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The post-oxidation treatment (POT) is an important step in the preparation of silicon carbide (SiC) ceramic membranes via recrystallization sintering, which is generally considered to remove any possible free carbon. It is found, however, that increasing the temperature of POT improves not only the hydrophilicity and flux of SiC ceramic membranes but also their mechanical properties, chemical stability, etc. Therefore, it is necessary to study the principle of POT in order to obtain SiC ceramic membranes with optimal properties. In this study, the principle of POT was characterized via scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, synchronous thermal analysis, and metallographic microscopy. According to the principle, the conditions, such as the temperature and air flow rate, of the post-oxidation process were optimized. Under the optimized conditions, POT improved the flux of SiC ceramic membranes from 1074 to 5118 L·m−2·h−1·bar−1, increased the bending strength from 26 to 35 MPa, and provided SiC ceramic membranes with high stability under acid and alkali conditions. [ABSTRACT FROM AUTHOR]

A laminated structure is applied to strengthen metal alloys by a microstructure design strategy. Explosive welding is an important manufacturing method to realize the joining of laminated composites. Therefore, in this research, three Ti layers and two Cu layers (the thickness of each layer is 0.5 mm) are stacked alternatively to produce a Ti/Cu laminated composite by underwater explosive welding. Different microstructural techniques (i.e., optical microscopy and scanning electron microscopy) all reveal that the Ti/Cu laminated composite has a characteristic laminated structure with remarkably fine and elongated grains but no cracks. Tensile testing results indicate that the Ti/Cu laminated composite has a high ultimate tensile strength that is approximately 2 times higher than the estimated value for Cu and Ti mixtures. Microscopy observations reveal that the deformed microstructure and strong bonding interface of the Ti/Cu laminated composite can activate work hardening strengthening, terminate cracks and inhibit necking simultaneously, which leads to the high ultimate tensile strength of the composite. The present laminated structure with an improved strength provides an approach to strengthen similar metal alloys. [ABSTRACT FROM AUTHOR]

High Nb-containing TiAl alloys have good oxidation resistance and mechanical properties, but the microstructure and the properties are substantially affected by the segregation. To quantitatively investigate the segregation behavior of Al during solidification, microstructures of directionally solidified (DS) Ti-45Al-8Nb (in atomic percent) alloy prepared at withdrawing rates of 30 μm/s and 200 μm/s and a temperature gradient of 4200 K/m were observed by optical microscope and electronic probe microanalyzer. The microsegregations were characterized by wave dispersive spectroscopy. The results show that the DS ingots include the no melting zone, directionally solidified zone with columnar grains, mushy zone, and quenched liquid zone. The primary dendritic arm spacings are 353 μm and 144 μm, respectively, for the two ingots. But the solidified microstructures of the ingots are large lamellar colonies, which contain a few B2 patches and γ bands induced by microsegregation. From dendritic zone to columnar zone, the volume fractions of B2 patches and γ bands decrease. The segregation extents of Al and Nb decrease with the increase of solidification rate. There exists an obvious back diffusion process of Al during solidification and cooling after solidification. According to evolution of Al concentration profiles from mushy zone to columnar grain zone, interdiffusion coefficient for Al in β-Ti at near solidus temperature is semiquantitatively calculated, and the value is (6 – 11) × 10−11 m2/s. [ABSTRACT FROM AUTHOR]

The friction weldability of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy has been investigated by optimizing process parameters and analyzing the microstructures and tensile properties of the joints. The as-cast alloy with a nearly lamellar (NL) microstructure and the as-forged alloys with a duplex (DP) microstructure have been successfully welded. All the joints have a severe deformation zone (SDZ) and a transition zone (TZ) between the parent metal (PM) and SDZ. SDZ, showing a biconcave lens geometry, has a maximum thickness of hundreds of micrometers at the periphery. TZ is hundreds of micrometers thick. All SDZs have a fine-grained DP microstructure with a grain size of a few micrometers. For the joint of the as-cast alloy, the TZ consists of deformed lamellar colonies as the major constituent and partially recrystallized grains as the minor constituent. For the joint of the as-forged alloy, the TZ is similar to both the PM and SDZ, showing a DP microstructure. The grain size, volume fraction of γ grains, and the remnant lamellar colonies all increase with the distance from the SDZ. All joints presented perfect metallurgical bonding. The strengths of the joints are higher than those of the corresponding PMs. This indicates that the studied alloy has good friction weldability. [ABSTRACT FROM AUTHOR]

The demand for separation and purification applications under harsh conditions has grown strongly in recent years. Silicon carbide (SiC) ceramic membranes have broad prospects in this aspect due to their unique characteristics, but its pore size control is a crucial problem. Therefore, it is of great significance to develop simple and feasible methods for precise control of the pore size of SiC membranes to improve membrane selectivity and expand their application range. This review describes the pore formation process in the preparation of SiC membranes, focusing on the selection of SiC particles, sintering temperature, sacrificial template, sintering aids, oxidation process and other factors affecting the pore size and analysis. Finally, the control of SiC membrane pore size is summarized and the outlook is proposed. [ABSTRACT FROM AUTHOR]

Abstract: Exploratory experiments of laser welding cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft were conducted. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. The corresponding mechanisms were discussed in detail. Results showed that the laser-welded seam had non-equilibrium solidified microstructures consisting of FeCr0.29Ni0.16C0.06 austenite solid solution dendrites as the dominant and some fine and dispersed Ni3Al γ′ phase and Laves particles as well as little amount of MC short stick or particle-like carbides distributed in the interdendritic regions. The average microhardness of the welded seam was relatively uniform and lower than that of the base metal due to partial dissolution and suppression of the strengthening phase γ′ to some extent. About 88.5% tensile strength of the base metal was achieved in the welded joint because of a non-full penetration welding and the fracture mechanism was a mixture of ductility and brittleness. The existence of some Laves particles in the welded seam also facilitated the initiation and propagation of the microcracks and microvoids and hence, the detrimental effects of the tensile strength of the welded joint. The present results stimulate further investigation on this field. [Copyright &y& Elsevier]

High Nb containing TiAl alloys have prospect to be used for turbo wheel and exhaust valve for vehicle engine. Friction welding is a suitable process for fabricating these components. For investigating the weldability of these alloys, cylindrical samples of Ti–45Al-8.5Nb-0.2W-0.2B-0.02Y (in at%) alloy with a diameter of 10 mm were welded by continuous drive friction welding process, and the microstructures and the mechanical properties of the joints were analyzed in this paper. The results showed that the weldability of the present alloy was very good. At parameters of rotational speed of 2400 r/min, friction pressure of 320 MPa, burn-off distance of 3 mm and upset pressure of 360 MPa, completely metallurgical bonding interfaces were obtained. The joint had a severely plastic deformation zone (SPDZ) with a biconcave lens geometry. The thicknesses of the center and periphery of SPDZ were about 40 μm and 800 μm, respectively. There existed a transition zone (TZ) between parent metal (PM) and SPDZ. SPDZ exhibited the deformation flow lines and had a randomly-oriented recrystallized duplex (DP) microstructure with the average grain size of 2.5 μm, in contrast, PM presented a nearly lamellar (NL) microstructure with a colony size of 150 μm. TZ showed a mixture microstructure combining deformed lamellar colonies and partially recrystallized grains. At room temperature, 750 °C and 800 °C, welding joints had better tensile properties than PM. • As-cast Ti–45Al-8.5Nb-0. 2W-0. 2B-0. 02Y alloy has good friction weldability. • The joint had a severely plastic deformation zone with a biconcave lens geometry. • Fine-grained DP microstructure of welding joint showed random orientation. • Welding joint had better tensile properties than parent metal. [ABSTRACT FROM AUTHOR]

Abstract: With the development of superconducting technology, high magnetic fields (HMF) provide unique opportunity for materials and physics scientists to extend or exploit high performance of existing materials. In this paper, we report our findings of the improvement of compressive strength and ductility in a NiAl based eutectic alloy induced by high uniform magnetic field treatment. It shows that the compressive strength and ductility of NiAl–Cr(Mo)–(Ti,Hf) eutectic alloy were improved by 100 MPa and 12% respectively after 11T uniform magnetic field treatment. Microstructural characterization and fractography observation suggested that the modification of morphology and dispersed distribution of constituent phases through atomic diffusion induced by the HMF treatment could be responsible for the improvement of mechanical performance. [ABSTRACT FROM AUTHOR]

A three dimension thermodynamic model is used to investigate the steady wetting state of droplet on a single microstructure surface based on the lowest principle of droplet free energy. The cone, frustum (upright and inverted) and sphere in micro-scale are selected as studied structures. The results show that both the intrinsic contact angle and morphology parameters of microstructure surface affect the wetting depth and normalized Helmholtz free energy (NFE) of droplet. The wetting maps for minimum points of NFE of droplet on cone and frustum surfaces with respect to intrinsic contact angle and morphology parameters have been presented. These wetting maps provide a guidance for fabricating microstructures that is used to enhance droplet adhesion. The effect of three phase contact line tension on droplet NFE has been studied. Results indicate that the line tension slightly increases the droplet NFE and that causes the decrease of droplet wetting stability. The absolute differences between F τ * (NFE with line tension) and F * (NFE without line tension) are small both for 1 μL and 1 nL droplets. But the relative differences, (F τ *- F *)/ F *, of 1 nL droplet are significantly larger than that of 1 μL droplet. Therefore, for a droplet with small volume, the line tension effect on droplet free energy is relatively large and that results in the increase of instability of droplet wetting state. [ABSTRACT FROM AUTHOR]

Abstract: A process of laser cladding Ni–Cr–C–CaF2 mixed powders to form a multifunctional composite coating on γ-TiAl substrate was carried out. The microstructure of the coating was examined using XRD, SEM and EDS. The coating has a unique microstructure consisting of primary dendrite or short-stick TiC and block Al4C3 carbides reinforcement as well as fine isolated spherical CaF2 solid lubrication particles uniformly dispersed in the NiCrAlTi (γ) matrix. The average microhardness of the composite coatings is approximately HV 650 and it is 2-factor greater than that of the TiAl substrate. [Copyright &y& Elsevier]