尺寸条件下Cu-Nb-Cu复合线材微观结构的演变机制

本文采用集束拉拔技术成功制备出了Nb管增强Cu-Nb的多芯复合线材,分别对不同尺寸(Φ2.5mm、Φ2.2mm、Φ2.0mm,Φ1.7mm)线材的微观形貌进行了表征,并测试了不同尺寸线材的纳米压痕实验曲线.探究了尺寸效应下,Cu-Nb-Cu复合线材微观形貌的变化规律,及引入Nb管后,材料的界面、芯丝、及不同铜层的硬度演变规律.最后通过对Φ1.7mm线材进行高分辨场TEM测试,结果显示,线材经过大塑型变形后,完整的Nb管已经发生了破裂,呈条带状分布.     

Nb 管增强Cu-Nb 复合线材的结构及力学性能研究

Cu-Nb 复合材料兼具高强度、 高电导率以及高热稳定性等优势, 已成为脉冲磁体和电磁线圈导体材料的首选. 本文通过集束拉拔技术成功制备出了 Nb 管增强 Cu-Nb 三次复合线材. 综合利用 TEM/HRTEM、EBSD 等表征和测试手段分析了冷拉变形后材料的微观组织, 揭示了大塑性变形后 Nb 管增强 Cu-Nb 复合线材的微结构演变特征. 并基于线材强度与 Nb 芯丝的量化关系分析, 探讨了 Nb 管增强 Cu-Nb 复合线材的强化机理.     

氧化石墨烯增强Cu-Nb 复合线材结构及性能影响研究

本文采用粉末套管法成功制备出氧化石墨烯增强的 GO/Cu-Nb 多芯(192 芯) 复合线材及未掺杂氧化石墨烯的 Cu-Nb 多芯(192 芯) 复合线材. 通过金相、SEM 及拉曼光谱等表征不同尺寸下两种复合线材的芯丝组态、 界面特征及特征峰. 结果表明, 氧化石墨烯由于良好的自润滑特性较好地协调了芯丝与基体的变形, 其弥散分布有效阻隔了 Nb 颗粒团聚及大尺寸晶粒的产生, 芯丝变形更均匀, 形态更规则. 力学和电学性能测试结果表明, 掺杂氧化石墨烯后,Cu-Nb 复合线材的力学与电学性能均明显提升, 分析认为, 氧化石墨烯的尺寸大小、 分散均匀性及热处理是影响线材综合性能提升的主要原因.     

MgB_2/Nb/Cu多芯线材的力学性能研究

实验中对采用原位法粉末装管工艺(in-situ PIT)制备的Nb芯增强,6芯、12芯和36芯等多种导体结构的Nb/Cu复合包套MgB2多芯超导线材的力学性能进行了针对性地研究;设计并加工了一套专门用于低温下MgB2线带材弯曲应力测试的样品架;研究了MgB2线带材的临界电流随弯曲应力的变化性能;同时研究了室温及低温条件下的拉伸对MgB2线材超导电性的影响。力学性能分析表明,所制备的Nb芯增强6芯MgB2超导线材在弯曲直径为80mm以上时,超导临界传输电流没有出现明显的退降,同时该线材的拉伸力学性能也比未增强线材有明显的改善。     

共沉淀Bi-2212粉末与商用粉末的性能对比研究北大核心CSCD

要实现Bi-2212超导线材的大规模应用必须提高线材的电流传输性能.为进一步改善Bi-2212前驱粉末的性能,采用共沉淀技术制备了Bi-2212前驱粉末.对比了共沉淀粉末与商用前驱粉末的微观结构、杂质含量以及相应线材的载流性能,结果发现共沉淀粉末中小且少的第二相粒子,有利于减少最终线材内的第二相粒子,提高最终线材的载流性能.共沉淀粉末中Bi-2212晶粒尺寸较大,存在更多的Bi-O滑移面,粉末更易加工,从而得到银超界面光滑、晶粒排列整齐、载流性能优异的Bi-2212线材.最终共沉淀粉末Bi-2212线材的载流性能比商用粉末线材的性能高约35%.     

共沉淀Bi-2212粉末与商用粉末的性能对比研究

要实现Bi-2212超导线材的大规模应用必须提高线材的电流传输性能.为进一步改善Bi-2212前驱粉末的性能,采用共沉淀技术制备了Bi-2212前驱粉末.对比了共沉淀粉末与商用前驱粉末的微观结构、杂质含量以及相应线材的载流性能,结果发现共沉淀粉末中小且少的第二相粒子,有利于减少最终线材内的第二相粒子,提高最终线材的载流性能.共沉淀粉末中Bi-2212晶粒尺寸较大,存在更多的Bi-O滑移面,粉末更易加工,从而得到银超界面光滑、晶粒排列整齐、载流性能优异的Bi-2212线材.最终共沉淀粉末Bi-2212线材的载流性能比商用粉末线材的性能高约35%.     

千米量级多芯MgB2/Nb/Cu超导线材的制备及性能研究

以无磁性的Nb作为中心增强体和阻隔层材料,无氧铜作为稳定体包套材料,采用原位法粉末装管工艺(in-situ PIT)制备了千米量级7芯导体结构的MgB2/Nb/Cu超导线带材,由于Nb/Cu包套材料具有良好的塑形加工性能,整个加工过程中未进行中间退火热处理,复合多芯线材最终加工到Φ1.4mm;在真空热处理炉中680℃保温2小时进行成相热处理;对烧结后的线材进行了微观结构、超导电性、纵向电流分布均匀性及常温力学性能等分析检测.线带材的工程临界电流密度在20K,1T磁场条件下达到2.5×104 A/cm2.结果表明该工艺能够制备实用化高性能的MgB2线带材.     

热处理温度对碳掺杂MgB_2/Nb/Cu超导线材性能的影响

采用Nb/Cu复合管作为外包套材料,通过原位法粉末装管工艺(PIT)制备了C掺杂MgB2/Nb/Cu线材.在高纯流动氩气保护条件下、650～950℃温度区间内烧结2h.微观结构分析显示,通过该工艺制备的MgB2/NbZr/Cu线材具有良好的晶粒连结性和较高的致密度.X射线衍射(XRD)分析表明在750℃左右可以生成纯度较高的MgB2相,在低温和高温下烧结后均有杂相峰出现,并且高温烧结后所生成的相结构较为复杂.采用四引线法超导临界电流的测试结果表明,低温烧结后的线材具有超导电流传输性能,而当热处理温度超过750℃时,样品中的电流传输状态表现为正常电阻态.实验结果证实采用Nb作为原位法MgB2超导线材的包套阻隔层时,成相热处理一般应该在低于750℃的温度下进行.     

Bi-2212/Ag-Mn多芯线材的制备

Bi-2212(Bi2Sr2CaCu2Ox)线材具有优异的低温高场性能,线材的各向同性为磁体绕制提供方便,使其成为超高磁场磁体内插线圈的首选材料.本研究采用PIT技术与部分熔化工艺相结合的工艺制备Bi-2212/AgMn线材,系统研究熔化温度、降温速率以及低温保温时间后处理气氛等参数对线材相成分、微观结构及超导电性的影响,最终制备出临界电流密度Jc达到14.7kA/cm2(77K,self)的Bi-2212多芯线材.     

尺寸对Cu-Nb微观复合材料磁滞特性的影响

本文研究了Cu-Nb多芯复合线微观结构对磁滞特性的影响.样品通过不同塑性应变和热处理获得不同显微结构.结果表明,经冷加工变形和800℃/1.5h退火,尺寸为Φ1.06mm样品的磁化曲线中只在高场下出现了一个峰,这个峰来源于Nb的体钉扎.Φ2.04mm样品在退火后的曲线中出现了三峰结构,低场下的两个峰是由于临界效应使Cu基体中产生了超导电性引起的,而高场下出现的第三个峰也与Nb的体钉扎有关.样品中Nb芯丝直径及芯丝间距等因素影响了复合材料直流磁化曲线的特征.     

Deformation-induced martensite and nanotwins by cryogenic laser shock peening of AISI 304 stainless steel and the effects on mechanical properties

Laser shock peening (LSP) of stainless steel 304 was carried out at room and cryogenic temperature (liquid nitrogen temperature). It was found that the deformation-induced martensite was generated by LSP only when the laser-generated plasma pressure is sufficiently high. Compared to room temperature laser shock peening (RT-LSP), cryogenic laser shock peening (CLSP) generates a higher volume fraction of martensite at the same laser intensity. This is due to the increase in the density of potential embryos (deformation bands) for martensite nucleation by deformation at cryogenic temperature. In addition, CLSP generates a high density of deformation twins and stacking faults. After CLSP, an innovative microstructure, characterised by networks of deformation twins, stacking faults and composite structure (martensite and austenite phases), contributes to material strength and microstructure stability improvement. The combined effect of higher surface hardness and a more stabilised microstructure results in greater fatigue performance improvement of the CLSP samples compared to that of the RT-LSP samples.     

Mechanical properties,density, and defect structure of VT1-0 titanium after intense plastic deformation due to screw and longitudinal rollings

The influence of screw rolling combined with standard methods of mechanothermal treatment on the homogeneity of the forming submicrocrystalline structure, density, and mechanical performance of VT1-0 commercial titanium is studied. It is shown that such a treatment carried out within optimal temperature and strain rate intervals (special deformation conditions) causes minor softening of the material and can be effectively used to form a homogeneous submicrocrystalline structure with high strength and elastoplastic properties.     

Accelerated formation of an ultrafine-grained structure in a two-phase Ti alloy during compression with decreasing temperatures

ABSTRACT

Ultrafine-grained (UFG) structure is beneficial for overcoming the strength-ductility trade-off and enhancing the superplasticity of two-phase Ti alloys. Recently, it has been demonstrated that compression with decreasing temperatures is effective for producing UFG two-phase Ti alloys initially with lamellar microstructures. However, the effect of lamellar thickness on the microstructural evolution during this process has not yet been fully elucidated. In this study, Ti-6Al-4V alloys with different lamellar thicknesses were compressed while the processing temperature was decreased from 800°C to 600°C. The thinner lamellar microstructure was preferable for preventing void/crack formation, while accelerating the continuous dynamic recrystallisation, thus providing a fully UFG structure at a relatively low strain of 1.4. In addition, the origin of different plastic flows in each sample was analysed in detail by analysing the microstructural evolution. These findings demonstrate that the processing method is effective for reducing the grain size of a two-phase Ti alloy without severe plastic deformation techniques, which require large strain (≥4). A reduction in the strain required to achieve the UFG structure would be beneficial because conventional metal-forming processes, i.e. rolling, extrusion, or forging, which are suitable for mass production, could be used.     

Thermodynamics of deformation of micro composites

A thermodynamic evaluation, based on simultaneous measurements of the mechanical work and the concomitant heat of deformation in a stretching micro calorimeter, was performed on polycarbonate/carbon fibre (PC/CF) micro composites. Single fibre composite specimens containing sized and unsized CF were subjected to different thermal treatments. Thermodynamic effects of deformation were measured for both elastic and plastic deformation ranges. In the elastic/beginning of plastic deformation ranges the internal energies and heat effects of deformation of micro composites exceeded those of matrices. The highest internal energy and, in turn, the highest fibre/matrix adhesion, was found in the case of the quenched micro composites with sized fibers. Thermodynamic effects of deformation and calculated material constants of PC were in reasonable agreement with data in the literature.     

Effect of elastic excitations on the surface structure of hadfield steel under friction

The structure of the Hadfield steel (H13) surface layer forming under dry friction is examined. The deformation of the material under the friction surface is studied at a low slip velocity and a low pressure (much smaller than the yields stress of H13 steel). The phase composition and defect substructure on the friction surface are studied using scanning, optical, and diffraction electron microscopy methods. It is shown that a thin highly deformed nanocrystalline layer arises near the friction surface that transforms into a polycrystalline layer containing deformation twins and dislocations. The nanocrystalline structure and the presence of oxides in the surface layer and friction zone indicate a high temperature and high plastic strains responsible for the formation of the layer. It is suggested that the deformation of the material observed far from the surface is due to elastic wave generation at friction.     

Forming a perfect cubic texture in thin copper–yttrium alloy strips during cold rolling and annealing

The structure of strips produced from the Cu–1 wt % Y binary alloy using cold deformation by rolling to the degree of deformation of ~99%, followed by recrystallization annealing, as well as the process of texture formation in these strips, is studied. The possibility of forming a perfect cubic texture in a thin strip made of a binary yttrium-modified copper-based alloy has been shown in principle, which opens the prospect of the use of this alloy to produce substrates for strip high-temperature superconductors of the second generation. The optimum conditions of annealing have been determined, which make it possible to form a perfect biaxial texture in the Cu–1 wt % Y alloy with a content of cubic grains {001}〈100〉 ± 10° on the surface of the textured strip of over 95%.     

A model of grain refinement and strengthening of Al alloys due to cold severe plastic deformation

This paper presents a model which quantitatively predicts grain refinement and strength/hardness of Al alloys after very high levels of cold deformation through processes including cold rolling, equal channel angular pressing (ECAP), multiple forging (MF), accumulative roll bonding (ARB) and embossing. The model deals with materials in which plastic deformation is exclusively due to dislocation movement within grains, which is in good approximation the case for many metallic alloys at low temperature, for instance aluminium alloys. In the early stages of deformation, the generated dislocations are stored in grains and contribute to overall strength. With increase in strain, excess dislocations form and/or move to new cell walls/grain boundaries and grains are refined. We examine this model using both our own data as well as the data in the literature. It is shown that grain size and strength/hardness are predicted to a good accuracy.     

图像处理技术在材料表面形变测量中的应用

测量系统是针对检测碳/碳复合材料高温下的力学性能,分析碳/碳复合材料的应力应变关系所设计。介绍了基于CCD图像处理技术,测量高温下碳/碳复合材料试件形变系统的结构及工作原理。在高温下连续实时获取碳/碳复合材料试件上合作目标图像,并应用索贝尔算子准确的计算出图像边缘,通过坐标变换得到了碳/碳复合材料在高温载荷下形变数值。分析了测量系统中图像处理的设计思路,给出了实验结果曲线。