Nanostructured Al–Al2O3 composite formed in situ during consolidation of ultrafine Al particles by back pressure equal channel angular pressing

Ultrafine pure Al particles were consolidated into fully dense bulk material using back pressure equal channel angular pressing (BP-ECAP). The consolidation was carried out at 400 °C with a back pressure of 200 MPa. A fully dense Al–Al₂O₃ composite consisting of mostly nanocrystalline Al and γ-Al₂O₃, a small fraction of ultrafine Al grains and amorphous alumina was produced after four passes from the freshly formed particles. In contrast, no consolidation was achieved from the aged particles which had been kept for between 18 and 24 months. The formation of the nanostructure was attributed to the interaction between severe shear deformation and in situ oxidation during ECAP. The ultimate strength of the nanostructured material reached ～740 MPa in compression with a plastic strain to fracture of the order of ～1%. It is demonstrated that ultrafine particles can be well consolidated by ECAP when they are sheared to change shape rather than to slide over each other.     

Preparation of Al2O3–ZrO2–SiO2 ceramic composites by high-gravity combustion synthesis

By a furnace-free technique of high-gravity combustion synthesis, Al₂O₃–ZrO₂–SiO₂ ceramic composites were prepared via melt solidification instead of conventional powder sintering. The solidification kinetics and microstructure evolution of the ceramic composites in high-gravity combustion synthesis were discussed. The phase assemblage of the ceramic composites depended on the chemical composition, where both (Al₂O₃ + ZrO₂) and (mullite + ZrO₂) composites were obtained. The ceramic composites consisted of ultrafine eutectics and sometimes also large primary crystals. In the (mullite + ZrO₂) composites, two different morphologies and orientations were observed for the primary mullite crystals, and the volume fraction of mullite increased with increasing SiO₂ content. The ceramic composites exhibited a hardness of 11.2–14.8 GPa, depending on the chemical composition and phase assemblage.     

Selective laser sintering of polymer-coated Al2O3／ZrO2／TiC ceramic powder

A type of polymer-coated Al2O3/ZrO2/TiC ceramic powder was prepared. The laser sintering mechanism of polymer-coated Al2 O3/ZrO2/TiC powder was investigated by studying the dynamic laser sintering process.It is found that the mechanism is viscous flow when the sintering temperature is between 80 ℃ and 120 ℃, and it is melting/solidification when the temperature is above 120 ℃. The process parameters of selective laser sintering were optimized by using ortho-design method. The results show that the optimal parameters include laser power of 14 W,scanning velocity of 1 400 mm/s, preheating temperature of 50 ℃ and powder depth of 0.15 mm. A two-step posttreatment process is adopted to improve the mechanical properties of laser sintered part, which includes polymer debinding and high temperature sintering. After vacuum sintering for 2 h at 1 650 ℃, the bending strength and fracture toughness of Al2O3/ZrO2/TiC ceramic part reach 358 Mpa and 6.9 Mpa · m1/2 , respectively.     

In-situ sintering reaction of Al2O3–LaAl11O18–ZrO2 composite

Al₂O₃–LaAl₁₁O₁₈–ZrO₂ composites were prepared by in situ sintering reaction of different proportions of Al₂O₃ and La₂Zr₂O₇. The studied batches were uniaxially pressed and pressureless sintered at 1600 °C up to 1725 °C for 1 h. Phase composition study reveals that the only present phases are alumina, lanthanum hexaluminates and zirconia. No other intermediate phases are present. Rodlike LaAl₁₁O₁₈ was observed in the sintered bodies containing more than 25 wt.% LaAl₁₁O₁₈. The effect of rodlike particles on the densification and mechanical behavior was discussed. It was found that increasing the LaAl₁₁O₁₈ content more than 25 wt.% enhances the fracture toughness, but reduces both the bending strength and the hardness of the sintered composites.     

Mechanical wet-milling and subsequent consolidation of ultra-fine Al2O3-（ZrO2＋3%Y2O3） bioceramics by using high-frequency induction heat sintering

Alumina/zirconia composites were synthesized by wet-milling technique and rapid consolidation with high frequency induction heat sintering(HFIHS). The starting materials were a mixture of alumina micro-powder (80%, volume fraction) and 3YSZ nano-powders (20%). The mixtures were optimized for good sintering behaviors and mechanical properties. Nano-crystalline grains are obtained after 24 h milling. The nano-structured powder compacts are then processed to full density at different temperatures by HFIHS. Effects of temperature on the mechanical and microstructure properties were studied. Al2O3-3YSZ composites with higher mechanical properties and small grain size are successfully developed at relatively low temperatures through this technique.     

Preparation of Al2O3–ZrO2–Y2O3 Composite Coatings by a Modified Sol–Gel Technique for Thermal Barrier Application

Spatial-network Al₂O₃–ZrO₂–Y₂O₃ composite coatings were prepared by a modified sol–gel technique, so-called thermal pressure and filtration of sol–gel paint. The composite coatings were derived from a composite paint of yttria partially stabilized zirconia (YSZ) particles, Al₂O₃ particles and Al₂O₃–Y₂O₃ sol. Their microstructure showed that YSZ particles were covered with spatial-network Al₂O₃–Y₂O₃ blanket. Cyclic oxidation at 1,050 °C in air for 200 h demonstrates that the oxygen diffusion rate in the coatings could be effectively inhibited. Meanwhile, suitable coefficients of thermal expansion (CTE) gave the composite coatings better spallation resistance than that of Al₂O₃–Y₂O₃ or ZrO₂–Y₂O₃ coatings. The positive results of cyclic oxidation indicated that the composite coating can be used as an interlayer between the bond coat and the top ceramic layer in traditional TBCs. Not only the depletion rate of aluminum-rich phase in MCrAlY alloy could be slowed down by spatial-network Al₂O₃–Y₂O₃, but also different thermal expansion between thermally grown oxides layer and top layer could be relieved by suitable CTE. In this paper, the mechanisms of the inhibition of oxygen diffusion and thermal match between ceramic coating and alloy are also discussed.     

The influence of precipitation on plastic deformation of Al–Cu–Li alloys

The plastic behaviour of two recently developed Al–Cu–Li alloys with different Cu/Li ratios, AA2198 and AA2196, was studied as a function of precipitation state by tensile tests, strain hardening rate analysis, Bauschinger tests, slip line imaging and atomic scale characterization of precipitate shearing by advanced electron microscopy. For early ageing times where δ′ and/or solute clusters are present, plasticity occurs heterogeneously, the strain hardening rate is high, and the Bauschinger effect is moderate. When T₁ precipitates are present, plasticity becomes more homogeneous, and the strain hardening rate shows a slower decrease compared with other aluminium alloys containing shearable precipitates. In addition, these ageing conditions show a high reversibility of plastic strain, and it is observed that precipitates are sheared only by single steps. The possibility of a specific shearing mechanism of the T₁ phase is discussed in light of these results.     

The influence of powder characteristics on mechanical properties of Al2O3–TiC–Co ceramic materials prepared by Co‐coated Al2O3/TiC powders

Ultrafine Al₂O₃–TiC–Co (ATC) ceramic is prepared in order to improve the bending strength and fracture toughness of ceramic materials. The ultrafine Co‐coated Al₂O₃ and TiC powders have been synthesized by electroless plating at room temperature, and the composite powders were sintered by hot‐pressing to compact ATC samples. The average bending strength, average hardness and average fracture toughness values of ATC ceramic with different particle sizes and Co contents were investigated. The toughening mechanism of the ultrafine ATC ceramic was studied by transmission electron microscopy (TEM) and ceramic performance testing methods. The results show that the relative density, bending strength and fracture toughness values increase remarkably with the increase of Co content. The ultrafine grain of original powders is beneficial to improve the relative density, strength and toughness values of ATC ceramic. The Co phase hinders the growth of ATC ceramic grains during sintering. The Co phase forms a three‐dimensional mesh skeleton structure during sintering, improving the fracture toughness and strength of the composite ceramic.     

Synthesis of Al2O3／WC powder by aluminothermic reduction and carbonization method

Al2O3/WC powder was synthesized by means of aluminothermic reduction-carbonization with metallic Al powder, yellow tungsten oxide and carbon black or graphite as raw materials under the protection of coke granules.The effects of Al2O3 content, temperature, C/WO3 molar ratio, and atmosphere on the synthesis of Al2O3/WC powder were studied. The results show that the relative content of WC and W2C is strongly influenced by the factors mentioned-above. Carbon black has higher reactivity than graphite. Al2O3-WC composite is easier to obtain under the protection of coke granules than under argon atmosphere. The CO in the coke layer can easily react witht ungsten to form WC and to transfer from W2C to WC.     

Synthesis of ZrO2-SiC composite powder and effect of its addition on properties of Al2O3-C refractories

ZrO2-SiC composite powder was synthesized by carbothermal reduction of zircon in argon atmosphere, and it was used as the additive to prepare Al2O3-C refractories. The effects of heating temperature on the synthesis process and the addition of the synthesized composite powder on the properties of the Al2O3-C refractories were investigated. The results show that the synthesized composite powder can be easily obtained by heating the mixture of zircon and carbon black at 1 873 K for 4 h in argon atmosphere, and the relative contents of ZrO2 and SiC in sample reach about 83.7% and 16.3%, respectively. The bulk density, crushing strength and thermal shock resistance of the Al2O3-C refractories can be improved obviously by the addition of the synthesized ZrO2-SiC composite powder.     

Structure of Al–Fe alloys prepared by different methods after severe plastic deformation under pressure

Al–Fe alloys prepared by casting, rapid quenching from the melt, and mechanical alloying from elemental powders have been studied using X-ray diffraction analysis, optical metallography, transmission electron microscopy, and microhardness measurements in the initial state and after severe plastic deformation by high-pressure torsion using Bridgman anvils. The relationship between the phase composition, microstructure, and the microhardness of the investigated alloys has been established.     

Mechanical properties and rapid consolidation of nanostructured WC and WC–Al2O3 composites by high-frequency induction-heated sintering

The short-term rapid sintering of nanostructured WC and WC–Al₂O₃ hard materials was fabricated using the high-frequency induction-heating sintering (HFIHS) process. The sintering behaviors, microstructure, and mechanical properties of the WC and WC–Al₂O₃ composites were investigated. The addition of Al₂O₃ to WC can facilitate sintering, and the grain size of WC decreases as the addition of Al₂O₃ is increased; furthermore, the hardness and fracture toughness of WC-15 vol% Al₂O₃ are greater than those of monolithic WC and Al₂O₃.     

Electrochemical Synthesis and Physicochemical Properties of Nanostructured Al2O3–ZrO2–MgO Oxide Systems

Protection of Metals and Physical Chemistry of Surfaces - Based on the use of electrogenerated reactants, a method for producing Al2O3–ZrO2–MgO oxide systems has been proposed. Their...     

Pressure effect on crystallization kinetics of an Al–La–Ni amorphous alloy

The pressure effect on the crystallization kinetics of an amorphous Al₈₉La₆Ni₅ alloy has been investigated by means of piston–cylinder measurements within a pressure range of 0–1.0 GPa. It was found that an applied pressure enhances the first primary crystallization, i.e. the precipitation of f.c.c.-Al from the amorphous phase. The crystallization temperature decreases from 495 to 440 K when the applied pressure increases from ambient to 0.9 GPa. Meanwhile, the thermal stability of the residual amorphous phase with the Al dispersion is elevated at higher pressures. The crystallization temperature of the residual amorphous phase increases by 35 K/GPa when the pressure increases. The observed pressure effect on the crystallization kinetics in the amorphous alloy, which cannot be interpreted by means of the pressure effect on atomic diffusion, may be well understood in terms of the volume change effect at the early stage of crystallization.     

Al2O3—ZrO2陶瓷的研究

本文主要对Ａｌ２Ｏ３－ＺｒＯ２陶瓷材料进行了研究。利用化学法制备了Ａｌ２Ｏ３和ＺｒＯ２微粉，并对两种微粉的性能进行了测试，利用常压氧化气氛烧结技术制备了Ａｌ２Ｏ３－ＺｒＯ２陶瓷材料，并对其力学性能进行了评价，在我们的试验条件下，研制的Ａｌ２Ｏ３－ＺｒＯ２陶瓷材料的力学性能分别为：维氏硬度Ｈｖ＝１２ＧＰａ，弯曲强度σ＝１０５０ＭＰａ，断裂韧性ＫＩＣ＝１２．２ＭＰａｍ＾１／２，杨氏模量Ｅ＝２５８ＧＰａ