The effect of volume fraction of WC particles on wear behavior of in-situ WC/Fe composites by spark plasma sintering

Tungsten carbide (WC) particles have been in-situ synthesized through the reaction between tungsten particles and carbide particles by spark plasma sintering (SPS). The composites with different WC content were comparatively observed by the techniques of scanning electron microscopy (SEM), high-resolution transmission electron microscope (HRTEM), X-ray diffraction, hardness and pin-to-disc abrasive wear test. The results showed that the formed WC particles were homogenously distributed in the iron matrix with the size of smaller than 25 μm. Additionally, with the increasing of the WC content, the hardness of composites, the microhardness of matrix and the wear resistance increased, but there was no change significantly between 32 vol% WC/Fe composites and 42 vol% WC/Fe composites. The composites possessed excellent wear resistance comparing the specific wear rate determined in the present work to the martensitic wear-resistant steel under the load of 80 N after a sliding distance of ~ 950 m. The specific wear rate of the martensitic wear-resistant steel was a factor of 24 and 48 times higher than WC/Fe composites, when the content of WC was 32 vol% and 42 vol% in WC/Fe composites, respectively. The main wear mechanism was synthetic of abrasion wear and oxidation wear. The wear performance of 32 vol% WC/Fe composites didn't appear to be much different from 42 vol% WC/Fe composites, due to the WC particles in the 42 vol% composites produced stress concentration easily, which could ultimately induce the creak initiation around WC particles in the subsurface (near wear surface) and propagation to wear surface promoting the breakup of surface film.     

TiNi-V共晶钎料钎焊Si3N4陶瓷接头界面结构及性能

采用真空电弧熔炼技术制备了TiNi-V高温共晶钎料合金,研究了该钎料在Si3N4陶瓷表面的铺展行为．随后采用TiNi—V钎料实现了Si3N4陶瓷的钎焊连接,利用SEM,EDS以及XRD等分析方法,确定了接头的典型界面结构为：Si3N4／TiN＋Ti-si化合物／NiV．重点研究了钎焊温度对接头界面结构及力学性能的影响．结果表明,随着钎焊温度的升高,熔融钎料与Si3N4陶瓷反应程度增加,导致钎缝中TiN＋Ti-Si化合物层厚度不断增加,且在接头残余应力的作用下形成了大量显微裂纹,降低了接头性能．当钎焊温度为1200℃,钎焊时间为10min时,接头室温抗剪强度达到最大为28MPa．断口分析显示接头断裂于TiN＋Ti-Si化合物层为脆性断裂．     

Effect of Nicalon SiC fibre heat treatment on short fibre reinforced β-sialon ceramics

SiC Nicalon fibre yarn was heat-treated at elevated temperature in a gas pressure furnace under CO atmosphere. Weak surface coating is essential for ceramic matrix composite (CMC) reinforcement. Therefore Nicalon SiC fibres were coated after CO heat treatment and then used for β-sialon ceramic reinforcement. The heat treated fibres were chopped about 1–2 mm, and β-sialon z = 1 starting powders were prepared with conventional ball milling. The sialon starting composition and the short fibres were mixed with the certain amount of water to obtain a plastically formable mud. This mud was unaxially cold-pressed to form green bodies and to decrease water content. The green bodies were hot pressed at elevated temperatures for half an hour to produce CMC samples. Vickers hardness test showed that heat-treated fibre reinforcement of β-sialon composites provided higher fracture toughness. Uniform fibre distribution, fibre coating, matrix densification and phase transformation were examined by SEM and XRD analysis.     

Reactive wetting and interfacial characterization of ZrO2 by SnAgCu-Ti alloy

The reactive wetting behavior of zirconia with SnAgCu-x%Ti (SAC-x%Ti, wt%, x?=?1, 4) alloy was investigated via the sessile drop method in isothermal experiments. As temperatures elevated, the final contact angle decreased and the minimum contact angle of 21° and 7° were obtained at 1000?°C for SAC-1%Ti and SAC-4%Ti droplets, respectively. Kinetic calculations indicated that the spreading of SAC-Ti droplets on zirconia was controlled by interfacial reaction and the wetting activation energy was 108.8?kJ/mol. The reaction products distribution and morphology in droplets were influenced vastly by the addition of Ti. Along with the increase of Ti content from 1% to 4%, a great deal of Ti-Sn intermetallic compounds (IMCs) were generated in droplets, thereby the outline of droplets were transformed from hemispherical into similar trapezoidal due to the limited spreading and fluidity of droplets. Owing to the interfacial reaction between active elements Ti and zirconia and the subsequent formation of the Ti-O layer, the wettability of SAC-Ti/zirconia was greatly promoted. According to transmission electron microscopy (TEM) analysis, the thin Ti-O reaction layer consisted of the Ti₂O, Ti₄O₇, Ti₇O₁₃ and TiO₂ phase.     

Enhanced electrical properties in lead-free NBT–BT ceramics by series ST substitution

Comprehensive electrical properties of 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ lead-free ceramics by doping series SrTiO₃ were investigated. High piezoelectric constant of 205 pC/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the rhombohedral–tetragonal morphotropic phase boundary at x=0.02–0.06. Very large recoverable strain of 0.34% was obtained at x=0.10 due to the coexistence of ferroelectric and relaxor pseudocubic phases. A large electrocaloric effect (ΔT_max=1.71 K and ΔT/ΔE=0.34 K mm kV⁻¹ at 50 kV cm⁻¹) which determined by indirect measurements method was obtained at 120 °C at x=0.02, which is significantly higher than that of lead-free ferroelectric ceramics reported so far. Moreover, lower operating temperatures of 50 °C and 30 °C were proposed when x=0.10 and 0.20 with ΔT_max=0.79 K and 0.6 K, respectively. These properties added together indicate a promising material for applications in cooling systems and actuators.     

Helium irradiation effects on MgO-Nd2Zr2O7 composite ceramics prepared by one-step method used for inert matrix fuel

In this work, the helium irradiation effects were systematically investigated on MgO-Nd₂Zr₂O₇ (hereafter M-NZO) composite ceramics used for inert matrix fuel, in which the ion fluences range from 5 × 10¹⁶ to 3 × 10¹⁷ ions/cm² at room temperature. It was demonstrated that M-NZO composite ceramic shows superior radiation tolerance after He implantation. Specifically, MgO had almost no phase transition and extensive amorphization. And Nd₂Zr₂O₇ phase possessed slightly pyrochlore-fluorite-amorphization structural transformation, while part of Nd₂Zr₂O₇ maintained the ordered pyrochlore structure and co-existed with the irradiation-induced disordered fluorite structure. Importantly, GIXRD results indicated that MgO phase presents higher resistance to radiated damage than Nd₂Zr₂O₇ phase. Besides, SEM images illustrated that MgO particles have substantially less fragmentation than Nd₂Zr₂O₇ particles due to the higher thermal conductivity of MgO. It reconfirmed that MgO phase resists irradiation damage better than Nd₂Zr₂O₇ phase in post-irradiated M-NZO composite ceramics.     

FIB-assisted TEM study of an oxide array in the root of a friction stir welded aluminium alloy

A local region in the preferentially etched line at the root part of a friction stir welded Al alloy was directly observed by TEM. TEM clarified that the preferentially etched line consisted of zigzag region having a high density of amorphous Al₂O₃ particles which originated from the initial butt surface oxide layer.     

Chemical reaction of SiCp/Al composites during multiple remelting

In the present study, the chemical reaction of SiCp/Al composites during multiple remelting above the liquidus was investigated using Differential Scanning Calorimetry and Transmission Electron Microscope. The experimental results indicated that the chemical reaction of SiC particles increased with increasing remelting temperature and the number of recycling runs. After reaching a certain degree of reaction, no further change occurred during subsequent remelting. The chemical reaction in SiCp/Al composite during multiple remelting could be suppressed by controlling the remelting temperature when silicon was present in the aluminum matrix. When 8 wt% silicon was added to the aluminum matrix, the chemical reaction between SiC particles and aluminum was completely suppressed even during multiple remelting above the liquidus.     

A Comparative Study on the Microstructure and Mechanical Properties of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> and Cu<Subscript>3</Subscript>Sn Joints Formed by TLP Soldering With/Without the Assistance of Ultrasonic Waves

In this study, the microstructure and mechanical properties of Cu₆Sn₅ and Cu₃Sn intermetallic joints, formed by the transient liquid phase (TLP) soldering process with and without the assistance of ultrasonic waves (USWs), were compared. After the application of USWs in the TLP soldering process, Cu-Sn intermetallic compounds (IMCs) exhibited a novel noninterfacial growth pattern in the molten solder interlayer. The resulting Cu₆Sn₅ and Cu₃Sn joints consisted of refined equiaxed IMC grains with average sizes of 3 and 2.3 µm, respectively. The Cu₆Sn₅ grains in the ultrasonically soldered intermetallic joints demonstrated uniform mechanical properties with elastic modulus and hardness values of 123.0 and 5.98 GPa, respectively, while those of Cu₃Sn grains were 133.9 and 5.08 GPa, respectively. The shear strengths of ultrasonically soldered Cu₆Sn₅ and Cu₃Sn joints were measured to be 60 and 65 MPa, respectively, higher than that for reflow-soldered intermetallic joints. Ultrasonically soldered Cu₆Sn₅ and Cu₃Sn joints both exhibited a combination of transgranular and intergranular fractures during shear testing.     

Charge transfer complexes in silicon-organic photoconductors with admixtures of acceptor molecules

The absorption spectra of the solutions of polysilanes (poly(methylphenylsilane) (PMPS) and poly(dihexylsilane) (PDHS)) with various acceptors (tetracyanoquinodimetane, tetracyano-naphtoquinodimetane and trinitrofluorenone) were investigated. It is shown that several new broad bands in visible region arise in the absorption spectra of polysilanes with addition of acceptors, which is accompanied by changes in the color of the solutions. These new bands are attributed to formation of charge transfer complexes (CT-complexes) between σ-donor (polysilane) and π-acceptor, i.e. polysilanes with admixture of acceptors can be used both as transport layers and photogeneration ones. The possible nature of the structure of the absorption bands of these CT-complexes is discussed.