Effects of sintering parameters on the microstructure and tensile properties of in situ TiBw/Ti6Al4V composites with a novel network architecture

In order to better understand the relationship of processing–structure–mechanical properties of in situ TiB whisker reinforced Ti6Al4V (TiBw/Ti64) composites with a novel network architecture, the effects of sintering parameters on the microstructure and tensile properties of the composites were investigated. TiB whiskers with the highest aspect ratio and the coarsest whiskers were obtained at 1100 °C and 1200 °C due to the skips of whisker growth speeds along the [0 1 0] direction and the [0 0 1] and [1 0 0] directions, respectively. Additionally, TiB whisker with a claw-like structure can be synthesized from one TiB₂ polycrystal parent. The quasi-continuous network architecture of TiBw/Ti64 composites can be achieved at higher sintering temperatures more than 1200 °C. The prepared composites with the quasi-continuous network architecture exhibit a superior combination of tensile properties.     

Toughness enhancement by polycarbosilane coating on SiC whiskers incorporated in Si3N4 matrix composite

Si₃N₄ matrix composite was fabricated by hot pressing with 20% SiC whiskers coated with polycarbosilane (PCS). The preceramic polymer on the whiskers was pyrolysed during sintering to form a carbon-rich layer at the whisker/matrix interface. Mechanical properties were measured, and compared to those of the composites with whiskers purified with HCl and HF. Elastic modulus and bending strength of the composite with PCS-coated whiskers were lower than those of the composites with other whiskers. Fracture toughness was measured by single-edge notched beam (SENB) and single-edge precracked beam (SEPB) methods. The toughness, including crack-growth resistance measured by the SEPB method, increased from 7.2 MPam^1/2 to 7.9 MPam^1/2 by PCS-coating on the whisker, while the toughness measured by the SENB method decreased from 6.5 MPam^1/2 to 5.7 MPam^1/2. The layer derived from PCS facilitated debonding at the whisker/matrix interface and activated the wake-toughening. Optical microscopic observation of the crack propagation near the interface confirmed enhancement of interfacial debonding by the PCS-coating.     

Effects of amorphous silica coatings on the sintering behaviours of SiC whisker-reinforced Al2O3 composites

In this study, pressureless sintering of silicon carbide whisker (SiC_w)-reinforced alumina composites was investigated. SiC whiskers or Al₂O₃ powders were coated with amorphous silica, and sintering behaviour was analysed according to the powder characteristics of the composite. It was found that amorphous silica coatings improved densification as compared with uncoated powders, because the viscous flow allows the release of any tensile stress due to differential shrinkage between the matrix and the silicon carbide whiskers. Mullite occurred when amorphous silica coatings reacted with alumina at 1500 °C, which resisted the viscous sintering of the amorphous silica coatings.     

Fabrication of 6061 aluminium alloy/Al18B4O33(w) composite by using sol–gel alumina binder

Abstract

For fabrication of aluminium borate whisker (Al₁₈B₄O_33(w)) reinforced 6061 aluminium alloy composites, a sol–gel alumina binder instead of conventional silica binder was used for preparing the whisker preforms of the squeeze cast composites. The results show that a sound whisker preform and a uniform composite can be made by this method. Unlike the reactive silica binder, the sol–gel alumina binder is rather stable throughout the entire high temperature fabrication process. Under appropriate conditions, the sol–gel alumina binder can also serve as a thermal barrier for minimising interfacial reactions between aluminium borate whiskers and the matrix alloy. With a binder concentration of 0.6 mol L^-1, the ultimate tensile strength of the composite is as high as 277.6 MPa at room temperature and moderate at elevated temperatures. The tensile fracture of the alumina bound composite shows a mixed mode of dimple fracture and interface debonding.     

Fabrication of aluminum matrix composites with enhanced mechanical properties reinforced by in situ generated MgAl2O4 whiskers

Aluminum matrix composite reinforced by in situ generated single crystalline MgAl₂O₄ whiskers was fabricated by chemical synthesis method in an Al-Mg-H₃BO₃ system. A large number of MgAl₂O₄ whiskers were generated during the sintering process and distributed homogeneously in the Al matrix. The whiskers penetrate into the matrix grains to form the framework of the materials, leading to an incredible increase in mechanical properties of the composites. The generation mechanism of the MgAl₂O₄ whiskers was also discussed.     

Aluminium-matrix silicon carbide whisker composites fabricated by pressureless infiltration

Al-matrix SiC whisker composites were fabricated by pressureless infiltration of liquid Al-Mg or Al-Si-Mg alloys at 830–950°C in the presence of N₂ into a preform of nickel coated SiC whiskers. The nickel coating on the whiskers was obtained by electroless plating and made pressureless infiltration possible. The composite made by pressureless infiltration exhibited slightly lower tensile strength and modulus and slightly higher coefficient of thermal expansion than the corresponding composite made by pressure infiltration. However, the differences were small in spite of the lack of prior evacuation in the pressureless infiltration case. On the other hand, the hardness decreased with increasing distance from the preform-melt interface much more significantly in composites made by pressureless infiltration than those made by pressure infiltration. The hardness decrease, which was attributed to a porosity increase, was larger for composites made by pressureless infiltration without prior evacuation than those made by pressureless infiltration with prior evacuation. The Al-SiC reactivity was larger for composites made by pressureless infiltration than those made by pressure infiltration, because the infiltration time was longer in pressureless infiltration.[/p]     

Characterization of metal-matrix composites fabricated by vacuum infiltration of a liquid metal under an inert gas pressure

Silicon carbide whisker-reinforced aluminium was fabricated by vacuum infiltration of liquid aluminium into a porous whisker preform under an argon gas pressure, using an infiltration temperature of 665 °C. The volume fraction of whiskers ranged from 11% to 37%. No whisker pull-out was observed on the fracture surface for an infiltration temperature of 665 °C, but some whisker pull-out was observed for an infiltration temperature of 720 °C. Both the tensile strength and ductility decreased with increasing infiltration temperature above 665 °C. Tensile test results from room temperature to 300 °C are reported. They showed that the quality of these composites was comparable to that of composites made by powder metallurgy or squeeze casting. The coefficient of thermal expansion at 100–150 °C was decreased by 45% by the addition of 37 vol % whiskers.     

Equicohesive temperature of the interface and matrix and its effect on the tensile plasticity of Al18B4O33 whiskers reinforced aluminum composite at elevated temperatures

A pure aluminum composite reinforced by Al₁₈B₄O₃₃ whiskers was fabricated by a squeeze casting technique. In the present study, it is found that the dependence of tensile plasticity on temperature in Al₁₈B₄O₃₃ whiskers reinforced aluminum composite is different from other discontinuously reinforced aluminum composites. The tensile elongation to fracture of the composite obtains its maximum value at about 573 K, which is considered to be related to the equicohesive temperature of interface and matrix. This will also establish a basis for the research of hot forming process of the composite at elevated temperatures.     

Improvement of the temperature resistance of aluminium-matrix composites using an acid phosphate binder

The use of phosphate binders instead of the widely used silica binder resulted in improved temperature resistance, increased tensile strength and decreased coefficient of thermal expansion. The effects were largest for the phosphate binder which contained the largest amount of phosphoric acid (P/Al atom ratio = 24 in the liquid binder). These effects were probably due to the protection of the SiC whiskers by the binder phases (aluminium metaphosphate or aluminium orthophosphate), the binder-SiC reaction product (SiP₂O₇) and the binder-aluminium reaction product (AIP) from further reaction between the SiC and aluminium. The tensile strength of the composite containing the SiC whisker preform made with the phosphate binder (P/Al atom ratio = 6 or 24 in the liquid binder) was increased after heating at up to 600 °C for 240 h. The silicon phosphate (SiP₂O₇) acted as an in situ binder and was primarily responsible for increasing the compressive strength of the preform and increasing the temperature resistance of the composite. The carbon fibre composite containing the preform made by using the phosphate binder (P/Al atom ratio = 24 in the liquid binder) with either water or acetone as the liquid carrier during wet forming of the preform had a higher tensile strength than the carbon fibre composite made by using the silica binder. After composite heat exposure to 600 °C for 14 h, the carbon fibre composite made by using this phosphate binder with acetone as the liquid carrier during wet forming of the preform showed the best temperature resistance, while the carbon fibre composites made by using this phosphate binder with water as the carrier showed the second best temperature resistance, and that made by using silica binder was the worst. The reason for the better effect of the phosphate binder than the silica binder is probably due to the ability of the phosphate binder and the binder-aluminium reaction product (AIP) to protect the carbon fibres from the undesirable reaction between the carbon fibres and aluminium. The lack of a binder-fibre reaction contributed to making the carbon fibre composites less temperature resistant than the SiC whisker composites. The use of a higher binder concentration is attractive for increasing the temperature resistance of the composites. The binder concentration in the preform can be increased by increasing the binder concentration in the slurry used in the wet forming of the preform.     

Improved sinterability of SiC(w)/Si3N4 composites by whisker-oriented alignment

Effects of whisker-oriented alignment on sintering behavior of SiC(w)/Si₃N₄ composites were investigated by sintering curves of SiC(w)/Si₃N₄ compacts with different whisker alignment (random and unidirectionally oriented). It was found that whisker-oriented alignment led to considerable anisotropy of linear shrinkage and sintering rate of SiC(w)/Si₃N₄ compacts during sintering. Whisker-oriented alignment could improve the sinterability of the SiC(w)/Si₃N₄ composites and consequently improve the maximum fraction of whisker addition and reduce the content of sintering additives without obviously lowering the density, which are more beneficial to exert the best reinforcing effects of whiskers and mechanical properties of SiC(w)/Si₃N₄ composites at elevated temperature.     

Quantitative determination of the stability of SiC whisker in reaction-bonded/hot-pressed Si3N4 composites by X-ray diffraction

A method was developed for the quantitative determination of weight fractions of the phases in SiC whisker-reinforced reaction-bonded Si₃N₄ composites using X-ray diffraction. Composites with different amounts of sintering additives and whiskers were fabricated using reaction bonding followed by hot pressing. The amount of whiskers remaining in each composite after each processing stage was determined. In order to study the degradation mechanism, the microstructural development after each processing step was examined using scanning and transmission electron microscopy. Finally, the effect of sintering additives on the microstructural development and whisker stability was also investigated.     

Microstructure and mechanical properties of titanium carbonitride whisker reinforced β-sialon matrix composites

The microstructure, hardness, fracture toughness and thermal shock resistance were investigated for 15 vol.% TiC_0.3N_0.7 whisker reinforced β-sialon (Si_6−zAl_zO₂N_8−z with z=0.6) composites with additions of three different volume fractions 2, 5 and 20 vol.%, of an yttrium-containing glass oxynitride phase. The composites were prepared by hot pressing at 1750°C for 90 min under a uniaxial pressure of 30 MPa in nitrogen atmosphere. The TiC_0.3N_0.7 whiskers were found to survive without deteriorating in morphology or reacting with the β-sialon matrix and/or the glass phase. The TiC_0.3N_0.7 whiskers had no obvious influence on the matrix microstructure, but their presence improved both the hardness and the fracture toughness of the composites. The highest hardness was obtained for the whisker composite with 2 vol.% glass phase (H_v=18.6 GPa). The fracture toughness and thermal shock resistance improved with increasing glass content. The whisker reinforced composite containing 20 vol.% glass showed the highest fracture toughness (K_1C=6.8 MPa m^1/2). No unstable crack extension occurred during the thermal shock test of the obtained composites in the temperature interval 90-700°C, but above 700°C severe oxidation of the whiskers precludes further evaluation of thermal shock properties by the indentation-quench method applied.     

团簇对粉末热挤压铝基复合材料力学性能的影响

为明确晶须团簇行为对材料力学性能的影响,采用粉末热挤压法制备了硼酸镁晶须增强铝基复合材料,对不同含量的晶须增强铝基复合材料进行了力学性能测试,并基于载荷传递模式提出相应的模型对材料强度进行预测.结果表明:随着硼酸镁晶须含量的增加,团簇加剧;当晶须体积分数大于10%时,材料力学性能降低;提出的模型考虑了团簇因素,成功预测了复合材料的实验强度.     

Analysis of the elastic properties of an interpenetrating AlSi12-Al2O3 composite using ultrasound phase spectroscopy

An interpenetrating composite fabricated by squeeze-casting a eutectic aluminium-silicon alloy into a porous alumina preform is studied in this work. The preform was fabricated by pyrolysis of cellulose fibres used as pore forming agent, pressing of the green ceramic body and subsequent sintering of alumina particles. The resulting preform had both micropores within the ceramic walls and macropores between those walls, which were infiltrated by the liquid metal. Composites with alumina contents varied in the range of 18-65 vol.% were studied. Three longitudinal and three shear elastic constants of the composites were determined using ultrasound phase spectroscopy on rectangular parallelepiped samples. Complete stiffness matrix of one sample was determined by modifying the sample geometry by cutting at the corners of the sample and subsequent ultrasonic measurements. All composites exhibit a moderately anisotropic behavior, which can be attributed to a non-random pore orientation distribution caused by uni-axial pressing of the preforms prior to sintering. The experimental results are compared with several theoretical micromechanical models.     

Development of ZrO2 interfaces for all-oxide composites

Development of a fibre coating process based on sol–gel synthesis for depositing ZrO₂ interfacial layers on Al₂O₃ fibres is described. The sol employed exhibited a shear-thinning behaviour and was used to infiltrate Nextel™ 610 fibre tows, forming minicomposites that are used as reinforcements in glass matrix composites (GMCs). The sol–gel method was investigated with respect to the rheological properties of the sol, and the thermal and sintering behaviour of the deposit. The dip-coating method was then optimised and uniformly coated fibres were obtained, which exhibited sufficient retained tensile strength (>50%) to be used as reinforcement in GMCs.     

Effect of sintering temperature on magneto-transport properties of La0.67Ba0.33MnO3/Ba0.7Sr0.3TiO3 composites

La_0.67Ba_0.33MnO₃-20 wt.%-Ba_0.7Sr_0.3TiO₃ composites were sintered at different temperatures in order to explore the possibility of improving the magneto-transport properties of the composites. Detail studies on the magnetic and electrical transport properties for the sintered composite samples have been performed. Results show that the sintered composites have identical ferromagnetic to paramagnetic transition temperature and filamentary feature of metallic phase. When sintering temperature higher than 1300 °C, the composites show Efros-Shklovskii-like variable-range hopping in the temperature range lower than Curie temperature. For samples sintered lower than 1100 °C, a dome-like resistance peak appears at a temperature well below the Curie temperature. Magnetoresistance behavior indicates the existence of spin polarized tunneling in the low temperature range. Considering the contributions from Efros-Shklovskii-like variable-range hopping and spin polarized tunneling, the resistance peak can be well fitted.     

Influence of Bi2O3 and CuO addition on low-temperature sintering and dielectric properties of Ba0.6Sr0.4TiO3 ceramics

In this study, we tried to lower the sintering temperature of Ba_0.6Sr_0.4TiO₃ (BST) ceramics by several kinds of adding methods of Bi₂O₃, CuO and CuBi₂O₄ additives. The effects of different adding methods on the microstructures and the dielectric properties of BST ceramics have been studied. In the all additive systems, the single addition of CuBi₂O₄ was the most effective way for lowering the sintering temperature of BST. When CuBi₂O₄ of 0.6 mol% was mixed with starting BST powders and sintered at 1100 °C, the derived ceramics demonstrated dense microstructure with a low dielectric constant (? = 4240), low dielectric loss (tan δ = 0.0058), high tunability (Tun = 38.3%) and high Q value (Q = 251). It was noteworthy that the sintering temperature was significantly lowered by 350 °C compared with no-additive system, and the derived ceramics maintained the excellent microwave dielectric properties corresponding to pure BST.     

Preparation and characterization of ZrB2-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Interfacial reactions in K2Ti6O13(w)/Al2O3/Al composite

Abstract

A pure Al matrix composite, reinforced by potassium titanate whiskers coated with sol–gel alumina, was fabricated by squeeze casting. Good interface bonding was achieved in the coated composite. Interfacial reactions in the composite were found to be less severe than those in an uncoated composite, owing to the barrier effect of sol–gel -Al₂O₃ coating. After the composite was thermally exposed at 530°C for 30 h, the alumina coating still restrained the decomposition of the whisker structure, and thus prevented a decrease in the strength of the composite. However, after a thermal exposure at 700°C for 10 h, the coating was no longer held on whisker surfaces. Mechanical testing showed that tensile fracturing of the coated composite tended to occur at the coating/matrix interface where the interfacial bonding is weaker than at the coating/whisker interface.     

Microstructural features of sintered Si3N4/SiC-whiskers composites: mechanical integrity of whiskers

A detailed microstructural analysis of slip-cast Si₃N₄/SiC-whisker composites has been made by TEM. In spite of the gentle forming process, SiC whisker breakage constitutes a fundamental feature of this material. The breaking takes place during the sintering process and could be associated with residual thermal stresses, as revealed by the experimental evidence. The dramatic decrease in the whiskers aspect ratio translates into little or no effect of whisker addition on the mechanical properties of the Si₃N₄ matrix.