Production of MgAl2O4 – Ti(C,N) composite ceramics by aluminothermic reduction in reducing atmosphere

Abstract

In the presence of N₂ and CO gases, MgAl₂O₄ – Ti(C,N) composite has been synthesised by aluminothermic reduction. The phase characterisation and microstructure of this novel composite were investigated by powder X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results show that the nitridation reaction begins at 1100°C. With an increase of temperature, TiC starts to appear and forms Ti(C,N) solid solution with TiN; the grain size of Ti(C,N) grows with increasing temperature and the final product is MgAl₂O₄ – Ti(C,N).     

Development of precipitate size and volume fraction of niobium carbonitrides in stabilised stainless steel

Abstract

Niobium carbonitride (NbX) precipitates have been studied in a niobium stabilised austenitic stainless steel AISI type 347 with approximate nominal composition Fe–0.06C–17.5Cr–11.4Ni–0.8Nb. The steel was extruded to seamless tube, solution annealed at 1100°C for 3 min, water quenched, and subsequently isothermally aged at 700°C for times up to 70 000 h. Size distribution and volume fraction of the coarse distribution (1–10 µm) of NbX particles were measured using scanning electron microscopy (SEM). The fine distribution (～30 nm) was investigated using energy filtered transmission electron microscopy (EFTEM). Size distribution and volume fraction were determined using jump ratio images. Coarse NbX (～0.3% volume fraction) precipitates were formed during solidification and extrusion, and were little affected by solution annealing and isothermal aging. Fine NbX (～0.5% volume fraction) precipitates formed during solution annealing and grew during the first 800 h of aging. Precipitate size determination using EFTEM appears to give accurate results, while volume fraction determination requires homogeneous material for good results.     

Structure evolution in annealed and hot deformed Al–V2 O5 composite

Abstract

Structural observations and hot deformation tests were carried out on mechanically alloyed Al-10 wt-%V₂ O₅ composite. Initial annealing experiments revealed a hardening of the material during the first stage of annealing. The material hardness increased from 114 HB for as extruded material to 167 HB after annealing at 873 K for 6 h. Differential scanning calorimetry tests conducted on as extruded material confirmed the development of an exothermic reaction during heating of the material within the temperature range 650–870 K. The amount of heat released was reduced with increasing annealing time at 873 K. Transmission electron microscopy (TEM) and X-ray analysis of annealed material revealed new intermetallic grains and very fine aluminium oxide particles, which resulted from the chemical reaction between the aluminium matrix and vanadium oxides. The development of voids in long aged specimens was found to be an undesirable effect of local specific volume reduction during the course of the chemical reaction that was not fully compensated by the local volume increase due to the growth of intermetallic particles. As a result, the material hardness was reduced in long time annealed specimens. The mechanical properties of as extruded and annealed specimens were investigated by means of hot compression testing within the temperature range 623–903 K. These tests revealed that the flow stress of as extruded material was reduced from 180 to 22 MPa when tested at 623 and 903 K, respectively. Annealed specimens exhibited higher flow stresses of 195 and 32 MPa at the same temperatures. The results indicate that the strength of the material can be effectively increased owing to a change of material structure as a result of the chemical reaction taking place during high temperature annealing.     

Direct observation of ternary solid state transformation in Bi–Cd–In alloy system

Abstract

Cast or solution treated specimens of a Bi–9·0Cd–26·7In (wt-%) alloy were observed to form a fine, three phase microstructure on aging at room temperature, replacing a single phase formed at a higher temperature. The three phases resulting from this solid state reaction were found to grow with a lamellar morphology into the high temperature phase, with a growth rate of 0·5–1·0 μm h^-1 at room temperature. The equilibrium temperature for the transformation was found to be ～25°C. Using a Hitachi S-4500 field emission SEM, the phase transformation was followed in progress at magnifications of 3000 and 10 000 times. It was noted that a volume change was associated with the transformation. It was concluded that the transformation is of the ternary eutectoid type.     

Magnesium matrix nanocomposites fabricated by ultrasonic assisted casting

Abstract

Magnesium matrix composites reinforced with nano-sized SiC particles (n-SiC_p/AZ91D) were fabricated by high intensity ultrasonic assisted casting. The microstructure of the nanocomposites was investigated by optical microscopy, scanning electronic microscopy (SEM), high resolution transmission electronic microscopy (HRTEM) and Energy Dispersive Spectroscopy (EDS) methods. The results showed that the dispersion and distribution of n-SiC_p in magnesium alloy melts were significantly improved by ultrasonic processing. Compared to the unreinforced AZ91D matrix, mechanical properties of the nanocomposites including tensile and yield strengths were remarkably improved and the yield strength increased by 117% after gravity permanent mould casting.     

Study of miscibility of melamine-formaldehyde resin and poly(vinyl acetate) blends for use as adhesives in engineered flooring

The objective of this research was to investigate the miscibility behavior of melamine-formaldehyde (MF) resin and poly(vinyl acetate) (PVAc) blends for their use as adhesives for bonding fancy veneer and plywood in engineered flooring, by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). Blends of various compositions of MF resin/PVAc were prepared. To determine and compare the effect of PVAc content, blends with PVAc to MF resin weight ratios of 0, 30, 50, 70 and 100% were prepared. These blends displayed a single cure temperature over the entire range of compositions indicating that this blend system was miscible in the amorphous phase due to the formation of hydrogen bonding between the amine groups of the MF resin and the carbonyl groups of PVAc.     

Influence of the interfacial resin layer on delamination initiation in broken ply composite laminates

The present study has investigated the influence of a resin layer on the delamination initiation at the interface of broken and continuous plies in the case of GR/E (graphite/epoxy) laminates with broken central plies. A full three-dimensional (3D) finite element (FE) analysis was performed with each layer of the laminate modelled as homogeneous and orthotropic. The interface between the broken and the continuous plies was modelled with a thin resin-rich layer. Eight-noded isoparametric layered elements were used to model the laminate specimen. Also, 3D contact elements were used to prevent inter-penetration of the delaminated faces at the interface. Based on the results of the 3D FE analysis, strain energy release rates were calculated at the delamination front using Irwin's 'crack closure integral'. Using the concepts of linear elastic fracture mechanics (LEFM), the strain energy release rate was used as a parameter for assessing delamination initiation. The effects of various factors such as resin layer stiffness, resin layer thickness, and fibre orientation at the interface on the three components of the strain energy release rates, namely G_I, G_II and G_III, were studied for laminates with various crack sizes of the broken ply, and the influence of the resin layer in the delamination initiation was established. It was observed that delamination initiation is a mixed-mode phenomenon even in the case of uniaxial loading and the dominance of the mode of delamination is governed by the resin layer stiffness, thickness, and lamina orientation at the interface. The present work also concludes that an increase in the resin layer modulus leads to an increase in the probability of mode I delamination while the probability of mode II delamination decreases. A 0/90 interface exhibits a higher chance of delamination in modes I and II, while mode III delamination is maximum for 0/30 and 0/60 fibre orientation interfaces. It was also observed that the larger the crack width, the greater the probability of delamination initiation at the interface.     

Thermal ageing and lifetime prediction for organic matrix composites

Abstract

During their thermal ageing in air, organic matrix composites undergo a superficial oxidation leading to a 'spontaneous' cracking. The aim of this article is to present a new strategy of investigation intended to minimise empirism in the field of lifetime prediction. It is based on a kinetic model starting from a radical chain mechanism, initiated by hydroperoxide decomposition, and coupling the oxygen diffusion and consumption kinetics. The model gives access to the thickness distributions of all the chemical changes happening during the exposure. All its parameters have a physical sense and are checkable experimentally. In addition, they obey Arrhenius law, which is not the case for the usual global properties. Some significant results showing the model efficiency are reported. Then, it is shown how, in the near future, the model will be usable to predict changes in mechanical properties.     

Effects of Different Silane Coupling Agent Monomers on Flexural Strength of an Experimental Filled Resin Composite

The hydrolytic stability of various silane combinations and their effects on biomechanical properties and water sorption of an experimental dental composite made of bis-GMA and TEGDMA and silane-treated fillers were evaluated. Four silane coupling agents and their blends with a cross-linker silane were used as coupling agents for the 0.7-μm BaSiO₃ fillers. The silanization was carried out in toluene containing 1% (v/v) of one of the four following organofunctional silane coupling agents: 3-acryloxypropyltrimethoxysilane, 3methacryloxypropyltrimethoxysilane, 3-styrylethyltrimethoxysilane and 3-isocyanatopropyltriethoxysilane. Blends of these functional silanes with 1% (v/v) of a cross-linker silane, 1,2- bis -(triethoxysilyl)ethane were also used for silanization. Composites were prepared by mixing 5.00 g Ba-glass filler with 2.00 g of a resin mixture consisting of bis-GMA (58.8 wt%) and TEGDMA (39.2 wt%) in a high-speed mixer. Threepoint bending test specimens (2.0 mm × 2.0 mm × 25.0 mm) were fabricated (n= 8) in a mould and photo-polymerized. The degree of conversion was measured with FT-IR. Biomechanical testing was carried out according to the ISO 10477 standard. Specimens were tested (flexural strength) after 30 days of water storage (37° C, distilled water). Water sorption and solubility (in wt%) were also measured on 1, 2, 3, 5, 7, 14, 21 and 30 days in water storage. Statistical analysis with ANOVA showed that the highest flexural strength was obtained when 3-acryloxypropyltrimethoxysilane + 1,2- bis -(triethoxysilyl)ethane (100.5 MPa; SD, 25.7 MPa) was used in the silanization step, and the lowest was obtained when 3isocyanatopropyltriethoxysilane + 1,2- bis-(triethoxysilyl)ethane (28.9 MPa; SD, 8.8 MPa) was used. The three-point bending strength was significantly affected by the functionality of the main silane tested (p< 0.05), but not by the addition of the cross-linker silane ( p> 0.05). The composite that had been silanized with 3-isocyanatopropyltriethoxysilane had the greatest amount of water uptake (1.75%), and the composite silanized with 3-methacryloxypropyltrimethoxysilane + 1,2- bis-(triethoxysilyl)ethane had the least (1.08%). In conclusion, selection of the functional silane monomer can be a significant factor in developing filled resin composites in dentistry.     

多层介质单模光纤传输特性：一种简便有效的数值法

本文提供了多层介质光纤传输特性分析的快速有效的数值法,即对近似的标量边界条件建立矩阵型特征方程,为2×2矩阵运算,这样可节省大量的计算机内存加快运算速度。对弱导光纤计算结果的精度是能满足要求的可靠的。通过大量分析提出了保证低损耗小色散单模光纤优化设计的思路及方法。