Improving the tensile properties of extruded Mg–Ga alloy by ageing treatment

A hot-extruded Mg-5Ga alloy was subjected to ageing treatment at 150 ?°C, 190 ?°C and 230 ?°C. The microstructures and mechanical properties of the extruded and aged alloy were examined in this study. Microstructure examinations suggested that particle-shaped and rod-shaped Mg₅Ga₂ were precipitated in the alloy after peak ageing treatment. The extruded alloy showed the yield strength, ultimate tensile strength and elongation to fracture of 157.6 ?MPa, 248.6 ?MPa and 17.5%, respectively. After peak ageing, the yield strength and ultimate tensile strength can be enhanced by as much as 15.7% and 8.6% reaching 182.3 ?MPa and 270 ?MPa, respectively. The improvement of the tensile strengths is mainly attributed to the enhanced precipitation strengthening by newly formed fine Mg₅Ga₂ precipitates. The ductility of the alloy was slightly increased by peak ageing at low temperatures (150 ?°C and 190 ?°C), but remarkably decreased by peak ageing at high temperature (230 ?°C) due to the formation of coarsened Mg₅Ga₂ particles which easily initiated the cracks during tensile deformation.     

Highly mechanical and high-temperature properties of Cu–Cu joints using citrate-coated nanosized Ag paste in air

Today,a growing number of third-generation semiconductor-based power devices are used in products that can continuously operate at high temperatures for extended periods of time.Hence,traditional tin-lead and lead-free solders are no longer suitable for modern electronic packaging.A common method is to apply Ag paste for bare Cu-Cu joints under an inert or reductive atmosphere.In this study,the citrate-coated nanosized Ag paste was utilized to generate robust bare Cu-Cu joints under atmospheric ...     

Studies on the effects of pre-firing and sintering temperature on NSDC nanocomposite electrolytes

Solid oxide fuel cells (SOFCs) technology, with fuel flexibility, is one of the most promising power generation technology. However, the high operating temperature of SOFCs has hindered their commercial applications. As a crucial requirement to enhance its performance, SOFCs electrolytes should operate at a low temperature. Carbonate/ceria composites are developed as electrolytes for low operating temperature SOFCs, and a better understanding of the mechanism of its ionic conductivity serves this purpose. In this work, ceria-carbonate composite electrolyte, Na₂CO₃/samarium doped ceria (NSDC) were synthesized by the co-precipitation method. The synthesized electrolytes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and UV–Vis spectroscopy. The XRD and SEM results showed that the sintered NSDC nanocomposite comprised a single-phase dense electrolyte structure. The crystallite size of the NSDC nanocomposite was greatly affected by the different pre-firing temperatures and different sintering temperatures. Also, the ionic conductivity of the prepared NSDC nanocomposite electrolytes was strongly dependent on the pre-firing and sintering temperatures. The NSDC nanocomposite electrolytes were pre-fired at 950 ?°C and 650 ?°C and sintered at 1200 ?°C and 900 ?°C respectively, had ionic conductivity in H₂ and air high as 0.36 ?S/cm and 0.3 ?S/cm.     

High-temperature oxidation resistance of ceramic coatings on titanium alloy by micro-arc oxidation in aluminate solution

Ceramic coatings with aluminum titanuate (Al₂TiO₅) were prepared on Ti–6Al–4V alloy using pulsed bi-polar Micro-arc Oxidation (MAO). The micromorphology and phase composition of the micro-arc-oxidition ceramic coatings on the titanium alloy were characterized by X-ray powder diffraction (XRD), and scanning electron microscopy (SEM) respectively. The results revealed that the distinct discharge channels and pores on the surface of the micro-arc-oxidition coatings appeared, and these channels were connected in the molten state. The electrolyte concentration was inversely proportional to the coating hardness; additionally, the coating prepared with sodium aluminate and sodium hypophosphite concentrations of 4 ?g/L and 0.5 ?g/L, respectively, was the most refined after high-temperature sintering, and it was demonstrated to better prevent oxidation. Increasing the electrolyte concentration coincided with fluctuating coating thermal shock resistance. The thermal shock resistance of the coating respectively prepared with sodium aluminate, and the sodium hypophosphite concentrations of 4 ?g/L and 0.5 ?g/L was the highest. Additionally, the high-concentration coatings performed significantly better than the low-concentration coatings. The oxidation resistance of the coating samples was also significantly higher than that of the TC4 titanium alloy substrate. The adhesion strength between the coatings and the substrate with and without the sealing treatment was measured by tensile tests. Then, the high-temperature oxidation performance of the coating samples with and without the sealing treatment was investigated by conducting a high-temperature oxidation experiment at a calcinating temperature of 500 ?°C. The results indicate that the adhesion strength between the coatings and substrate was high for the as-prepared and sealed micro-arc oxidation samples regardless of whether they were calcined. The high-temperature oxidation mass increase curves for the sealed and unsealed coating samples calcined at 500 ?°C for 500 ?h revealed that the high-temperature-oxidation-induced mass increase of the coating samples sealed with a sodium silicate solution was much lower than that of the titanium alloy substrate. Thus, the sealing treatment significantly improved the high-temperature oxidation resistance of the TC4 titanium alloy. Lastly, the high-temperature oxidation behavior at 500 ?°C was analyzed and discussed.     

A multi-component nanocrystalline FeCrV alloy with improved mechanical properties and excellent irradiation resistance

An equiatomic low-activated FeCrV ternary alloy was prepared by mechanical alloying and spark plasma sintering. The BCC solid solution alloy exhibited an excellent strength-ductility synergy properties, which was 1.85 ?GPa compressive yield strength, 3.04 ?GPa fracture strength and more than 28% plasticity at room temperature. Even at an elevated temperature of 600 ?°C, its yield strength was still more than 1 ?GPa, revealing a pronounced high temperature resistance. The FeCrV alloy had a homogeneous microstructure with high-density dislocations. A nanocrystalline characteristic was demonstrated and the average grain size was about 322 ?nm, leading to the improved mechanical properties. The strengthening mechanism was discussed in detail and the contributions of strengthening factors were calculated. The FeCrV alloy was irradiated up to 160 dpa with 2 ?MeV Au²⁺ ions at 573 ?K. The nanoindentation test was carried out to simply evaluate the hardening degree under such a high dose irradiation with heavy ions. The semi-quantitative analysis results show that FeCrV alloy has an excellent irradiation tolerance compared with pure Fe.     

Optimization of mechanical properties of Sn-3.8Ag-0.7Cu alloys by the additions of Bi,In and Ti

Bi, In and Ti were added to Sn-3.8Ag-0.7Cu (SAC387) solder alloy to optimize the mechanical performance. The alloying effects of Bi, In and Ti on the microstructure, thermal and mechanical properties of SAC387 based solder alloys were investigated. The results demonstrate that adding 3.5 ?wt % of Bi could refine the microstructure, optimize the thermal properties, and improve the tensile strength. Meanwhile, the ductility of the solder alloys reduced evidently. Adding 2.8 ?wt % of In into SAC387–3.5 ?wt %Bi alloy could increase both the strength and ductility, which is attributed to the beneficial effect of In addition, as adding In could improve the solubility of Bi in the β-Sn matrix. Meanwhile, the melting point was reduced, and the wettability improved with the addition of In. Introducing amounts of Ti into SAC387–3.5 ?wt % Bi-2.8 ?wt % In alloy could further increase the strength. However, the ductility was significantly reduced when 0.8 ?wt % of Ti was added due to the formation of the coarse Ti₂Sn₃ phase. The undercooling was remarkably reduced with the addition of Ti. The nanoindentation tests demonstrate that the hardness increased mainly due to the hardening effect of the Bi addition. Among all the samples prepared, alloy SAC387–3.5 ?wt % Bi exhibited the highest creep resistance at the ambient temperature. Further adding In and Ti into SAC387–3.5 ?wt % Bi alloys reduced the creep resistance of the solder alloys. The mechanism associated with the different mechanical responses is also discussed in this study.     

Effect of long-term aging treatment on the tensile strength and ductility of GH 605 superalloy

Aging treatment is an effective way to optimize the mechanical properties of Co-based superalloys. In this study, commercial GH 605 superalloy was subjected to aging treatment at 650 ?°C in a wide time range up to 1000 ?h. The effects of aging time on the tensile characteristics, microstructure evolution and mechanical properties were systematically investigated at room temperature (RT) and 900 ?°C. The results showed that the volume fractions of M₆C and M₂₃C₆ carbide increased with the aging time. After long-term aging treatment, the yield strength (YS) at RT was enhanced from 490.3 ?MPa to 805.9 ?MPa, while the alloy still had high tensile ductility (above 20%). Microscopic observations by transmission electron microscopy (TEM) indicated that the strengthening mechanism was related to carbide precipitation inside the grains and the change in the dislocation slipping mode. Moreover, long-term aging treatment can increase the elongation from 24.1% to 47.3% at 900 ?°C accompanied by a slight increase of YS from 299.3 ?MPa to 313.9 ?MPa. Based on detailed microstructure analysis the strengthening mechanism can be attributed to the refined grains as well as carbide precipitation inside the grains and around the grain boundaries.     

Stability of thermal cycling and high temperature mechanical properties for Ti–V–Al–B lightweight shape memory alloy

The microstructure of the Ti–V–Al shape memory alloy with refined grain and in-situ TiB phase was modified by doping minor Boron (B), which contributes to the superior mechanical performances and strain recovery characteristics. Compared with other quaternary Ti–V–Al-X alloys, the Ti–V–Al–B alloy showed the largest ultimate tensile stress due to the solution strengthening, grain refinement and precipitation strengthening of in-situ TiB phase. Moreover, the Ti–V–Al alloy added 0.1 ?at.%B possessed the maximum yield stress of 701 ?MPa and the largest tensile fracture strain of 27.6% at the temperature of 150 ?°C. Meanwhile, the excellent strain recovery characteristics with fully recoverable strain of 4% could be obtained due to B addition. Besides, B addition suppressed the precipitation of ω phase during thermal cycling and further improved the thermal cycling stability of the Ti–V–Al alloy.     

烧结温度和粒度分布对多孔氧化硅陶瓷型芯材料性能的影响

采用热压注法制备了纯氧化硅多孔陶瓷型芯材料样品, 研究了烧结温度和陶瓷粉末粒度分布对陶瓷材料烧结后的组织和性能的影响. 结果表明, 随着烧结温度的升高, 样品的气孔率逐步降低, 室温和高温抗弯强度均相应提高. 当烧结温度为1 200°C时, 烧结收缩率为2.75%, 气孔率为24.69%, 室温抗弯强度达到25.3 MPa, 高温抗弯强度达到44.23 MPa; 当烧结温度超过1 200°C时, 室温和高温抗弯强度均明显降低, 而收缩率和气孔率变化不明显. 通过样品断口形貌和相应物相分析发现, 不同烧结温度下样品致密度和方石英含量的不同是造成陶瓷型芯室温和高温抗弯强度变化的主要原因, 而粒度分布能够显著影响型芯材料的气孔率、收缩率和抗弯强度. 在本实验中, 具有如下粒度分布的型芯材料的综合性能最佳: 10 μm以下约为25.33%, 10∽30 μm约为38.16%, 30∽50 μm约为28.74%, 50 μm以上约为7.77%, 最大粒径不超过95 μm.     

Mangosteen peel-derived activated carbon for supercapacitors

This work reports the effects of activation temperatures on the porous development and electrochemical performance of activated carbons. Herein, activated carbons were prepared from the biowaste of mangosteen peel by using KOH activation at temperatures of 400, 600, and 800 ?°C. The results demonstrate that the specific surface area increases with increasing the activation temperatures in which the well-developed porous structure after KOH activation at 800 ?°C provides the highest specific surface area of 1039 ?m² ?g^?1. At 600 ?°C, the activated carbon delivers the highest specific capacitance value of 182 ?F ?g^?1 ?at a current density of 0.5 ?A ?g^?1 in 3 ?M KOH aqueous electrolyte. This is correlated well with its high micropore fractions (99%). Moreover, it was found that the activation temperature changes the major contribution of oxygen-containing functional group on surface of activated carbon, which is beneficial for the enhancement of the specific capacitance value of activated carbon at the temperature of 600 ?°C. This work suggests that the activation temperature is a key to optimizing the electrochemical performance of activated carbons. Overall, our activated carbons can be considered as a strong candidate for use as electrode materials in supercapacitors.     

Effect of temperature and stress on high temperature creep behavior of Ni3Al-based single crystal superalloy

The creep behavior and dislocations mechanism of the Ni₃Al-based single crystal alloy IC6SX with [001] orientation were investigated under the testing conditions of 1100 ?°C/137 ?MPa, 1100 ?°C/120 ?MPa and 1070 ?°C/137 ?MPa. It was observed that the temperature and stress had a significant effect on the high temperature creep life of the single crystal alloy. As the temperature was reduced from 1100 ?°C to 1070 ?°C, the creep life increased from 65.07 ?h to 313.8 ?h. As the stress was reduced to 120 ?MPa, the creep life increased to 243.3 ?h. Under the high temperature and low stress condition the dislocations entered the γ′ phase by climbing caused by the atomic diffusion, instead of slipping.     

Rapid reactive synthesis of TiAl3 intermetallics by thermal explosion and its oxidation resistance at high temperature

Porous TiAl₃ intermetallics were synthesized from Ti-75 at.% Al elemental powder mixtures using an energy-saving and rapid reactive method of thermal explosion (TE). The results demonstrated that the actual temperature of the compact climbed rapidly from 673 °C to 1036 °C within 24 s, indicating that an obvious TE reaction occurred during sintering process. The video graphs suggested that the TE in Ti–Al system behaved instant occurrence and overall heating whether from axial or radial direction. The silver wires and NaCl particles that pressed on the surface of the sample disappeared due to the heavy heat released during TE reaction. Only pure TiAl₃ phases were synthesized in TE products and the open porosity of 55.4% was easy to obtain. After high-temperature treatment at 1000 °C, large amounts of sintering-neck formed and then improved the compressive strength of porous TiAl₃ materials. Moreover, the mass gain curve of porous TiAl₃ intermetallics oxidized at 650 °C for 120 h exhibited the parabolic oxidation rate law. XPS analysis confirmed that the strong O 1s peak was 531.4 eV which was the typical binding energy of Al₂O₃. Therefore, the excellent oxidation resistance of porous TiAl₃ foams would be considered as good candidate materials for prolonging the service life at high temperatures.     

Porous SiC ceramics fabricated by quick freeze casting and solid state sintering

Porous SiC ceramics with uniform microstructure were fabricated by quick freezing in liquid nitrogen and solid state sintering.Poly(vinyl alcohol)(PVA) was added as binder and pore morphology controller in this work.The microstructure and mechanical properties of porous SiC ceramics could be controlled by the composition of the aqueous slurries.Both solid content of the slurries and PVA content impacted on the pore structures and mechanical properties of the porous SiC ceramics.The solid content of slurries and PVA content varied from 60 to 67.5 wt%and 2-6 wt%,respectively.Besides,the grain morphology of ceramics was also tailored by changing the sintering temperature from 2050 to 2150 ℃.Porous SiC ceramics with an average porosity of 42.72%,flexural strength of 59.28 MPa were obtained at 2150 ℃ from 67.5 wt% slurries with 2 wt% PVA.     

圆柱形空腔多孔材料辐射效应研究

 多孔材料具有孔隙率高、密度小和比表面积大等特征,在消声、减震、隔热和电磁屏蔽等应用方面具备多种优异性能。由于其在气孔中存在低热导率的空气介质,可以用作绝热材料。与其他相应材料相比,它具有耐高温和强度高的优点,已被广泛应用于航空航天、原子能和交通运输等行业。本文研究了热辐射效应对圆柱形空腔多孔材料传热性能的影响,推导了多孔材料局部有效热导率的方便工程应用的封闭公式。与已有理论结果比较,给出了本文公式的精度。数值算例揭示了孔洞对于热传输的有意义的影响规律。多孔材料的局部有效热导率可以分为两部分：一部分由固相和气相的纯传导引起,无量纲化的传导热导率不依赖于温度和孔径;另一部分由热辐射引起,随温度和孔径的变化而剧烈变化。在温度较低和孔径较小的情形下,辐射影响可以忽略不计。     

Effect of Zr and Ti on isothermal oxidation behavior of Nb–Si based alloys

The isothermal oxidation behavior of 56Nb-16Si-(20-x)Ti–3Cr–3Al-2Hf-xZr (x ?= ?0, 2, 5, 10 ?at. %) alloys was investigated at 800 ?°C and 1250 ?°C, respectively. The results show that increasing the Zr content evidently increased the oxidation rates at 800 ?°C, accompanied by the obvious occurrence of pesting oxidation. The alloys showed alike linear oxidation kinetics at 1250 ?°C. With the increase of Zr content, the adherence and integrity of oxide scales were improved, but the overall oxidation resistance was slightly deteriorated. The observed oxidation behavior may be attributed to the composition variation of Zr and Ti in the alloys. The oxidation mechanism associated with the composition variation is discussed in this study.     

Microstructure and electrical conductivity of 10% Yb-doped SrCeO3 ceramics

As a candidate material for hydrogen separation, Yb-doped SrCeO₃ has attracted increasing attention in recent decades. In the present study, Yb-doped SrCe_0.9Yb_0.1O_3-α ceramics were prepared by the dry pressing and sintering approach, with the microstructure evolution and the micro morphology investigated. It was indicated that the ceramics sintered in air were of a pure perovskite structure, and that the sintering temperature had a significant effect on the growth of ceramic grains. The average grain size increased from 1 ​μm to 10 ​μm with an increase in sintering temperature from 1300 to 1500 ​°C. Further investigation of the thermodynamics and kinetics of grain growth revealed that the grain boundary diffusion was the main driving force of grain growth during solid phase sintering, with a grain growth index of 4 and an activation energy of approximately 61.23 ​kJ ​mol⁻¹. These results illustrate an obvious tendency of grain size growth. By electrochemical workstation with different atmospheres the effects of sintering temperature on the conductivity were characterized in the temperature range of 700–900 ​°C. The electrical conductivities σ of SrCe_0.9Yb_0.1O_3-α ceramics in different atmospheres were as follows: σ(wet hydrogen) ​> ​σ(dry hydrogen) ​> ​σ(dry air) ​> ​σ(wet air). In the test atmosphere containing water and hydrogen the conductivity of protons increased with increasing temperature because of the protons jump between lattices in the form of interstitial hydrogen ions or bare protons.     

WOOD/PCM composite with enhanced energy storage density and anisotropic thermal conductivity

Phase change materials (PCMs) have great potential in energy-saving and environmental field due to their high latent heat. The deficiencies that restrict the application of PCMs are their poor thermal conductivity and liquid leakage after phase change. To shoot these problems, a thermally-induced flexible WOOD/PCM composite with enhanced energy storage density and anisotropic thermal conductivity has been proposed. This composite consisted of polyethylene glycol 6000 (PEG6000), delignified balsa wood and boron nitride (BN). The results revealed that the melting enthalpy and freezing enthalpy of WOOD/PEG6000 composite were 209.3 ?J/g and 214.9 ?J/g, which had an augment of about 8% comparing with pure PEG6000. After adding 33 ?wt% BN to the composite, its thermal conductivity in the out-of-plane direction reached 0.96 ?W/(m·K), while the radial thermal conductivity was 0.36 ?W/(m·K). The controllable anisotropic thermal conductivity implies a good application in the environment where forced unidirectional heat dissipation is needed. Furthermore, the composite also shows excellent thermal induced flexibility, such as bending and compression, which is significant in reducing thermal contact resistance with substrates in application. This work indicates that the prepared PCM composite has a great potential application in thermal energy storage and thermal management.     

Effect of sintering parameters on the microstructure and thermal conductivity of diamond/Cu composites prepared by high pressure and high temperature infiltration

Pure Cu composites reinforced with diamond particles were fabricated by a high pressure and high temperature (HPHT) infiltration technique. Their microstructural evolution and thermal conductivity were presented as a function of sintering parameters (temperature, pressure, and time). The improvement in interfacial bonding strength and the maximum thermal conductivity of 750 W/(m·K) were achieved at the optimal sintering parameters of 1200℃, 6 GPa and 10 min. It is found that the thermal conductivity of the composites depends strongly on sintering pressure. When the sintering pressure is above 6 GPa, the diamond skeleton is detected, which greatly contributes to the excellent thermal conductivity.     

Effect of carbon addition on microstructure and mechanical properties of a typical hard-to-deform Ni-base superalloy

In this paper, the as-cast microstructure, microsegregation, and mechanical properties of GH4151 superalloys with a carbon addition were studied by scanner electron microscopy(SEM), transmission electron microscope(TEM), electron probe microanalysis(EPMA), differential thermal analyzer(DSC), and electron backscattered diffraction(EBSD). The results show that the solid solution effect of carbon in alloys is limited, the addition of C from 0.01 to 0.08 wt% significantly refines the secondary dendr...     

Sprayable coating based on fluorinated silica nanocomposites with superhydrophobic and antibacterial properties for advanced concrete

The porous hydrophilic nature of concrete structures makes them highly susceptible to chemical attack and bacterial adhesion, and the durability of concrete structures gradually decreases. In this work, a kind of sprayable coating with superhydrophobic and antibacterial properties was developed, which was used for surface engineering of concrete via the hydrophilic property of concrete and capillary action. The treated surface of concrete exhibited significant superhydrophobicity, with a static water contact angle of 162.4° and a sliding angle of 5°. The water absorption values of the treated concrete samples in merged in deionized water and 3.5 ?wt% NaCl solution for 7 days were measured as 1.85% and 2.71%, indicating the enhancement of water repellency by 72.03% and 80.11%, respectively. The antibacterial characterization revealed that the superhydrophobic coating enabled significantly reduced adhesion of bacteria, with the antibacterial rates of 84.57 ?± ?2.5% for E. coli and 90.43 ?± ?1.6% for S. epidermidis, respectively. The superhydrophobic coating provides hydrophobic protection for porous substrate structures and effectively reduce the corrosion damage of substrate materials, which is of practical significance for improving the durability of concrete structures.