Cfiber/SiCfiller/Si–B–C–Nmatrix composites with extremely high thermal stability

Precursor-derived Si–B–C–N ceramics are well known for their outstanding thermal stability up to 2000 °C. However, if they are integrated with long ceramic fiber fabrics, the thermal stability of the respective fiber–matrix composites decreases, and the associated thermomechanical properties worsen. A method of improving the thermal stability of a fiber-reinforced Si–B–C–N-based composite up to 1700 °C by the application of SiC filler particulates is reported. The mass loss of such composites is very low even after heating to 2100 °C. Remarkably, a pre-heat treatment of the SiC filler is essential in order to achieve the thermal stability of the ceramic matrix composites by removing surface SiO₂. The composite described here retained 96% of its room-temperature strength and possessed non-brittle fracture behavior after heating at 1700 °C for 10 h in Ar. The flexural creep deformation of the composite at 1400 °C was only 0.25% after 60 h under 100 MPa pressure.     

Enhanced cycling stability of Mg–F co-modified LiNi0.6Co0.2Mn0.2–yMgyO2–zFz for lithium-ion batteries

The layered LiNi_0.6Co_0.2Mn_0.2–yMg_yO_2–zF_z (0≤y≤0.12, 0≤z≤0.08) cathode materials were synthesized by combining co-precipitation method and high temperature solid-state reaction, with the help of the ball milling, to investigate the effects of F–Mg doping on LiNi_0.6Co_0.2Mn_0.2O₂. Compared with previous studies, this doping treatment provides substantially improved electrochemical performance in terms of initial coulombic efficiency and cycle performance. The LiNi_0.6Co_0.2Mn_0.11Mg_0.09O_1.96F_0.04 electrode delivers an high capacity retention of 98.6% during the first cycle and a discharge capacity of 189.7 mA·h/g (2.8–4.4 V at 0.2C), with the capacity retention of 96.3% after 100 cycles. And electrochemical impedance spectroscopy(EIS) results show that Mg–F co-doping decreases the charge-transfer resistance and enhances the reaction kinetics, which is considered to be the major factor for higher rate performance. It is demonstrated that LiNi_0.6Co_0.2Mn_0.11Mg_0.09O_1.96F_0.04 is a promising cathode material for lithium-ion batteries for excellent electrochemical properties.     

High-Speed Friction Stir Welding of SiC_p/Al–Mg–Si–Cu Composite

A 17 vol% SiCp/Al–Mg–Si–Cu composite plate with a thickness of 3 mm was successfully friction stir welded(FSWed) at a very high welding speed of 2000 mm/min for the first time. Microstructural observation indicated that the coarsening of the precipitates was greatly inhibited in the heat-affected zone of the FSW joint at high welding speed, due to the significantly reduced peak temperature and duration at high temperature. Therefore, prominent enhancement of the hardness was achieved at the lowest hardness zone of the FSW joint at this high welding speed, which was similar to that of the nugget zone. Furthermore, the ultimate tensile strength of the joint was as high as 369 MPa, which was much higher than that obtained at low welding speed of 100 mm/min(298 MPa). This study provides an effective method to weld aluminum matrix composite with superior quality and high welding efficiency.     

Interfacial reactions of Sn–Zn–Ag–Cu alloy on soldered Al/Cu and Al/Al joints

This work shows the effect on the soldering process of the addition of Ag and Cu to Sn–Zn alloys. Soldering of Al/Cu and Al/Al joints was performed for a time of 3?min, at a temperature of 250°C, with the use of flux. Aging was carried out at 170°C for Al/Cu and Al/Al joints for 1 and 10 days. During the aging process, intermetallic layers grew at the interface of the Al/Cu joint at the Cu substrate. Intermetallic layers were not observed during wetting of Al/Al joints. On the contrary, dissolution of the Al substrate and migration of Al-rich particles into the bulk of the solder were observed. The experiment was designed to demonstrate the effect of Ag and Cu addition on the dissolution of Al substrate during the soldering and aging processes. In the solder alloys, small precipitates of AgZn₃ and Cu₅Zn₈ were observed.     

C/SiC/MoSi2–SiC–Si multilayer coating for oxidation protection of carbon/carbon composites

C/SiC/MoSi₂–SiC–Si oxidation protective multilayer coating for carbon/carbon (C/C) composites was prepared by pack cementation and slurry method. The microstructure, element distribution and phase composition of the as-received coating were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The results show that the multilayer coating was composed of MoSi₂, SiC and Si. It could effectively protect C/C composites against oxidation for 200 h with the mass loss of 3.25% at 1873 K in static air. The mass loss of the coated C/C composites results from the volatilization of SiO₂ and the formation of cracks and bubble holes in the coating.     

Electrochemical Properties and Microstructure of Al/Pb–Ag and Al/Pb–Ag–Co Anodes for Zinc Electrowinning

The Al/Pb–0.8%Ag and Al/Pb–0.75%Ag–0.03%Co(in mass fraction) anodes used in zinc electrowinning are prepared through the electrodeposition of lead methanesulfonate electrolyte onto an aluminum matrix.The results of anode polarization curves,Tafel curves,and EIS characterizations indicated that the Al/Pb–0.75%Ag–0.03%Co anode has higher electrocatalytic activity and corrosion resistance than the Al/Pb–0.8%Ag anode.SEM observations on the fruit surfaces demonstrated the crystals on the Al/Pb–0.8%Ag anode are larger than on the Al/Pb–0.75%Ag–0.03%Co anode.After 24 h of anodic polarization,SEM observations and XRD analysis showed that the MnO2–PbO2layer on the Al/Pb–0.75%Ag–0.03%Co anode surface is characterized by dendritic crystals,and the PbSO4–PbO2layer under the MnO2–PbO2layer is characterized by uniform and chaotic orientation tetragonal symmetry crystallites of PbSO4.However,the MnO2–PbO2layer on the Al/Pb–0.8%Ag anode surface is characterized by granular crystals,and the PbSO4–PbO2layer under the MnO2–PbO2layer is characterized by well-organized orientation crystallites of PbSO4,which are concentrated in certain zones.     

Mullite/molybdenum ceramic–metal composites

Dense (>98 th%) homogeneous mullite/Mo (32 vol.%) composites with two different Mo average grain sizes (1.4 and 3 μm) have been obtained at 1650°C in vacuum and in reducing condition. Depending on the Mo grain size and processing atmosphere, the K_IC ranges from 4 to 7 MPa m^1/2 and σ_f from 370 to 530 MPa. The MoO₂–2SiO₂·3Al₂O₃–Mo system was found to be compatible in solid state, and a solid solution of ≈4 wt% of MoO₂ in mullite at 1650°C was detected. A solid state dewetting of MoO₂ from the surface of the Mo particle takes place during sintering. It was found that the absence of MoO₂ in the mullite/Mo composites by processing in reducing conditions increases the strength of the metal/ceramic interface and the plasticity of the Mo metal particles, thus strengthening the composite by a crack bridging mechanism. As a result, the K_IC and the σ_f values of the ceramic–metal composite were found to be ≈4 times and ≈2 times higher than the ones corresponding to the mullite matrix.     

Electrodeposition of Co–Sb thermoelectric film from ethylene glycol–CoCl2–SbCl3 solution

The electrodeposition of cobalt–antimony thermoelectric semiconductor films was investigated in a non-aqueous solution of ethylene glycol (EG)–CoCl₂–SbCl₃ at 393 K. By controlling the bath composition and cathodic current density, Co–Sb alloys containing 0–94 mol% of Co were obtained at 50–300 Am^− 2. The resistance polarization caused by the formation of semiconductor film was observed in the polarization curve measured in the EG–CoCl₂–SbCl₃ (90.0–9.3–0.7 mol%) bath. In this bath, the Co–Sb (23.6–76.4 mol%) alloy was obtained by maintaining the constant potential electrolysis at 0.1 V. This alloy included CoSb₃ and exhibited a p-type thermoelectric conversion by the given temperature difference.     

Fatigue Behavior of Dissimilar Al–Mg–Si/Al–Zn–Mg Aluminum Alloys Friction Stir Welding Joints

In this study, fatigue properties and fracture mechanism of dissimilar Al–Mg–Si/Al–Zn–Mg aluminum alloys friction stir welding(FSW) joints were investigated and the effect of the sheet configuration on the fatigue behavior of the FSW joints was also discussed. Results showed that the joints owned better fatigue properties when the Al–Zn–Mg aluminum alloy was placed at the advancing side(AS). At 10~7 cycles, the fatigue strengths of Al–Zn–Mg–AS and Al–Mg–Si–AS joints were, respectively, 105.6 and 90.1 MPa. All joints fractured at the heat-affected zone at the Al–Mg–Si alloy side. Transmission electron microscopy results showed that better fatigue property of the Al–Zn–Mg–AS joint was associated with the bridging effect of the bigger secondary phase particles.     

Microstructural characterization of in situ TiC/Al and TiC/Al–20Si–5Fe–3Cu–1Mg composites prepared by spray deposition

An innovative spray-deposition technique has been applied to produce in situ TiC/Al and TiC/Al–20Si–5Fe–3Cu–1Mg composites. This technique provides a new route to solve the problems of losses and agglomeration of the reinforcement particles when they are injected into the spray cone of molten droplets during spray forming process. Experimental results have shown that the presence of needle-like Al₃Ti and Al–Si–Fe compounds, which are detrimental not only to the fracture toughness, but also to the stability of the microstructure, can be eliminated completely from the final product by using a proper Ti:C molar ratio of 1:1.3 in the Ti–C–Al preforms and adding 5 wt% TiC particles to Al–20Si–5Fe–3Cu–1Mg alloy. Moreover, another major problem of coarsening of silicon particles usually encountered in the hypereutectic Al–Si alloys has also been solved by the technique. The silicon particles in the spray-deposited 5 wt% TiC/Al–20Si–5Fe–3Cu–1Mg composite were much refined (∼2 μm) compared to those (∼5 μm) obtained in the matrix alloy without TiC addition. The formation and elimination mechanisms of Al₃Ti phase in TiC/Al composites can be explained based on thermodynamic theory. The modification of the microstructures in the spray-deposited Al–20Si–5Fe–3Cu–1Mg alloy can be interpreted in the light of the knowledge of atomic diffusion. The experimental results also showed that the ultimate tensile strength of the TiC/Al composites was improved over that of the unreinforced Al matrix.     

The influence of Cu/Li ratio on precipitation in Al–Cu–Li–x alloys

The impact of the Cu/Li ratio on the sequence and kinetics of solid-state precipitation is studied for two recently developed Al–Cu–Li–Mg–Ag alloys: AA2198 and AA2196. A quantitative evaluation of the alloy microstructure is carried out using small-angle synchrotron X-ray and neutron scattering, conventional and aberration-corrected (scanning) transmission electron microscopy and differential scanning calorimetry. It is shown that small modifications to alloy chemistry profoundly change the phases formed during natural ageing: Cu-rich clusters in the Li-lean alloy and the δ′(Al₃Li) phase in the Li-rich alloy. Despite this difference in early ageing, the peak aged microstructures of the two alloys are similar, dominated by high-aspect-ratio T₁ plates with a thickness of 1.3 nm and a diameter up to 50 nm. However, the incubation time for nucleation and saturation size after growth of the precipitates is found to be significantly different between the two alloys. Small amounts of other phases from the binary Al–Cu sequence are also observed. Mechanisms of formation of the T₁ phase are discussed in view of these experimental results.     

SiC–MoSi2/ZrO2–MoSi2 coating to protect C/C composites against oxidation

To improve the oxidation resistance of carbon/carbon (C/C) composites in air at high temperatures, a SiC–MoSi₂/ZrO₂–MoSi₂ coating was prepared on the surface of C/C composites by pack cementation and slurry method. The microstructures and phase compositions of the coated C/C composites were analyzed by scanning electron microscopy and X-ray diffraction, respectively. The result shows that the SiC–MoSi₂/ZrO₂–MoSi₂ coating is dense and crack-free with a thickness of 250–300 μm. The preparation and the high temperature oxidation property of the coated composites were investigated. The as-received coating has excellent oxidation protection ability and can protect C/C composites from oxidation for 260 h at 1773 K in air. The excellent anti-oxidation performance of the coating is considered to come from the formation of ZrSiO₄, which improves the stability of the coating at high temperatures.     

Quality index and hot tearing susceptibility of Al–7Si–0.35Mg–xCu alloys

The effects of Cu addition (0.5%, 1%, 1.5%, 2%, and 3%, mass fraction) on the quality index (Q_i) and hot tearing susceptibility (HTS) of A356 alloy were investigated. According to the results, Cu addition up to 1.5% increases the Q_i by almost 10%, which seems to be due to its solid solution strengthening and dispersion hardening effect of Cu-rich Al₂Cu and AlMgCuSi compounds. However, further addition of Cu (up to 3%) decreases the Q_i by almost 12%, which is likely due to the reduction of tensile strength and elongation caused by increased volume fraction of brittle Cu-rich intermetallics and microporosities in the microstructure. It is also found that Cu increases the HTS of A356 alloy measured by constrained rod casting method. According to the thermal analysis results, Cu widens the solidification range of the alloy, which in turn, decreases its fluidity and increases the time period during which the mushy-state alloy is exposed to the hot tearing susceptible zone. SEM examination of the hot tear surfaces in high-Cu alloys also demonstrates their rough nature and the occurrence of interdendritic/intergranular microcracks as convincing evidences for the initiation of hot tears in the late stages of solidification in which there is not enough time for crack healing.     

High-Temperature Corrosion of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C Alloy in N2/0.1%H2S Gas

Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy was corroded at 800–1100 °C for 200 h in N₂/0.1%H₂S gas to characterize its corrosion behavior in an aggressive H₂S-containing environment. The alloy displayed superior corrosion resistance because Ti and Al preferentially reacted with impurity oxygen in the gas to form TiO₂ and Al₂O₃. It corroded primarily by outward diffusion of Ti, Al, W, and Cr in addition to inward transport of sulfur, nitrogen, and oxygen. Scales were adherent and consisted of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂, Al₂O₃)-mixed layer. TiN and Ti₂AlN formed at the scale/matrix interface where sulfur, Nb, W, and Cr segregated.

     

B2O3含量对SiO2–B2O3–Al2O3–Na2O系陶瓷结合剂结构与性能的影响

制备不同B₂O₃含量的SiO₂–B₂O₃–Al₂O₃–Na₂O系玻璃试样和陶瓷结合剂试样,利用电子多功能实验机、扫描电镜、显微硬度仪、平面流淌法、热膨胀系数测试仪等分别测试不同玻璃试样的密度和显微硬度,陶瓷结合剂试样的抗折强度、微观形貌和热膨胀系数等,并用X射线衍射仪、傅里叶变换红外光谱仪对陶瓷结合剂的结构和成分变化进行分析。结果表明:将B₂O₃引入陶瓷结合剂中可有效降低其烧结温度,提高其热稳定性并调节其热膨胀系数等。在陶瓷结合剂中加入摩尔分数为15%的B₂O₃时,其样条抗折强度最高为78.11 MPa,密度和硬度最高分别为2.45 g/cm3和856 MPa,且该陶瓷结合剂的热膨胀系数与金刚石最匹配。X射线衍射分析结果表明陶瓷结合剂是典型的玻璃相结构,且对磨料有良好的包覆效果。      

High temperature oxidation response of Al/Ce doped Mo–Si–B composites

A systematic investigation has been performed to study the oxidation response of Mo₇₆Si₁₄B₁₀ alloy doped with traceable amount of Ce and Al at temperature in the range of 900–1300 °C. The resistance to degradation of Mo₇₆Si₁₄B₁₀ ≥ 900 °C has increased due to the dissociation of Mo-oxides into Mo in protective glassy borosilicate layer. Addition of Ce aggrieves oxidation initially, but protection is achieved upon exposure within 3 h at all the different temperatures. No passivation has occurred in Al containing alloy due to the dissolution of Al₂O₃ in the borosilicate scale leading to the precipitation of aluminum-borate and mullite along with unreacted cristoballite.     

Cu/Li Ratio on the Microstructure Evolution and Corrosion Behaviors of Al–xCu–yLi–Mg Alloys

The microstructure evolution and the corrosion feature of Al–x Cu– y Li–Mg alloys( x : y = 0.44, 1.65 and 4.2) were systematically investigated under the same artificial aging conditions. The relationships between types of precipitates and mechanical performance, as well as electrochemical behaviors, were discussed. Our results show that different types of precipitates can be obtained in alloys with different Cu/Li mass ratios, which significantly influences the mechanical performance of the alloys and substantial corrosion behaviors. Specifically, the analogous corrosion evolution in the aging Al– x Cu– y Li–Mg alloys was first ascertained to be derived from the growth mechanism of the precipitates at the grain boundary(GB). Moreover, a small number of GB precipitates can be obtained in the aged alloy with the lowest Cu/Li mass ratio, thereby resulting in the largest intergranular corrosion resistance. A higher proportion of the GB T_1 phase in the continuous precipitates induces higher corrosion sensitivity in alloy with a high Cu/Li mass ratio.     

Corrosion behaviour and catalytic effectiveness of Pb–Ca–Sn,RuO2–IrO2/Ti and IrO2–Ta2O5/Ti anodes for copper electrowinning

Abstract

The dependence of the corrosion rate on cell current density (CD) for three anode materials (Pb–Ca–Sn, RuO₂–IrO₂/Ti and IrO₂–Ta₂O_5/Ti) in a laboratory scale copper electrowinning cell has been studied by means of short term weight loss tests, scanning electron microscopy observations and energy dispersive spectroscopy analysis. The lead anodes (Pb–Ca–Sn) corroded at all the studied cell CDs, and their corrosion rate increased with increasing cell CD. The precious metal oxide anodes (RuO₂–IrO₂ and IrO₂–Ta₂O₅) only exhibited corrosion at the highest tested cell CD (1000 A m^?2), and their corrosion rates were about a quarter of the lead corrosion rate at the same cell CD. The electrocatalytic properties of the three anode materials were characterised by means of potentiodynamic experiments. The overall results pointed to IrO₂–Ta₂O₅/Ti as the best anode material of choice, although plant tests would be required before deciding on any specific commercial use.     

Achieving higher strength in Cu–Ag–Zr alloy by warm/hot rolling

High-strength Cu–3Ag–0.5Zr alloy plates were produced by multi-pass rolling in the temperature range of500–800 °C. An increase in strength was observed by rolling in the aforementioned range without significant loss in ductility. All the rolled samples show higher strength than solution-treated and aged samples. The maximum strength was observed for plates rolled at 500 °C with a yield strength and ultimate tensile strength of 311 and385 MPa, respectively, and retaining a ductility of 23 %.Transmission electron microscopy(TEM) studies showed uniform distribution of fine silver precipitates and high dislocation density in the rolled samples. Nevertheless, the size of precipitates and dislocation density varied with the rolling temperature. The superior strength achieved in the rolled samples is attributed to grain refinement, dislocation strengthening, and precipitation hardening. This method can be employed to produce high-strength plates of precipitation hardenable copper alloys.