Microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy produced by rapid solidification powder metallurgy

The microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy was investigated. Both rapid solidified powders and cast billets were extruded at 573, 623 and 673 K to optimize the processing conditions for obtaining better mechanical response. Powder was consolidated to prepare the extrusion billets using both cold compaction and Spark Plasma Sintering at 473 K. The tensile properties of the extruded alloy were then evaluated and correlated to the observed microstructure. The results show that the use of rapid solidified powder could lead to effective grain refinement, which in turn resulted in the improved mechanical response, especially compared to the extruded conventional cast material.     

Microstructures and Mechanical Properties of Rapidly Solidified Mg-Al-Zn-MM Alloys

Mg-Al-Zn-M M （misch metal） alloy powders were manufactured by inert gas atomization and the characteristics of alloy powders were investigated.In spite of the low fluidity and easy oxidation of the magnesium melt,the spherical powder was made successfully with the improved three piece nozzle systems of gas atomization unit. It was found that most of the solidified powders with particles size of less than 50μm in diameter were single crystal and the solidification structure of rapidly solidified powders showed a typical dendritic morphology because of supercooling prior to nucleation.The spacing of secondary denrite arms was deceasing as the size of powders was decreasing.The rapidly solidified powders were consolidated by vacuum hot extrusion and the effects of misch metal addition to AZ91 on mechanical properties of extruded bars were also examined.During extrusion of the rapidly solidified powders,their dendritic structure was broken into fragments and remained as grains of about 3μm in size.The Mg-Al-Ce intermetallic compounds formed in the interdendritic regions of powders were finely broken,too.The tensile strength and ductility obtained in as-extruded Mg-9 wt pct Al-1 wt pct Zn-3 wt pct MM alloy wereσ-（T.S.） =383 MPa andε=10.6%,respectively.All of these improvements on mechanical properties were resulted from the refined microstructure and second-phase dispersions.     

Effect of powder characteristics on Cr internal oxidation for preparation of Cr2O3/Cu composite

Abstract

Cr O₃/Cu composite was prepared by the internal oxidation of Cu–Cr pre-alloyed powders formed by high energy milling. Effects of milling time on the internal oxidation characteristics of Cu–Cr pre-alloyed powders were also discussed in this paper. The results indicate that the degree of the internal oxidation continually increases with prolonged milling time. At the initial stage, external oxidation rather than internal oxidation occurs, resulting in coarse Cr₂O₃ particles. With further milling, the internal oxidation becomes more complete and the sizes of Cr₂O₃ particles also become finer and well distributed. The properties of the composite are therefore improved. A high quality composite specimen from Cu–1·0Cr pre-alloyed powders after 40 h milling was prepared by the internal oxidation process. The Cr₂O₃ particles with an average size of 2–5 μm in diameter and about 5–10 μm in particles space were found by a microstructure examination, and they were uniformly dispersed in the Cu matrix.     

Densification,microstructure, and properties of W-Mo-Cu alloys prepared with nano-sized Cu powders via large electric current sintering

This study rapidly fabricated a novel W-Mo-Cu alloy by large current electric field sintering at a relatively low temperature, and the effects of the powder size of Cu on the densification, microstructure, and properties were comprehensively investigated. The particle size of Cu did not influence the phase type but significantly affected the densification, microstructure, and properties. XRD and TEM results showed that the alloy contained three new phases aside from W, Mo, and Cu phases, i.e., Mo-W ordered phase, Mo-Cu solid solution, and Cu_0.4W_0.6 intermetallic compound. Copper powders with smaller sizes were beneficial to improving the distributional homogeneity of elements and the sintering densification. Therefore, the alloy prepared with 100 nm Cu powders had a denser and more homogeneous microstructure and better comprehensive properties than that prepared with 5 μm Cu powders. Overall, the W-Mo-Cu alloy prepared with 100 nm Cu powders at 980 °C proposed the best comprehensive properties, and its relative density can reach 98% approximately.     

微结构对Bi_2Sr_2Co_2O_y化合物电性能的影响

通过溶胶-凝胶法制备单相Bi2Sr2Co2Oy化合物,通过添加PEG20000和超声分散对化合物粉体微结构进行调控,采用SPS烧结得到了致密的块体.探索了微结构对Bi2Sr2Co2Oy化合物电性能的影响规律.结果表明,添加PEG20000和超声分散可以明显降低Bi2Sr2Co2Oy化合物粉体的晶粒尺寸,使烧结块体的晶粒尺寸大幅度减小,从而显著提高材料的电性能.温度为873K时,添加PEG20000并超声处理所制备样品烧结块体获得了最高ZT值0.041.     

Microstructure and properties of Cu–Mg alloy treated by internal oxidation

A Cu–0.24Mg alloy bar was treated by internal oxidation using Cu₂O as an oxidiser at 1123–1273?K for 10?h. The thermodynamic diagram for oxidation of a Cu–Mg alloy was confirmed by the critical oxygen pressure. The results show that the thickness of the MgO/Cu layer and the electrical conductivity of the Cu–0.24Mg alloy increase with increasing internal oxidation temperature, whereas the average hardness of the MgO/Cu layer initially increases and subsequently decreases. Examination of the microstructure of the MgO/Cu layer revealed that Mg precipitation in the form of MgO particles and their uniform dispersion in the Cu matrix were the primary reasons for increases in the comprehensive properties of the Cu–Mg alloy treated by internal oxidation.     

Influence of silver additions on electrical, mechanical and structures properties of rapidly solidified Sn–0.7%Cu alloy from melt

The eutectic alloy Sn_99.3-xCu_0.7Ag_x has been examined as one of the lead free solder alloys. Melting point, electrical resistivity, internal friction, elastic moduli, microhardness and the microstructure of the Sn_99.3Cu_0.7, Sn_95.8Cu_0.7Ag_3.5 and Sn_95.3Cu_0.7Ag₄ rapidly solidified lead free solder alloys have been investigated. The examined physical properties are improved by increasing silver contents in the studied lead free solder alloys. These improved properties indicate that these alloys are adequate for low temperature soldering applications.     

Effect of ceramic powder particle size on the electrical properties of Li0.03Na0.97Ta0.05Nb0.95O3 ceramics

We compare the microstructure, electrical properties, and Raman spectra of ceramic samples of the Li_0.03Na_0.97Ta_0.05Nb_0.95O₃ ferroelectric solid solution prepared by a conventional ceramic processing technique using ceramic powders with different particle sizes. The Li_0.03Na_0.97Ta_0.05Nb_0.95O₃ solid solutions prepared from the powders of different particle sizes are shown to differ only slightly in average grain size, pore size, and porosity, but the particle size of the ceramic powder has a significant effect on their electrical properties. The difference in properties between the samples is due not so much to the difference in particle size between the parent ceramic powders as to the fact that the diffusion processes involved in solid-state synthesis may give rise to composition gradients, and grains that differ in size may differ in microstructure and composition.     

Processing and properties of zirconia-toughened alumina ceramics

Al₂O₃:ZrO₂ ceramics have been prepared from physically mixed pure oxide powders. The results indicate that careful processing of the starting powders and a two-stage sintering process can avoid expensive processing methods like hot pressing/hot isostatic pressing used for achieving high densification. The mechanical properties were measured and the resultant microstructure studied to explain the toughening behaviour of this material.     

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 2 Tensile properties of extruded and Conformed gas atomised powders

Abstract

The effects of aging treatments on the tensile properties and microstructure of Al–Cr–Zr–Mn powder metallurgy aluminium alloys prepared from high pressure gas atomised powders were investigated. The alloy compositions were designed to give powders with or without Al₁₃Cr₂ intermetallics in the <45 μm size fraction. The Al–5·2Cr–1·4Zr–1·3Mn alloy is typical of the former (concentrated alloy) and the Al–3·3Cr–0·7Zr–0·7Mn alloy of the latter (dilute alloy). The alloys were prepared using a canning/degassing/extrusion sequence or the Conform consolidation process. Measurements of micro hardness and electron microscopy were used to correlate the microstructure with the tensile properties. The extruded powders of both alloys exhibited better properties than those of the Conformed powders. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The dilute alloys had consistently better ductility. Neither alloy retained its strength after prolonged aging at 400°C, but the results indicate that a service temperature of 300°C may be possible.

MST/1247b     

Polymer-coated metal and alloy particles for magnetic recording

Fine particles of cobalt and an iron-cobalt alloy were prepared and encapsulated in a plastic film to protect them from oxidation. The magnetic properties of the coated powders were determined and compared with the properties of coated iron powders studied previously. The thermal degradation of magnetic properties was also investigated. Polymer-coated iron and cobalt powders were incorporated in a Hypalon rubber matrix to obtain additional magnetic property data and to study recording characteristics. The experimental materials exhibited better high-frequency response in magnetic recording than a standard γ-Fe2O3material, and one material had a higher signal output as well.     

Metal oxide powder synthesized with amorphous metal chelates

Chelate powder consisting of amorphous particles was synthesized through the process in which the droplet of the chelate solution is dried in the gas phase and solidified in a moment using a splay-dry technique. To investigate the advantage of the use of amorphous chelate powder in the processing of metal oxide powder, this study provides following two routes: a conventional route of mechanically mixing of crystalline metal-ethylenediaminetetraacetic acid (EDTA) powders and spray-dry mixing of metal-EDTA. These routes were followed by calcinations of metal-EDTA powder to form metal oxide powder. In this study, morphology, crystallization and metal composition of resulting (Ba,Sr)TiO₃ and YBa₂Cu₃O₇ powders were investigated. The amorphous metal-EDTA powder involving several metal elements is appropriate for successful calcination at lower temperatures rather than the mixture of crystalline powders prepared by mechanically mixing.     

Synthesis and growth mechanism of ThO2–W–Mo alloy nanocomposite powder

Nanocomposite ultrafine powders of thorium oxide and tungsten molybdenum alloy were prepared by arcing between a pure molybdenum cathode and a thoriated tungsten anode under an argon–hydrogen atmosphere. The microstructure, morphology and chemical analyses of the ultrafine powders were examined by X-ray diffraction, transmission electron microscopy and associated techniques. The powders are composed of thorium oxide single-crystalline nano-grains decorated by tungsten molybdenum alloy particles which are crystallized in a metastable A15 structure. Two morphologies were observed: octahedra and needle-like particles. Every octahedral particle bigger than 15 nm had on each corner a metallic grain generally less than 8 nm in size. The needles had a length below 600 nm and their width to length ratio ranged from 1/3–1/15. They all presented on one of their ends a metallic particle which was comparable in size to the needle width. Other smaller particles (<10 nm) may be encountered along the edges. The metallic particles were found to be epitaxial on the thorium oxide grains according to the relation: (100) [011]_ThO2∥(100) [011]_A15. The extensive study of the microstructural and morphological properties of the powders leads us to suggest a growth mechanism of the nanocomposite particles. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of atomized powder size on microstructures and mechanical properties of rapidly solidified Al–Cr–Y–Zr aluminum alloys

Rapidly solidified powders of Al–5.0Cr–4.0Y–1.5Zr (wt%) were prepared by using a multi-stage atomization-rapid solidification powder-making device. The atomized powders were sieved into four shares with various nominal diameter level and were fabricated into hot-extruded bars after cold-isostatically pressing and vaccum degassing process. Influence of atomized powder size on microstructures and mechanical properties of the hot-extruded bars was investigated by optical microscopy, X-ray diffraction, transmission electronic microscopy with EPSX and scanning electron microscopy. The results show that the fine atomized powders of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy attains supersaturated solid solution state under the exist condition of multi-stage rapid solidification. With the powder size increasing, there are Al₂₀Cr₂Y (cubic, a = 1.437 nm) and Ll₂ Al₃Zr (FCC, a = 0.407 nm) phase forming in the powders, and even lumpish particles of Al₂₀Cr₂Y appearing in the coarse atomized powders, as can be found in the as-cast master alloy. Typical microstructures of the extruded bars of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy can be characterized by fine grain FCC α-Al matrix with ultra-fine spherical particles of Al₂₀Cr₂Y and Al₃Zr. But a small quantity of Al₂₀Cr₂Y coarse lumpish particles with micro-twin structures can be found, originating from lumpish particles of the coarse powders. The extruded bars of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy by using the fine powders eliminated out too coarse powders have good tensile properties of σ_0.2 = 403 MPa, σ_b = 442 MPa and δ = 9.4% at room temperature, and σ_0.2 = 153 MPa, σ_b = 164 MPa and δ = 8.1% at high temperature of 350 °C.     

Characterisation of oxide scales formed on nickel superalloy using impedance spectroscopy

Abstract

Impedance spectroscopy has been used to measure the electrical properties of oxide scales formed from oxidation of IN738LC superalloy at high temperature. Electrical resistance and capacitance of the oxide scales were obtained from the simulation of the measured impedance diagrams based on the equivalent circuit model, which represents the features of the oxide scales. For oxidation of IN738LC superalloy, the electrical resistance of oxide scales increased with increasing oxidation time for the specimens exposed to air at 900°C. However, for the specimens oxidised at 1,200°C, the oxide scales showed very low electrical resistance, which indicated that cracking and spallation in oxide scales occurred continuously. By using scanning electron microscopy and X-ray diffraction techniques, the composition and microstructure of the oxide scales were examined. It was found that electrical properties were determined, not only by the microstructure of oxide scales, but also by the composition of the oxide scales. By determining the relationship between electrical properties, microstructure and composition of oxide scales, impedance spectroscopy could be used as a non-destructive technique for monitoring the oxidation of metallic alloys at high temperature.     

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.     

Mechanism of surface modification of a porous-coated Ti-6Al-4V implant fabricated by electrical resistance sintering

A porous-coated Ti-6Al-4V implant was fabricated by electrical resistance sintering, using 480 F capacitance and 1.5 kJ input energy. X-ray photoelectron spectroscopy (XPS) was used to study the surface characteristics of the implant material before and after sintering. There were substantial differences in the content of O and N between as-received atomized Ti-6Al-4V powders and the sintered prototype implant, which indicates that electrical resistance sintering alters the surface composition of Ti-6Al-4V. Whereas the surface of atomized Ti-6Al-4V powders was primarily TiO₂, the surface of the implant consisted of a complex of titanium oxides as well as small amounts of titanium carbide and nitride. It is proposed that the electrical resistance sintering process consists of five stages: stage I – electronic breakdown of oxide film and heat accumulation at the metal-oxide interface; stage II – physical breakdown of oxide film; stage III – neck formation and neck growth; stage IV – oxidation, nitriding, and carburizing; and stage V – heat dissipation. The fourth stage, during which the alloy repassivates, is responsible for the altered surface composition of the implant.     

Densification behaviour of oxide-coated silicon nitride powders

Powder coating has been explored as a method of incorporating sintering additives into a ceramic powder. This procedure has been explored in the case of Si₃N₄ powders coated with thin layers of MgO.The effectiveness of the powder coating technique has been evaluated by comparing the powder properties, densification behaviour, microstructure and mechanical properties of coated Si₃N₄ powders with identical powders in which the additive oxide has been added in particulate form. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder. The coated powder exhibited improved homogeneity, and gave good green compact density, high green strength, and faster densification rate. Moreover, coated powders densified more easily by pressureless sintering and showed a more homogeneous microstructure, higher strength and faster densification rates, compared with materials prepared using mixed oxide powders. Significant improvements in hardness and fracture toughness were observed for the coated powders.     

Machinable mica-based glass-ceramic

A glass-ceramic in the K₂O-MgO-Al₂O₃-MgF₂-SiO₂ system was prepared through the sol-gel and alkaline precipitation routes. The powders were hot pressed into workable discs. The glass-ceramic was found to be machinable with conventional tools. Its physical and mechanical properties compare favourably with commercial macor. A dependence of flexure strength on microstructure is also indicated.     

Microstructure and mechanical properties of directionally solidified Mg–Zn alloy as a biomaterial

Directionally solidified Mg-4wt-% Zn alloy was prepared and the effect of growth rate on its microstructure evolution and mechanical properties was investigated. A typical cellular structure was observed when the growth rate was lower than 60?µm?s^?1. The microstructure evolved from cell to columnar dendrite as the growth rate increased. The ultimate tensile strength of the directionally solidified alloy was found to be higher than that of the alloy ingot with the same cooling rate. The ultimate tensile strength of the directionally solidified alloy increased with increasing growth rate but it decreased during the cell–dendrite transition. The results indicate that the mechanical properties of the directionally solidified alloy with fine cellular and columnar dendritic structures meet the requirements of biomaterials.