Microstructure and mechanical properties of laser beam welded Al–Li alloy 2060 with Al–Mg filler wire

Alloy 2060-T8 is a newly developed high-strength Al–Li alloy for applications in aircraft industry. Crack-free welds were obtained in laser beam welding with 5087 filler wire under optimized welding conditions. In this paper, fusion zone microstructure and joint mechanical properties were investigated. Microstructure typical for the weld metal consists of α-Al matrix with a few nanoscale precipitates inside and a coarse icosahedral quasicrystalline T₂ phase at the dendritic and grain boundaries. The quasicrystalline occurred normally in Al–Li–Cu alloys with higher Li contents. Our investigations show that the icosahedral quasicrystalline phase T₂ phase forms in the laser-welded Al–Li alloy 2060 with lower Li content as a result of segregation and replacement of Mg element. The joint tensile strength in as-welded condition is around 317 MPa, about 63% of that of the base metal, and fracture occurs within the fusion zone.     

Fibrous porous ceramics with devisable phenolic resin reinforcing layer

Fibrous porous ceramics with devisable phenolic resin reinforcing layer were fabricated using low cost atmospheric impregnation technology at room temperature. In combination with additional sealing method, phenolic resin reinforcing layer with controllable thickness could be obtained on the surface of fibrous porous ceramics. Typical gradient profile was observed along the thickness direction of impregnation. The effects of the phenolic resin reinforcing layer on mechanical properties and thermal insulation properties were studied. The results revealed that compressive strength increased from 1.70?MPa to 2.61?MPa, tensile strength increased from 0.78?MPa to 0.91?MPa, and flexural strength increased from 9.55?MPa to 10.89?MPa with the phenolic resin layer increasing from 0?mm to 9?mm. Simultaneously, room-temperature thermal conductivity increased from 0.051?W/(m·K) to 0.055?W/(m·K). In addition, the impact resistance of the surface of the material was obviously improved. The contact angel of the surface of the material exceeded 125°, which effectively improved the environmental adaptability.     

Broadband,high-sensitivity self-powered ferroelectric LuMnO3-based photodetector with large photocurrent output

The broadband spectrum detection from ultraviolet to near-infrared is hankered in the photoelectric applications of imaging, sensing and communication. Here, a new self-powered photodetector based on ferroelectric LuMnO₃ thin film with a narrow bandgap of 1.46 eV exhibits high-sensitivity ultraviolet–visible–near infrared photodetection properties. The responsivity (R) and detectivity (D*) in sunlight are 0.4 A/W and 7.05 × 10¹¹ Jones, which are much higher than that of other ferroelectric photodetectors. Moreover, under the monochromatic light (900 nm), the R and D* can reach 0.39 A/W and 6.89 × 10¹¹ Jones. The outstanding photodetection performances owed to the large photocurrent output, where the short-circuit current density can reach 10.5 mA/cm² under 1 sun illumination. The synergistic effect of ferroelectric photovoltaic effect and interface barrier effect demonstrates that the multi-driving forces can achieve high dissociation efficiency for photon-generated carriers. The excellent photodetection performances open up new application of ferroelectric materials in broadband self-powered photodetectors.     

Microstructure and flexural strength of C/HfC-ZrC-SiC composites prepared by reactive melt infiltration method

C/HfC-ZrC-SiC composites were fabricated via reactive melt infiltration (RMI) of the mixed HfSi₂ and ZrSi₂ alloys. The microstructure, infiltration behavior of the hybrid silicide alloys infiltrating C/C composites, and flexural strength of C/HfC-ZrC-SiC composites was studied. Inside composites, there were more Hf-rich (Hf, Zr)C phases distributed in the exterior region, while more SiC and Zr-rich (Zr, Hf)Si₂ in the interior region. There was compositional segregation in (Hf, Zr)C, with the HfC content decreasing from the exterior region to interior region. The RMI process was performed at different temperatures to investigate the structural evolution, and a model for the reactive melt infiltration of the mixed HfSi₂ and ZrSi₂ alloys into C/C composites was established. Compared with C/HfC-SiC and C/ZrC-SiC prepared by same process, C/HfC-ZrC-SiC had the highest flexural strength of 247Mpa and 213Mpa after oxidation at 1200 ℃ for 15 min. Both the unoxidized and oxidized samples presented a pseudo-plastic fracture behavior.     

Opacifier embedded and fiber reinforced alumina-based aerogel composites for ultra-high temperature thermal insulation

A novel method was developed to uniformly disperse sub-micron TiO₂ opacifier into fiber reinforcements using agar and silica as binders via freeze drying. TiO₂ opacifier/ fiber/ alumina-based aerogel ternary (TFA) composites with high strength and excellent high-temperature thermal insulation were successfully prepared by sol-gel route, impregnation process and supercritical fluid drying. The microstructure, mechanical and thermal insulation properties of TFA composites were investigated comprehensively. The results show that the mechanical property of TFA composites can be significantly enhanced by mullite fiber felt and the incorporation of SiO₂ binder. The effect of TiO₂ opacifier on the high-temperature thermal conductivity was studied by adjusting the content of TiO₂ from 0 to 15 wt%. The obtained TFA composites exhibit high Young's modulus of up to 3.58 MPa and low high-temperature thermal conductivities of 0.129 W/m·K at 800 °C and 0.168 W/m·K at 1000 °C, respectively. The mechanism of heat transfer in TFA composites at high-temperature was also analyzed.     

Effect of composition and heat treatment conditions on the tensile strength and creep resistance of SiC-based fibers

Polymer-derived SiC-based fibers with fine-diameter (∼10–15 μm) and high strength (∼3 GPa) were prepared with carbon-rich and near-stoichiometric compositions. Fiber tensile strengths were determined after heat treatments at temperatures up to 1950 °C in non-oxidizing atmospheres and up to 1250 °C in air. The creep resistance of fibers was assessed using bend stress relaxation measurements. Fibers showed excellent strength retention after heat treatments in non-oxidizing atmospheres at temperatures up to 1700 °C for the carbon-rich fibers and up to 1950 °C for the near-stoichiometric fibers. The near-stoichiometric fibers also showed considerably better strength retention after heat treatments in air. Creep resistance of the as-fabricated fibers was greatly improved by high-temperature heat treatments. Heat-treated near-stoichiometric fibers could be prepared with ∼3 GPa tensile strengths and bend stress relaxation creep behavior which was significantly better than that reported for the Hi-Nicalon™ Type S near-stoichiometric SiC fibers.     

Heat transfer from multiple row arrays of low aspect ratio pin fins

Pin fin arrays are used in many applications to enhance heat transfer. In modern gas turbines, for example, airfoils are designed with sophisticated internal and external cooling techniques. One method for cooling is routing air from the compressor through intricate cooling channels embedded in turbine airfoils. Heat transfer from the blade to the coolant air can be increased by installing arrays of cylindrical pedestals often referred to as pin fins. Pin fin arrays increase heat transfer by increasing the flow turbulence and surface area of the airfoil exposed to the coolant.For the current study, experiments were conducted to determine the effects of pin spacing on heat transfer and pressure loss through pin fin arrays for a range of Reynolds numbers between 5000 and 30,000. Results showed that spanwise pin spacing had a larger effect than streamwise spacing on array pressure loss while streamwise spacing had a larger effect than spanwise spacing on array heat transfer.     

应用于传声器阵列定位校准的空间点声源声场拟合方法

介绍了一种用于传声器阵列声源定位精度校准的空间点声源声场模拟方法,并基于该方法设计了一套空间点声源模拟系统,完成了一个传声器阵列的定位位置精度校准。文章采用多通道点声源空间声场合成算法模拟了一个位于自由场空间的点声源,根据传声器阵列中每一个传声器的空间位置坐标,计算出传感器所处位置声场的动态声信号。通过耦合腔标准声源将对应的多通道电压信号输入被校准阵列系统,完成点声源的模拟。然后,该阵列运用波束形成算法进行声源定位,得出点声源的位置,并与模拟点声源的位置进行比对,实现对阵列定位准确性的校准。     

Evolution of thermal insulation of plasma-sprayed thermal barrier coating systems with exposure to high temperature

The thermal insulation potential of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings is generally assessed via the evaluation of the ceramic layer. However, ageing of the complete system leads to microstructural transformations that may also play a role in the heat transport properties. This study thus investigated the microstructure-heat insulation relationships of different TBC systems in their as-deposited state and when aged under various conditions, through the systematic analysis of both microstructure and thermal diffusivity. The latter was measured from room temperature up to 1100 °C using the laser-flash technique, while the porous microstructure was assessed using image analysis. The different coatings exhibited relatively similar thermal diffusivity values that were shown to be mostly influenced by the thin porosities in contrast to larger defects. The thermal insulation of the TBC systems after exposure to high temperature was shown to be stable despite the microstructural variations introduced by cracks, oxidation and chemical degradations.