Effects of copper and Al2O3 particles on characteristics of Cu–Al2O3 composites

High-energy milling was used for production of Cu–Al₂O₃ composites. The inert gas-atomized prealloyed copper powder containing 2 wt.%Al and the mixture of the different sized electrolytic copper powders with 4 wt.% commercial Al₂O₃ powders served as starting materials. Milling of prealloyed copper powders promotes formation of nano-sized Al₂O₃ particles by internal oxidation with oxygen from air. Hot-pressed compacts of composites obtained from 5 and 20 h milled powders were additionally subjected to the high-temperature exposure in argon at 800 °C for 1 and 5 h. Characterization of processed material was performed by optical and scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness, as well as density and electrical conductivity measurements. Due to nano-sized Al₂O₃ particles microhardness and thermal stability of composite processed from milled prealloyed powders are higher than corresponding properties of composites processed from the milled powder mixtures. The results were discussed in terms of the effects of different size of starting copper powders and Al₂O₃ particles on the structure, strengthening of copper matrix, thermal stability and electrical conductivity of Cu–Al₂O₃ composites.     

Synthesis and properties of a Cu-Ti-TiB2 composite hardened by multiple mechanisms

Multiple hardening mechanisms of a copper matrix have been presented and discussed. The gas atomized Cu-0.6 wt.%Ti-2.5 wt.%TiB₂ (Cu-Ti-TiB₂) powders were used as starting materials. Dispersoid particles TiB₂ were formed in situ in the copper matrix during gas atomization. The powders have been consolidated by hot isostatic pressing (HIP). Optical microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) analysis were performed for microstructural characterization of powders and composite compacts. High hardening of the Cu-Ti-TiB₂ composite achieved by aging is a consequence of the simultaneous influence of the following factors: the development of modulated structure with metastable Cu₄Ti_(m) particles and in situ formed TiB₂ dispersoid particles.     

Particle fracture in metal-matrix composite friction joints

The influence of welding parameters, reinforcing particle chemistry and shape, matrix condition and silver interlayers on particle fracture during similar and dissimilar friction welding of aluminium-based metal-matrix composite (MMC) base material was investigated. Two composite base materials were examined, one containing Al₂O₃ particles and the other containing 72 wt% Al₂O₃–7 wt % Fe₂O₃–17 wt % SiO₂–3 wt % TiO₂ particles. The different material combinations comprised MMC/MMC, MMC/alloy 6061, MMC/AISI 304 stainless steel and MMC/1020 mild steel joints. Particle fracture was confined to a narrow region immediately adjacent to the dissimilar joint interface. The calculated normal pressure for fracture of Al₂O₃ particles ranges from 0.56–17.58 MPa and is in agreement with an experimentally measured pressure of 1.06 MPa found during sliding wear testing of aluminium-based composite base material. Because the lowest normal pressure applied during friction joining was 30 MPa, particle fracture occurs very early in the joining operation (immediately following contact between the two substrates). The application of a silver interlayer during dissimilar MMC/AISI 304 stainless steel joining decreased the particle fracture tendency. It is suggested that the presence of a silver interlayer decreased the coefficient of friction and lowered the stresses applied at the contact region. The particle fracture tendency was markedly increased when the MMC material contained blocky alumina particles. However, there was negligible particle fracture when the MMC base material contained spherical 72 wt % Al₂O₃–7 wt % Fe₂O₃–17 wt % SiO₂–3 wt % TiO₂ particles. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of high energy ball milling on the displacement reaction in particulate reinforced Al–Si–Cu alloy matrix composite powders

The displacement reaction between Al and SiO₂ in an Al–3wt%Cu–3wt%Si–9wt%SiO₂ powder mixture has been studied when the mixture had been ball-milled, and compared with the reaction in the as-mixed powder. Diffusion couples consisting of Al/SiO₂ were formed during ball milling. The size of the composite powder particles and the diffusion couples was reduced by increasing the ball milling time. Differential thermal analysis and X-ray diffraction results showed that the displacement reaction between Al and SiO₂ did not occur in the as-mixed powder, but occurred in the as-milled powders in the temperature range of 640–680 °C. Furthermore, the onset temperature of the displacement reaction shifted to lower temperatures after increasing the ball milling time. On the basis of these results the milled powder was sintered at 640 °C in order to produce an Al–Cu–Si matrix composite reinforced with homogeneously distributed submicron-sized Al₂O₃ particles. This is much lower than the temperature required for the same reaction in other processes which are used to produce such composites, such as the melting infiltration process.     

Consolidation of ultrafine-grained Cu powder and nanostructured Cu–(2.5–10) vol%Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite powders by powder compact forging

An as-received ultrafine-grained Cu powder and four nanostructured Cu–(2.5–10) vol%Al₂O₃ composite powders produced by high-energy mechanical milling of mixtures of the Cu powder and an Al₂O₃ nanopowder were consolidated using warm powder compaction followed by open die powder compact forging. The circular discs produced in the experiments achieved full densification. Tensile testing of the specimens cut from the forged discs showed that the Cu-forged disc had a fairly high yield strength of 330 MPa, UTS of 340 MPa and a plastic strain to fracture of 15%, but the Cu–Al₂O₃ composite-forged discs did not show any macroscopic plastic yielding. The fracture strength of the composite-forged discs decreased almost linearly with the increase of the volume fraction of Al₂O₃ nanoparticles. This study shows that a high level of consolidation of the ultrafine-grained Cu powder and the nanostructured Cu–2.5 vol%Al₂O₃ composite powder has been achieved by warm powder compacting at 350 °C and powder compact forging at 500 and 700 °C. However, this is not true for the nanostructured Cu–(5, 7.5 and 10) vol%Al₂O₃ composite powders, possibly due to their higher powder particle hardness at elevated temperatures in the range of 350–800 °C.     

Effects of ball milling and high-pressure torsion for improving mechanical properties of Al–Al2O3 nanocomposites

Pure Al powders were mixed with a 30 % volume fraction of Al₂O₃ powders having particle sizes of ~30 nm. The mixed powders were first subjected to ball milling (BM) and thereafter consolidated by high-pressure torsion (HPT) at room temperature under a pressure of 3 GPa for 10 turns. The Al–Al₂O₃ composite produced by BM and HPT (BM + HPT) had a more uniform dispersion of the nano-sized Al₂O₃ particles in the Al matrix. Hardness values of the BM + HPT composites were higher than those of the composites without BM. It is shown that the use of BM powders for HPT is more effective in achieving a uniform dispersion of the nano-sized Al₂O₃ particles and in improving mechanical properties of the Al–Al₂O₃ nanocomposites.     

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.     

Densification and dielectric properties of SrO–Al2O3–B2O3 ceramic bodies

The influence of SrO (0·0–5·0 wt%) on partial substitution of alpha alumina (corundum) in ceramic composition (95 Al₂O₃–5B₂O₃) have been studied by co-precipitated process and their phase composition, microstructure, microchemistry and microwave dielectric properties were studied. Phase composition was revealed by XRD, while microstructure and microchemistry were investigated by electron-probe microanalysis (EPMA). The dielectric properties by means of dielectric constant (ε _r ), quality factor (Q × f) and temperature coefficient of resonant frequency (τ _f ) were measured in the microwave frequency region using a network analyser by the resonance method. The addition of B₂O₃ and SrO significantly reduced the sintering temperature of alumina ceramic bodies to 1600 °C with optimum density (∼ 4g/cm³) as compared with pure alumina powders recycled from Al dross (3·55g/cm³ sintered at 1700 °C).     

Refinement of primary Si and modification of eutectic Si for enhanced ductility of hypereutectic Al–20Si–4.5Cu alloy with addition of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

It is very difficult to simultaneously refine and modify Si particles in hypereutectic Al–Si–Cu alloys to enhance their ductility. This study investigates how nanoparticles affect Si particles during solidification in hypereutectic Al–Si–Cu alloys. 0.5 wt% γ-Al₂O₃ nanoparticles were added in hypereutectic Al–20Si–4.5Cu alloy melt and further dispersed through an ultrasonic-cavitation-based technique. The as-cast Al–20Si–4.5Cu–Al₂O₃ nanocomposites showed marked enhancements in both ductility and strength. The ductility of Al₂O₃ nanocomposite was more than two times higher than that of the monolithic alloy without the nanoparticles. Microstructural analysis with optical and scanning electron microscopy revealed that both the primary and eutectic Si particles were significantly refined. The primary Si particles were refined from star shapes to polygon or blocky shapes, and their edges and corners were much smoother. The large plate eutectic Si particles were also modified into the fine coralline-like ones. The porosity of alloy was also reduced with the addition of γ-Al₂O₃ nanoparticles. Study suggests that γ-Al₂O₃ nanoparticles simultaneously refine and modify Si particles as well as reduce porosity in cast Al–20Si–4.5Cu, resulting in unusual ductility enhancement that could have great potential for numerous applications.     

Microstructures formed during devitrification of Na2O·Al2O3·B2O3·SiO2·Fe2O3 glasses

Soda alumina borosilicate glasses of composition (24-y)Na₂O·yAl₂O₃·14B₂O₃·37SiO₂·25Fe₂O₃, y = 8, 12, 14, 16 mol%, were melted using Fe₂O₃ as raw material. Besides, samples with y = 12 and Fe₂O₃ concentrations of 14.32, 17.8, and 25.0 mol% were prepared from FeC₂O₄·2H₂O as raw material. The X-ray diffraction analyses showed the presence of magnetite for the samples from all the investigated compositions. Transmission electron microscopy (TEM) evidenced that all the samples are phase separated and droplets in the diameter range 100–1000 nm, enriched in iron, are formed. Inside these droplets, numerous small magnetite particles, with size in the 25–40 nm interval, are crystallized.     

Evolution of microstructure induced by calcination in leached Al–Cu–Fe quasicrystal and its effects on catalytic activity

Effects of calcination on catalytic activity for steam reforming of methanol (SRM) over an Al–Cu–Fe quasicrystal (QC) leached with NaOH aq. have been investigated in terms of microstructure with X-ray diffraction and transmission electron microscope (TEM). Calcination at 600 °C in air drastically improved the catalytic activity of the leached QC. TEM observations on cross-section of the samples revealed that cubic Cu _x Fe_3-x-y Al _y O₄ spinel was formed at the outermost layer of the leached QC after calcinations. Prior to the catalytic reaction, the Cu _x Fe_3-x-y Al _y O₄ spinel decomposed to a composite where fine Cu nanoparticles dispersed in (Fe,Al)₃O₄ matrix under H₂ treatment at 300 °C. Drastic increase in catalytic activity is responsible for the fine Cu nanoparticles in the composite. The Cu nanoparticles sit along the same orientation with (Fe,Al)₃O₄, e.g., Cu [013]//(Fe,Al)₃O₄ [013] and Cu [200]//(Fe,Al)₃O₄ [400]. This orientation relationship may stabilize the Cu nanoparticles through a bonding of Cu–O–Fe.     

Low-temperature sintering of SiC reticulated porous ceramics with MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> additives as sintering aids

SiC reticulated porous ceramics (SiC RPCs) was fabricated with polymer replicas method by using MgO–Al₂O₃–SiO₂ additives as sintering aids at 1,000∼1,450 °C. The MgO–Al₂O₃–SiO₂ additives were from alumina, kaolin and Talc powders. By employing various experimental techniques, zeta potential, viscosity and rheological measurements, the dispersion of mixed powders (SiC, Al₂O₃, talc and kaolin) in aqueous media using silica sol as a binder was studied. The pH value of the optimum dispersion was found to be around pH 10 for the mixtures. The optimum condition of the slurry suitable for impregnating the polymeric sponge was obtained. At the same time, the influence of the sintering temperature and holding time on the properties of SiC RPCs was investigated. According to the properties of SiC RPCs, the optimal sintering temperature was chosen at 1,300 °C, which was lower than that with Al₂O₃–SiO₂ additives as sintering aids.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> on the properties of nanocrystalline ZrO<Subscript>2</Subscript> + 3 mol % Y<Subscript>2</Subscript>O<Subscript>3</Subscript> powder

We have studied the properties of nanocrystalline ZrO₂〈3 mol % Y₂O₃〉 and 90 wt % ZrO₂〈3 mol % Y₂O₃〉-10 wt % Al₂O₃ powders prepared via hydrothermal treatment of coprecipitated hydroxides at 210°C. The results demonstrate that Al₂O₃ doping raises the phase transition temperatures of the metastable low-temperature ZrO₂ polymorphs and that the structural transformations of the ZrO₂ and Al₂O₃ in the doped material inhibit each other.     

Aging behavior of Cu–Ti–Al alloy observed by transmission electron microscopy

Aging behavior of Cu–3 at.%Ti–4 at.%Al alloy at 723 K has been examined from mechanical, electrical, and microstructural points of view. Compared with binary Cu–3 at.%Ti alloy, the electrical conductivity improved six times to about 6%IACS (international annealed copper standard); whereas the peak hardness decreased from 280 to 180 Hv. The major strengthening phase is the tetragonal α-Cu₄Ti, which forms not via spinodal decomposition but based on the nucleation and growth mechanism. The precipitates grow in the c direction of the tetragonal phase, which lies along one of the axes of the matrix fcc Cu phase. This growth mode minimizes the strain energy arising from the lattice mismatch of about 2% between the matrix and precipitate; and results in a square rod shape, which reaches about 50 nm in length after 100 h anneal. Another precipitating phase is AlCu₂Ti (D0₃, Strukturbericht notation), with the major habit plane close to {110} of the fcc Cu matrix. The orientation relationship was not definitely determined, but it was found that the angle between the 100 and 110 poles of the matrix and precipitates, respectively, is about 5°, while the angle between the two 001 axes being about 7°. It was suggested that the formation of this ternary phase reduced the solute Ti concentration, leading to the decrease in the resistivity.     

Microstructural parameters of dispersion strengthened Cu–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> materials

The aim of the article was to evaluate the microstructural parameters of Cu–Al₂O₃ dispersion strengthened materials with different volume fraction of Al₂O₃ phase. For analyses of dispersoids Al₂O₃, the extraction carbon replica was used. The distribution of Al₂O₃ particles in the matrix was estimated by three methods (quadrant count method, polygonal method, and by interparticle distances), these methods showed that particle distribution in material with 1 vol.% of Al₂O₃ is very close to the Poisson point process (PPP), which is a model of randomly distributed points. Particle distributions in materials with 8 and 10 vol.% of Al₂O₃ achieve features of regularity proved mainly by the spherical contact distance.     

Glass formation and phase transformations in plasma prepared Al2O3-SiO2 powders

The structure of Al₂O₃-SiO₂ sub-micron powders prepared by oxidation of mixed aluminium-silicon halides in an oxygen-argon high frequency plasma flame has been studied. The powders were completely amorphous up to at least 52 wt % Al₂O₃ and partially amorphous in the range 52 to 88 wt % Al₂O₃. The crystalline phase was mullite up to 75 wt % Al₂O₃ but at higher Al₂O₃ contents a metastable solid solution of SiO₂ in -Al₂O₃ was observed in addition to mullite. Amorphous particles crystallized to mullite on heating to 1000°C, independently of composition. Extension of glass formation towards the high Al₂O₃ end of the Al₂O₃-SiO₂ system as the cooling rate is increased and particle size decreased, may be explained by the effect of viscosity on the nucleation rate of mullite from liquid, for Al₂O₃ contents up to 60 wt %. The viscosity change is relatively small as the Al₂O₃ content is increased beyond 60% and it is suggested that the change in nucleus-liquid interfacial energy with composition is the predominant factor controlling nucleation rate in this range. At Al₂O₃ concentrations greater than approximately 80 wt %, -Al₂O₃ is the phase which nucleates from the melt. A double DTA peak was observed for powders containing more than 80 wt % Al₂O₃. The lower temperature peak is believed to arise from the formation of mullite from a metastable solution of SiO₂ in -Al₂O₃, and the higher temperature peak from crystallization of mullite from the amorphous phase. The presence of SiO₂ in solution in metastable Al₂O₃ increases the temperature of transformation to -Al₂O₃ to greater than 1500° C compared with 1230° C for pure Al₂O₃.     

Effect of aluminum on precipitation hardening in Cu–Ni–Zn alloys

The effect of aluminum on the precipitation hardening of Cu–Ni–Zn alloys with varying aging temperatures and times was investigated in this article, in the hope to achieve better mechanical properties. Vickers hardness, tensile, and electrical conductivity tests were carried out to characterize the properties of the Cu–Ni–Zn alloys with or without an addition of aluminum subjected to different aging treatments. The results show that an addition of 1.2 wt% aluminum can play an influential role in the precipitation hardening of the Cu–Ni–Zn alloys. For example, it can increase the peak hardness from 58 Hv for the solution-treated Cu–10Ni–20Zn alloy to 185 Hv for the solution-treated Cu–10Ni–20Zn–1.2Al alloy during aging at 500 °C. The yield strength, tensile strength, and electrical conductivity of the Cu–10Ni–20Zn–1.2Al alloy subjected to suitable treatments under prior cold-rolled and aged conditions can reach 889 MPa, 918 MPa, and 10.96% IACS, respectively, being much higher than those of the relevant alloy without aluminum and comparable to those of the Cu–Be alloys (C17200 and C17510). According to the transmission electron microscope observations, it was found that formation of nanosized precipitates with the L1₂-type ordered lattice results in precipitation hardening, and an orientation relationship of [011]_\textp//[011]_\textm [011]_{\text{p}}//[011]_{\text{m}} and (100)_\textp//(200)_\textm (100)_{\text{p}}//(200)_{\text{m}} exists between the precipitates and the α-Cu matrix.     

High-pressure high-temperature transitions in nanocrystallineγ Al2O3,γ Fe2O3 and TiO2

We report first observation of new polymorphs of Al₂O₃ and Fe₂O₃ in specimens of xerogelγ Al₂O₃ andγ Fe₂O₃ quenched from high pressures and temperatures. At about 5 GPa and 1400°C, xerogel gamma alumina (XGA) transformed into a polymorphic mixture of phasesα Al₂O₃, B Al₂O₃ and C Al₂O₃, while XGA containing 1 wt% Cr₂O₃ transformed into a mixture of phasesαAl₂O₃, H Al₂O₃ and k′ Al₂O₃. The phases B Al₂O₃, C Al₂O₃ and H Al₂O₃ have the monoclinic-, cubic- and hexagonal-rare earth sequioxide (Ln₂O₃) type structure, respectively. At 5·2 GPa and 1450°C, XGA yielded a mixture ofα Al₂O₃ and hexagonalμ Al₂O₃. At STP, the phaseμ Al₂O₃ was found to transform to another hexagonal phaseλAl₂O₃ over a 10 week period. At 5·2 GPa and 900°C,γ Fe₂O₃ showed transition to a new phase H Fe₂O₃ which probably has an 8 layer close packed structure. In nanocrystalline TiO₂, only the anatase to rutile transition was found. The results are discussed using the free energy vs temperature diagram for xerogel and nanocrystalline materials.     

DeNO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> activity of Ag/γ–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposites prepared via the solvothermal route

Ag/γ–Al₂O₃ with silver loading of 3 wt.% were prepared by the solvothermal-calcination reaction of AgNO₃ in mixed water-alcohol solutions at 50–250 °C for 0–120 min, followed by calcinations at 550 °C for 2 h using γ-Al₂O₃ (SSA: 120–409 m² g⁻¹), γ-AlOOH (SSA: 270 m² g⁻¹), Al(OH)₃ (SSA 10 m² g⁻¹), Al(OCH(CH₃)₂)₃ and Al(NO₃)₃ as an aluminum source. The resultant product produced by the solvothermal reaction was Ag/γ–AlOOH even though a different aluminum source was used and Ag/γ–AlOOH was converted to Ag/γ–Al₂O₃ by the following calcinations. However, the characteristics of them changed greatly depending on the alumina source. The deNO _x catalytic performance of Ag/γ–Al₂O₃ also greatly changed depending on the aluminum precursor and solvothermal solvent in the order γ-AlOOH = γ-Al₂O₃ >> Al(OCH(CH₃)₂)₃ >> Al(NO₃)₃ > Al(OH)₃ and methanol = ethanol > 1-propanol > butanol >> hexanol, since the amount and size of silver particle impregnated and specific surface area of the product changed markedly. Ag/γ–Al₂O₃ prepared by solvothermal-calcination method consisted of homogeneously dispersed fine particles of silver and showed better performance for NO _x decomposition than that by conventional impregnation-calcination method.     

An alternative approach of solid-state reaction to prepare nanocrystalline KNbO<Subscript>3</Subscript>

A simple preparation of KNbO₃ powders was proposed by an alternative approach of solid-state reaction. Stoichiometric niobium oxalate and potassium acetate were mixed in water and then dried. It was demonstrated that an ion-exchange reaction occurred with the formation of K[NbO(C₂O₄)₂]·nH₂O intermediate. The single-phase KNbO₃ powders were synthesized when K[NbO(C₂O₄)₂]·nH₂O intermediate was calcined between 500 and 800 °C for 3 h. KNbO₃ powders obtained at 500 °C are determined as orthorhombic structure with an average particle size of 20–50 nm by X-ray diffraction, scanning electron microscope (SEM), and transmission electron microscopy (TEM) analysis. The morphologies of KNbO₃ obtained at different temperatures were observed by SEM and TEM analysis. The average band gap energy is estimated to be 3.16 eV by UV–vis diffuse reflectance spectra.