Effects of temperature on fracture behavior of Al-based in-situ composites reinforced with Mg_2Si and Si particles fabricated by centrifugal casting

An aluminum-based in-situ composites reinforced with Mg2Si and Si particles were produced by centrifugal casting A1-20Si-5Mg alloy. The microstructure of the composites was examined, and the effects of temperature on fracture behavior of the composite were investigated. The results show that the average fraction of primary Si and Mg2Si particles in the composites is as high as 38%, and ultimate tensile strengths （UTS） of the composites first increase then decrease with the increase of test temperature. Microstructures of broken specimens show that both the particle fracture and the interface debonding affect the fracture behavior of the composites, and the interface debonding becomes the dominant fracture mechanism with increasing test temperature. Comparative results indicate that rich particles in the composites and excellent interface strength play great roles in enhancing tensile property by preventing the movement of dislocations.

In-situ observation of porosity formation during directional solidification of Al-Si casting alloys

In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in the melt far away from the eutectic solid-liquid (S/L) interface and then grow and coagulate during solidification.Some pores can float and escape from the solidifying melt front at a relatively high velocity.At the end of solidification,the remaining pores maintain spherical morphology in the near eutectic alloy but become irregular in the hypoeutectic alloy.This is attributed to different solidification modes and aluminum dendrite interactions between the two alloys.The mechanism of the porosity formation is briefly discussed in this paper.

Heat transfer behavior of AZ80-1%Y alloy during low-frequency electromagnetic casting

Heat transfer behaviors of AZ80?1%Y alloy during low frequency electromagnetic casting (LFEC) and direct chilling casting were studied by in-situ temperature measurement. The results demonstrated that the low frequency electromagnetic field (EM) caused forced convection in the melt during LFEC. The forced convection led to uniform solidification velocity and temperature field. EM frequency, excitation current intensity and casting temperature could control the heat transfer behavior. The forced convection could improve the microstructure and degrade the difference in microstructure between the edge and center of billet. Appropriate parameters of low frequency EM for casting Mg alloy are 20 Hz of frequency and 60 A of electric current intensity.

Pressureless all-solid-state Na/S batteries with self-supporting Na_(5)YSi_(4)O_(12) scaffolds

The development of reliable and affordable all-solid-state sodium metal batteries(ASS-SMBs)requires suitable solid-state electrolytes with cost-efficient processing and stabilized electrode/electrolyte interfaces.Here,an integrated porous/dense/porous Na_(5)YSi_(4)O_(12)(NYS)trilayered scaffold is designed and fabricated by tape casting using aqueous slurries.In this template-based NYS scaffold,the dense layer in the middle serves as a separator and the porous layers on both sides accommodate the active materials with their volume changes during the charge/discharge processes,increasing the contact area and thus enhancing the utilization rate and homogenizing the current distribution.The Na/NYS/Na symmetric cells with the Pb-coated NYS scaffold exhibit significantly reduced interfacial impedance and superior critical current density of up to 3.0 mA cm^(-2)against Na metal owing to enhanced wettability.Furthermore,the assembled Na/NYS/S full cells operated without external pressure at room temperature showed a high initial discharge capacity of 970 mAh g^(-1)and good cycling stability with a capacity of 600 mAh g^(-1)after 150 cycles(based on the mass of sulfur).This approach paves the way for the realization of economical and practical ASS-SMBs from the perspective of ceramic manufacturing.

High temperature mechanical properties of low carbon silicon-bearing steel by twin-roll strip casting

The tensile equipment of the advanced confocal scanning laser microscope （CSLM） was used to research the high temperature mechanical properties of low carbon silicon-bearing steel by twin-roll strip casting. The results show that, at the strain rate of 0. 000 5 s ^-1, the strip clearly shows signs of brittleness at around 600 ℃ and its plasticity falls considerably between 750 ℃ and 600 ℃. This is because during the transformation from austenite to ferrite, the low strength ferrite at the austenite grain boundaries greatly reduces the steel＇ s high temperature plasticity. The subsequent strip coiling process should be controlled at less than the brittle temperature of approximately 600 ℃, so cracks at the coiling stage can be prevented.

马鞍山长江公铁大桥主航道桥中塔下横梁施工关键技术

马鞍山长江公铁大桥主航道桥Z4号中塔采用空间四肢A形钢-混组合塔,横向两塔肢间设下横梁,下横梁间设3道系梁,纵向两塔肢间不设置横梁。下横梁采用支架法分层浇筑施工。下横梁施工支架采用落地式钢管支架配套模板体系,受力性能满足要求;通过有限元计算比选下横梁混凝土分层浇筑方案并进行优化,采取(5.5+2.5)m分层方案,上、下层混凝土分别分为2块、3块分层分块浇筑;通过低温升、易泵送、高抗裂的C60混凝土研制、下横梁与塔柱倒弧结合面抗裂措施优化、分块间接缝构造形式及施工工艺优化、下横梁与系梁混凝土浇筑施工组织优化、混凝土温度智能化监控、预应力束张拉时机控制等混凝土防裂措施,有效解决了下横梁大体积混凝土易开裂的难题。

工具式液压卸料平台在现浇高层住宅中的应用

某工程为全现浇住宅工程,建筑层数为9~18层,其中3号、4号、6号、8号楼地上采用大钢模+轮扣支撑体系,结构外防护架体采用三角组合型钢悬挑脚手架。为了解决周转材料运输问题,项目部创新性地应用了工具式液压卸料平台,此平台采用液压油缸提升机并借助附着在结构墙上的2根钢导轨实现自动爬升,不仅操作简便、爬升速度快,而且自带智能荷载监控系统,便于日常安全管理。

浅析全现浇外墙结构非标准层悬挑外架逆作法施工技术

针对案例工程中全现浇混凝土外墙、铝合金模板、爬架等快穿插施工体系非标准层施工难的问题,采用全现浇外墙结构非标准层悬挑外架逆作法施工方法。论文详细介绍了该施工施工方法的施工流程和操作要点,并提出质量和安全控制措施。

浅析球墨铸铁管脚手架扣件的应用前景

介绍了球墨铸铁管脚手架扣件与可锻铸铁管脚手架扣件的力学性能、铸造性能及使用性能,分析了采用球铁代替可锻铸铁来生产脚手架扣件的可行性。通过球墨铸铁扣件与可锻铸铁扣件力学性能试验对比,发现球墨铸铁扣件的抗滑移、抗破坏、扭转刚度等力学性能均优于可锻铸铁扣件,特别是扭转刚度性能指标优势表现得更加明显。同时,应用球墨铸铁扣件和可锻铸铁扣件生产成本对比降低了6.8%,实现了节能减排。

公路桥梁工程中满堂支架现浇箱梁施工技术

现浇箱梁施工是公路桥梁施工中的重要内容,其施工质量对整个工程的质量及使用寿命有着至关重要的影响。本文结合实际工程,就现浇箱梁施工技术进行探讨,明确其操作流程,分析施工中可能出现的问题及应对措施,以提高施工质量。

混凝土连续梁挂篮悬臂施工技术的研究

本连续梁0#段采用在墩顶现浇施工并与墩身固结的方式,悬臂段从已经浇筑的0#段开始,采用挂篮逐步向前悬臂施工。本文针对该连续梁挂篮悬臂施工的要求对挂篮进行设计,以及对挂篮悬臂施工工艺和混凝土悬臂浇筑展开系统论述,为全面提高连续梁建设效率和安全性提供技术参照。

In-situ SEM observation on fracture behavior of austempered silicon alloyed steel

Crack initiation, propagation and microfracture processes of austempered high silicon cast steel have been investigated by using an in-situ tensile stage installed inside a scanning electron microscope chamber. It is revealed that micro cracks always nucleate at the yielding near imperfections and the boundary of matrix-inclusions due to the stress concentration. There are four types of crack propagations in the matrix: crack propagates along the boundary of two clusters of bainitic ferrite; crack propagates along the boundary of ferrite-austenite in bainitic ferrite laths; crack propagates into bainitic ferrite laths; crack nucleates and propagates in the high carbon brittle plate shape martensite which is transformed from some blocky retained austenite due to plastic deformation. Based on the observation and analysis of microfracture processes, a schematic diagram of the crack nucleation and propagation process of high silicon cast steel is proposed.

Study on in-situ WC particles/tungsten wire reinforced iron matrix composites under electromagnetic field

By applying electromagnetic field to a system consisting of tungsten wires and grey cast iron melt,the grey cast iron matrix composite reinforced by either in-situ WC particles or the combination ofin-situ WC particles and the residual tungsten wire was obtained.By means of differential thermal analysis(DTA),the pouring temperature ofiron melt was determined at 1,573 K.The microstructures of the composites were analyzed by using of X-ray diffraction(XRD),scanning electron microscopy(SEM) equipped with an energy dispersive spectrum(EDS) and pin-on-disc abrasive wear test.The obtained results indicated that,with the enhancing frequency of electromagnetic field,the amount ofin-situ WC particles gradually increases,leading to continuous decrease of the residual tungsten wires.When the electromagnetic field frequency was up to 4 kHz,tungsten wires reacted completely with carbon atoms in grey cast iron melt,forming WC particals.The electromagnetic field appeared to accelerate the elemental diffusion in the melt,to help relatively quick formation of a series of small FeW-C ternary zones and to improve the kinetic condition ofin-situ WC fabrication.As compared with the composite prepared without the electromagnetic field,the composite fabricated at 4 kHz presented good wear resistance.

Fabrication and Interfacial Structure of Ni-YSZ-LSM Porous Scaffold for Solid Oxide Fuel Cells

A novel process route using tape casting and stacking for fabricating porous scaffold of solid oxide fuel cells （SOFC） was demonstrated. The linear shrinkages of anode （Ni-YSZ, YSZ stands for 3% Y2O3 （mole fraction） stabilized ZrO2 ） and cathode （LSM-YSZ, LSM stands for La0.8Sr0.2MnO3 ） were optimized to be uniform with that of electrolyte during sintering, by controlling the content of pore former. The micromorphology and interface microstructure of the cross-section of the porous scaffold were observed by optical microscope and scanning electron microscope, respectively. The element distribution and phase composition were analyzed by energy dispersive spectrometer and X-ray diffraction, respectively. The results showed that the porous scaffold with regular pore shape and high specific surface area was obtained after sintering at 1 350℃. The fabricated porous scaffold had defect free interracial structures due to the uniform shrinkage of anode, cathode and electrolyte layers. In addition, it was shown that diffusions of Zr, Ni and La caused a progressive boundary between YSZ, Ni-YSZ and LSM-YSZ layers. The interface between anode and electrolyte （Ni-YSZ/YSZ） was mainly composed of Ni, YSZ and a small amount of NiO, and the interface between cathode and electrolyte （LSM-YSZ/YSZ） was mainly composed of YSZ, LSM and a small amount of La2Zr2O7.

高铁桥梁上跨既有营业线简支箱梁横移施工技术

新建福州至厦门铁路上行联络线湖边特大桥上跨既有福厦线湖地里大桥,设计为32 m简支混凝土箱梁。为确保该简支箱梁施工时既有营业线运营安全,简支箱梁施工采用墩旁线外支架现浇、横移就位施工技术。综合考虑天窗点作业时间限制、营业线高空施工安全,在常规的支架现浇、横移施工技术基础上,采用现浇支架与横移系统联合设计,增加了支架上箱梁横移的稳定性;设计了鱼腹式异形滑道梁,滑道梁分段设计、吊装,降低了营业线吊装难度,确保吊装安全;改进了滑轮导向限位装置,利用滑座两侧的钢轮引导及限位滑移,确保了横移就位的准确性。采用的施工技术降低了上跨施工安全隐患,节约了施工成本,加快了施工进度,有效解决了新建桥梁上跨既有营业线的施工难题。

In-situ Observation of Tensile Fracture in A357 Casting Alloys

The mechanism of damage evolution and fracture in A357 casting alloys was investigated by in-situ scanning electron microscopy （SEM） tensile testing. Different microstructures of A357 casting alloys were produced by eutectic Si modification and T6 heat treatment. It is shown that microcracks in these alloys are predominantly formed in eutectic Si particles. Large and elongated eutectic Si particles in unmodified alloy show the greater tendency to cracking, whereas cracking of small and round eutectic Si particles in Sr modified and T6 heat treated alloys is relatively lag. The crack mainly propagates along the broken eutectic Si particles in unmodified and Sr modified alloys or along the deepened shear bands in T6 heat treated alloy with accumulating the applied strain. The results were discussed in terms of Weibull statistics and the fracture models were established.

基于Midas Civil的钢管-贝雷组合支架优化设计

高架桥由于离地面较高、施工周期长,往往采用支架法进行施工,常见的有满堂支架法和钢管-贝雷组合支架法两种形式。针对组合支架法施工缺乏科学准确的分析手段,根据传统经验采取过于保守的搭设方案这一问题,采用Midas Civil软件辅助建立模型和力学理论分析相结合的方法,得出钢管-贝雷组合支架搭设形式的优化办法。

溶剂浇铸/颗粒沥滤技术制备组织工程支架材料

生物可降解多孔三维细胞支架是组织工程化组织构建的基础。溶剂浇铸 粒子沥滤技术是最简便、也是研究最广泛的一种多孔三维细胞支架制备技术 ,随着各种改进方法的出现 ,溶剂浇铸

改进溶剂浇铸/粒子沥滤法制备聚ε-己内酯组织工程支架

对经典的溶剂浇铸/粒子沥滤法进行适当改进,结合致孔剂直接黏结工艺制备三维多孔聚ε-己内酯(PCL)组织工程支架材料.以氯化钠为致孔剂,利用预成型-真空浇铸-脱溶剂工艺制得支架材料.对支架的微结构、孔径、孔隙率和亲水性等性能进行表征.结果表明,黏结工艺中加入水量在2%～8%时都可形成均匀多孔黏结结构,且支架孔隙率几乎不变;支架孔径基本分布在10～80μm;孔隙率可达(89±1)%;采用抽真空与加热相结合的脱溶剂工艺可以提高孔隙率.制得的PCL支架厚度>5mm且为三维高孔隙率海绵体结构,可适用于软组织工程的修复.

制备PDLLA组织工程支架的致孔剂研究

为了制备孔隙呈球形、孔隙间相互连通的高孔隙率三维细胞支架,研究了一种新型水溶性球形致孔剂及性质。探讨了间歇离心湿度粘技术与水溶性球形致孔剂相结合,制备聚乳酸(PDLLA)三维多孔细胞支架,获得了高度为5·0mm±0·5mm,直径为8·3±0·4mm的三维多孔支架。实验结果表明,致孔剂颗粒呈球形、水溶性良好,在三维支架中无残留;并且支架的孔隙结构均匀、孔隙率高(94·68%±0·52%)、孔隙呈球形、孔隙间相互完全连通,孔隙的大小可以人为控制。由此表明,在组织工程细胞支架制备中,这类水溶性球形致孔剂具有广泛的潜在应用价值。