalline powders

A high-strain-rate superplasticity at strain rates from 10^–1 to 1 s^–1 (which was many orders of magnitude higher than the strain rates in typical commercial superplastic alloys) was found in a new aluminium-based crystalline alloy, as-extruded Al-Ni-Mm crystalline and Al-Ni-Mm-Zr crystalline alloys (Mm = misch metal) fabricated by warm consolidation of their amorphous or nanocrystalline powders. The alloys were developed with very fine grained structures less than 100 nm in size, with a uniform distribution of both the Al₃Mm and the Al₃Ni particulates which were 70 nm in diameter. As a result of these specific microstructures, these alloys have superior mechanical properties at room temperature; for example the tensile strength is greater than 800 MPa and the Young's modulus is equal to 96 GPa. As-extruded workpieces of the alloys could be forged superplastically at a commercial production speed (less than 1 s) into complicated components with a con-rod shape, which exhibited good postdeformation mechanical properties.     

Low temperature synthesis of silver-palladium alloy powders internal electrodes for multilayer ceramic devices

Physical and electrical properties of three types of Ag-Pd pastes, which consist of different metal fine powders, i.e., a coprecipitated powder, an agglomerated alloy powder made by heat treatment and a pulverized alloy powder produced by improved pulverization method, have been studied. The paste prepared from pulverized alloy powder showed a higher film packing density (6.3 g/cm³) than those made of the other powders. The film consisting of pulverized alloy powder showed a lower expansion at around 500 °C, a lower shrinkage from 700 °C to 1100 °C and a lower electric resistivity. The results indicated that the paste which consists of an pulverized Ag-Pd alloy powder was superior in performance to the other two pastes for an internal electrode material of multilayer ceramic device.     

Rapid forming of superplastic aluminium alloy 5083

Abstract

Decreasing the cycle time significantly for forming the commercially available superplastic aluminium alloy 5083 has been achieved. Forming results and conditions are compared with previous relevant works which are actually scarce. A circular cup having a depthdiameter ratio of 1:2 can be formed in 70 s. This ratio requires flat sheet to be stretched in area by up to three times, which should be large enough when dealing with actual industrial sheet forming. On average, the thickness is decreased by two-thirds; in fact, the thickness distribution is not uniform and the gradient is concentrated at the wall of the cup. The location of minimum thickness in rapid forming is different from that in conventional forming. Disregarding the traditional approach, the pressure-time profile employed in this work was not restricted to yield the so called optimum strain rate, which is usually low. Following the same processing profile, but proceeding in stages of partial forming, a series of progressive forming configurations was obtained in order to analyse the strain rate path leading to the successful rapid forming. For a specimen processed at 500C, the maximum volume fraction of cavities is 4 existing at the location of minimum thickness.     

Rheology of silicone-coated ceramic powders

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Tensile strength of ceramic powders

The tensile strength of granular materials has been extensively studied and is of great technological importance. Several theoretical or experimental relationships between density and strength have been proposed. This paper compares these models with experimental data from zirconia powders. It is shown that the most important parameter determining the strength of a green compact is not density but compaction pressure. A micromechanical model, which gives a qualitative understanding of ceramic powder behaviour under tensile stress, is also presented.     

Cavitation in alloy steels during superplastic deformation

A study of cavitation during superplastic tensile straining of two microduplex steels has been made using density measurements and quantitative optical metallography. The steels were of basically similar composition with the exception of a trace addition of boron made to one alloy. During deformation cavities formedα/γ boundaries and matrix-carbide interfaces; the growth and coalescence of these cavities led to failure. Density measurements showed that the extent of cavitation increased with increasing strain and decreasing strain-rate, but the level of cavitation was reduced by the presence of boron. A time dependence of overall void volume of 1.4 to 2.0 was observed. Quantitative metallographic studies of the nucleation and growth contributions to the overall rate of void formation showed that boron inhibited each of these processeS. However, both the nucleation rate and the magnitude of the time exponent of void volume increase suggested that a substantial number of voids grew from pre-existing nuclei which were probably present as non-coherent carbide-matrix interfaces.     

Brittleness estimation of ceramic particulate composites

The possibility of calculating the brittleness parameter of ceramic particulate composites from indentation test data is investigated. Three different concepts are considered — based on threshold crack length, transition scratch depth and simple ratio of hardness to toughness. Threshold crack length based models predict higher values compared to the other two.     

Creep-sintering of polycrystalline ceramic particulate composites

The sintering of particulate composites consisting of a polycrystalline zinc oxide matrix with 10 vol % zirconia inclusions of two different sizes (3 and 14 m) was investigated at a constant heating rate of 4 °C min^–1 under an applied stress of 300 kPa. The presence of the inclusions produced a decrease in both the creep rate and the densification rate but the ratio of the densification to creep rate remained constant during the experiment. The ratio of the densification rate to creep rate for the composites was 1.5 times greater than that of the unreinforced matrix regardless of inclusion size. The creep viscosity of the composites was higher than that of the unreinforced matrix and increased slightly with decreasing inclusion size.     

Dynamic behavior of selected ceramic powders

An experimental setup has been developed on the continuous recording of the stress profiles in ceramic powders subject to shock loading with manganin gauges. A series of plate impact experiments on highly porous ceramic powders such as Al₂O₃, SiC and B₄C were conducted at the laboratory's single stage powder gun facility. The relationship between shock wave velocity and particle velocity was measured to obtain the Hugoniot data. Plate impact onto powder sample experiments were conducted at loading stresses ranging from 1.6 to 4.2 GPa. The experimental results show that the shock wave speeds in various ceramic powders vary between 1 and 2 km/s. Linear Hugoniot relations between shock velocity (D) and particle velocity (u) are observed. The loading stress–specific volume form of Hugoniot relations (P–V) was constructed using the data from quasistatic compression test results, Hopkinson bar dynamic compression test results and powder gun plate impact experiment results. The P–V diagram shows that the crush strength of ceramic powders is comparable to the loading stress level. The B₄C and SiC powders with bigger particle size more easily reach the solid state Hugoniot than the powders with smaller particle size at the same loading condition. In the case of Al₂O₃, the material shows less sensitivity to particle size difference at the same level of loading rate as compared to B₄C and SiC.     

BaTiO3陶瓷粉体凝胶注模成型的研究

为了研究BaTiO3粉体的凝胶注模成型，以聚甲基丙稀酰胺(PMAA—NH4)为分散剂，通过球磨制备稳定分散的BaTiO3浆料．采用正交实验设计原则，讨论了有机单体、交联剂和引发剂的用量对浆料的固化时间、坯体强度和密度的影响，确定最佳加入量(质量分数)分别为4％，1．5％和0．02％．在1340℃下烧结得到的BaTiO3陶瓷的晶粒尺寸均匀，气孔较少，致密度高，具有良好的PTC特性。     

水热法制备ZST微波陶瓷粉体

采用水热法制备了(Zr0.8Sn0.2)TiO4粉体,并借助于XRD、SEM、热分析等手段对反应后的结晶粉末进行了表征.实验结果表明,粉末性能受反应溶液pH值的影响显著.随着pH值的增大,晶粒的生长更加充分,完整.反应后的晶粒粒度为纳米尺度,表面活性大,有利于烧结.烧结温度在1260-1320℃之间,比传统固相法制备的ZST陶瓷烧结温度降低了300多度.     

Microstructure of diffusion-bonded joints in Al-Li 8090 alloy

Joints were produced between Al-Li 8090 alloy sheet by solid state (SSDB) and transient liquid-phase (TLPDB) diffusion-bonding techniques. The bond interface was a planar, thermally stable, large-angle grain boundary in the SSDB joint. Non-planar grain boundaries in a band of coarse grains were present in the TLPDB joint. The origin of these microstructures and the measured shear strengths of the joints relative to that of the parent sheet are discussed.     

Superplastic behaviour of annealed AA 8090 Al-Li alloy

Abstract

High temperature flow behaviour and microstructural evolution were investigated in an annealed AA 8090 Al - Li alloy over the temperature range 623 - 803 K and strain rate range ~ 6 × 10⁶ - 3 × 10² s^-1. Stress - strain rate data, obtained using a differential strain rate test technique and plotted in log - log scale, exhibited three regions I, II, and III, with increasing strain rate. In these regions, the values of strain rate sensitivity index m and the activation energy for deformation were determined to be 0.17, 0.43, and 0.17; and 758.8, 93.3, and 184.3 kJ mol^-1, respectively. The stress - strain curves obtained from constant strain rate tests exhibited flow hardening at lower strain rates and higher temperatures whereas flow softening occurred at higher strain rates and lower temperatures. The microstructural evolution revealed the dominance of grain growth under the former conditions and dynamic recrystallisation under the latter conditions. Ductility and m were found to increase with temperature, with the maximum values of 402% and 0.55, respectively, at a temperature 803 K and strain rate 2 × 10^-4 s^-1.     

Dispersion quality measurement and particulate magnetic coatingproperties

The use of a rheomagnetic instrument which provides an indirect measure of the dispersion quality of a magnetic ink suspension used for coating application in the manufacture of magnetic recording disks is described. The instrument was designed, tested, and put on a pilot line to study the relationship between the dispersion quality measurement and functional characteristics of the magnetic coating. The results of tests performed during pilot production suggest that it is possible to predict some of the disk-coating properties based on the dispersion quality of the magnetic particle suspension measured prior to coating. There is a qualitative correlation between the in situ measurements made on the magnetic particle suspension at the coater with the surface roughness due to flocs, the on-disk orientation ratio, and the signal-to-noise ratio (SNR)