【作者】 王鹏飞；

【导师】 张平祥；

【作者基本信息】 西北工业大学 ， 材料学， 2021， 博士

【摘要】 高强高导铜合金材料是集优良物理性能和力学性能为一体的有色金属材料,能够同时具备高强度、高电导率以及高热导率等优势,被广泛用于脉冲强磁场、转换开关、电接触器、引线框架及电车和电力火车导线等领域。Cu-Nb复合材料作为一种典型的高强高导铜基复合材料,是制备超高场脉冲磁体的首选导体材料。随着近年来脉冲磁体技术的快速发展,对导体材料的综合性能提出了越来越苛刻的要求,如抗拉强度、电导性能以及热稳定性等技术指标不断提高。目前,在Cu-Nb复合材料的制备过程中,纳米Nb芯丝强化确实能显著增强体系的机械性能,但是其带来的严重塑性衰退,限制了芯丝充分纳米化,是阻碍性能进一步提升的关键因素。在纳米结构材料的研究中,结构形态、尺寸效应、热处理、以及梯度纳米结构等特征决定了材料的综合性能。基于这一理念,本文针对脉冲磁体用高强高导Cu-Nb复合材料开展系统研究,从材料的结构调控入手,揭示了大塑性变形条件下,纳米尺度材料的微观结构演变规律。获得了Cu-Nb纳米复合材料的强化机理,实现了材料由纳米结构到综合性能的有效调控。表征了材料的热稳定性,获得了热变形条件下材料的本构关系模型和热加工图。另一方面,通过引入具有高硬度、高电导的石墨烯进行掺杂,同时获得了复合体系强度和电导率的提高。通过建立材料结构与性能的耦合,达到调控强度与电导倒置关系的目的,为我国脉冲磁场强度突破100T提供必要的导体材料支撑。本文主要研究结果如下:（1）采用集束拉拔技术成功制备出Nb管增强Cu基复合线材（称为Cu-Nb-Cu复合材料）,在具有4.91 mm²的横截面线材上获得了室温抗拉强度1.03 GPa的机械强度,同时电导率保持为74%IACS。在此基础上,系统阐明了Nb的组态差异对结构演变的影响规律。经历大塑性变形后,与传统Cu-Nb复合材料相比,Cu-Nb-Cu复合材料的衍射峰半高宽宽化明显,表明Cu-Nb-Cu复合材料的晶粒尺寸细化效果更为显著。微结构表征结果发现,Cu-Nb-Cu复合材料中Nb管发生了破裂,多以条带状分布,长径比约为5:1,大于Cu-Nb复合材料中芯丝的长径比（～4:1）,这是体系丝织构增强的关键。此外,部分位错在Cu/Nb界面处堆积和增殖。并且随着应变量的不断增加,（110）_Nb晶面位错密度急剧增加。另外,在加工过程中引入了孔型轧制工艺,结合700℃/4h的退火工艺,在传统Cu-Nb复合材料中获得了室温抗拉强度达到1.03 GPa,电导率为68%IACS,强度提高了约5%,延伸率提高了24%。基于Ludwik-Holloman方程对Cu-Nb-Cu复合材料的加工硬化指数（n）进行了计算,结果表明,随着应变量的增加,n值呈现先增大后减小趋势,这与加工硬化的强化作用与动态回复的软化作用大小关系有关。（2）对比分析了热处理对Cu-Nb和Cu-Nb-Cu复合材料微观结构及性能的影响。研究发现,两种结构的复合材料在较低的400℃条件下退火,Nb芯丝结构未发生显著变化,但当退火温度提高到700℃以上,晶粒发生了明显的再结晶长大,Nb芯丝呈现出一种“竹节状”结构。相比Cu-Nb复合材料,Cu-Nb-Cu复合材料中Nb芯丝的球化和粗化更为剧烈。采用晶粒生长模型计算得到两种结构材料的晶粒生长活化能分别为33.30 k J/mol和27.70 k J/mol,表明Cu-Nb-Cu复合材料具有较大的晶粒生长速率。材料的晶面位错密度计算结果表明,随着退火温度的升高,复合体系中（111）_Cu和（200）_Cu晶面上位错密度缓慢下降,而（110）_Nb晶面上的位错密度下降速率更快,但Cu-Nb-Cu复合材料的晶面位错密度远高于Cu-Nb复合材料。考虑到Nb的磁化率随其微结构会发生一定的变化,对材料的H-M曲线进行了测量。结果表明,磁滞特征峰的明显变化可能与不同复合材料中芯丝的微观结构变化有关。因此,该结果为磁化方法表征Cu-Nb复合材料结构和尺寸效应提供有效的数据支持。（3）系统开展了Cu-Nb复合材料的热变形研究。热压缩变形后,材料呈现出独特的颈缩形貌。700℃以上热压缩,纳米纤维发生快速再结晶,材料性能迅速衰减,材料硬化和软化过程的平衡被打破,很难观察到流变区域。700℃以下热压缩,应变速率提高经过峰值后迅速下降,流变应力趋于平稳,位错集中和位错湮灭达到动态平衡。以Arrhenius型双曲正弦本构模型为依据,构建了Cu-Nb复合材料的热压缩本构关系模型,模型数据和试验数据高度重合,重合度大于90%。采用动态材料模型（DMM）绘制出Cu-Nb复合材料的热加工图。并基于此获得了复合体系的最优加工参数,即当应变ε=0.6时,在620℃-750℃区域,可加工区域对应较高的应变速率,而ε=0.2时,在650℃-730℃区域,较低应变速率具有更好的热变形能力,此时材料具备获得良好的热加工性能。（4）采用粉末套管法制备出石墨烯掺杂Cu-Nb复合线材（Cu-Nb/G）。与传统线材相比,其室温抗拉强度提高10%,电导率提高15%。结构分析表明,石墨烯有效抑制了Nb颗粒尺寸的增大、团聚以及裂纹的产生,在晶粒细化方面具有显著的效果。同时,750℃/60h的热处理,有利于界面结合强度的增强和塑性的显著提高。导电机理研究表明,石墨烯掺杂Cu-Nb复合线材的导电性与石墨烯的尺寸、及形态分布有关。更多还原

【Abstract】 High-strength and high-conductivity copper alloys are typical non-ferrous metallic materials with excellent physical and mechanical properties,which exhibit ultra-high strength,good electrical conductivity,and thermal conductivity.They are now widely used for the fabrication of Ultra-high field Pulsed Magnets,Transfer Switches,Electrical Contractors,Lead Frames,and wires for trams and electric trains.As a typical high-strength and high-conductivity composite material,Cu-Nb composites are the preferred conductor materials for achieving higher pulsed magnetic fields.With the rapid development of pulse magnets fabrication technology,requirements of the conductor materials become more and more stringent.The materials with higher tensile strength,better electrical conductivity,and higher thermal stability are required.Since the nano-Nb filaments can indeed greatly enhance the mechanical properties.The decline of plasticity appearing as side-effect seriously restricts the further plastic deformation of the Cu-Nb composite.Thus the insufficient nano-sized filaments hindered the further enhancement of mechanical performances.The structure morphology,size effect,heat treatment（HT）,and gradient nanostructures are the keys to determine the overall performance.Therefore,in this thesis,the microstructure evolution of the Cu-Nb composite under large plastic deformation was analyzed.The strengthen mechanism involved in this Cu-Nb composites was revealed,and the modulation from nanostructures to the comprehensive performance was realized.By analyzing the thermal stability of the Cu-Nb composites,the constitutive relationship model under the thermal deformation was established,and the thermal processing maps of obtained Cu-Nb composites was also plotted.On the other hand,graphene with large hardness and high conductivity was introduced as dopant.The strength and electrical conductivity of Cu-Nb composites have been improved simultaneously.The study of this thesis provided a high-performance Cu-Nb composite material.Thus can be recognized as the solid conductor materials foundation for the further improvements of ultrahigh pulsed field magnets（100T）in China.The main research contents and results are listed as follows:（1）Nb tube-reinforced Cu-based composites（Cu-Nb-Cu composites）were successfully prepared by accumulative drawing and bundling（ADB）process,and the UTS of 1.03 GPa and electrical conductivity of 74%IACS were achieved on the wires wit cross-section area of 4.91 mm²at room temperature.The influences of different Nb configuration among Cu-Nb-Cu composites and traditional Cu-Nb composites were studied.After undergoing a large plastic deformation,comparing with the Cu-Nb composites,the FWHM of the diffraction peaks in Cu-Nb-Cu composites increased significantly,indicating the more effective grain size refinement in Cu-Nb-Cu composites.The microstructure observation results showed that the Nb tubes were broken into slender strips during the drawing process,the aspect ratio of strips was about 5:1,which was higher than that in Cu-Nb composites about 4:1.It was clearly observed that many edge dislocations accumulated and multiplied at the Cu/Nb interfaces.The calculation of the dislocation density showed that the dislocation density of the（110）_Nbcrystal plane had increased rapidly.Groove rolling process was applied on Cu-Nb compositesduring the deformation.Under the annealing process at 700℃for4 h,Cu-Nb composites with the tensile strength of 1.03 GPa,and electrical conductivity of 68%IACS was achieved with the mechanical strength improving by 5%,and the elongation enhancing by 24%.Based on Ludwik-Holloman equation,the work hardening index（n）of Cu-Nb-Cu material was obtained.The results showed that the n value first increased and then decreased with the increasing applied strain,which was related to the growth rate of the dislocation density inside the composites.（2）The HT effect on both Cu-Nb and Cu-Nb-Cu composites were studied in detail.Experienced the annealing process at 400℃,the Nb filaments remained stable.While HT over 700℃,the grains in matrix recrystallized and grown significantly,and the Nb filaments exhibited as a"bamboo-like"structure,the spheroidization and coarsening of Nb filaments in Cu-Nb-Cu composites were more severe than that in Cu-Nb composites.The grain growth model was used and the activation energies of two composites were calculated to be 33.30 k J/mol and 27.70 k J/mol,respectively,attributed to a large grain growth rate in Cu-Nb-Cu composites.The calculation showed that the dislocation density on the（111）_Cuand（200）_Cucrystal planes decreased more slowly comparing with the rapid decrease rate of dislocation density on the（110）_Nbcrystal plane.It is found that the magnetization properties depended on the microstructures.Hysteresis loops of composites were measured.Obvious change of characteristic peaks can be related with the microstructure change of Nb filaments in different composites.Therefore,the results declare a novel method to characterize Cu-Nb material structure under heavily deformation states.（3）The model of thermal deformation behavior of Cu-Nb composites was established.After the thermal compression tests,a special necking morphology was observed on composites.During the compression process under the heating temperature above 700℃,the nanofibers recrystallized rapidly,and the properties of Cu-Nb composites rapidly attenuated in the dynamic instabilty,and it was difficult to observe the rheological area.During the compression process below 700℃,the strain rate increased at first and reached the peak value,then rapidly dropped.The flow stress was tended to be stable,and the dynamic equilibrium was stable by the dislocation adjustment.Based on the modified Arrhenius-type hyperbolic sine constitutive model,the thermal compression constitutive relationship of Cu-Nb material was established,and the overlap between model calculation and experimental data was higher than 90%.The dynamic material model（DMM）was adopted,and the thermal processing maps of Cu-Nb composites with the strain of 0.2 and the strain of 0.6 were plotted.Based on these maps,materials with the strain of 0.6 prefers to a higher strain rate in the620℃-750℃region.However,lower strain rate was beneficial to a better deformability and good hot workability in the 650℃-730℃region with the strain of 0.2.（4）Graphene powders dopants were introduced in Cu-Nb composites by powder in tube method.Compared with Cu-Nb composite wires,the mechanical and electrical properties of Cu-Nb/G wires were both significantly improved,with the tensile strength at room temperature increased by about 10%,and the electrical conductivity by nearly15%.The growth,agglomeration,and cracks formation were effectively inhibited by graphene,and the refinement of grain was more obvious in graphene doping wires.The HT at 750℃/60h was beneficial for the enhancement of interface bonding strength as well as the plasticity.Conductive mechanism research shows that the conductivity of graphene-doped Cu-Nb composite wire was related to the size and shape distribution of graphene.更多还原