【作者】 姚大伟；

【导师】 孟亮；

【作者基本信息】 浙江大学 ， 材料加工工程， 2013， 博士

【摘要】 通过冷拉拔应变制备了原位纤维相强化的Cu-6%Ag合金,研究了不同应变条件下合金的显微组织和电阻率变化规律,讨论了应变对Cu-6%Ag合金导电性能的影响机制。随应变程度的增加,原始组织中的Cu基体晶粒、不平衡共晶体及次生相粒子最终演变成细密的纤维结构,合金电阻率上升。次生相界面、共晶体与Cu基体界面及位错对电子散射作用程度的变化导致了合金电阻率在不同变形程度范围内有不同的变化规律。当变形超过一定程度后,电阻率升高规律与来自较高Ag含量合金中纤维相尺度进入纳米数量级的界面散射模型相符。考察了Cu-12%Ag合金中Ag析出相对合金性能的影响。通过引入两种不同的热处理制度在Cu-12%Ag中获得不同含量的Ag析出相。通过多种手段系统地观察了Ag析出相的微观结构。Ag析出相与Cu基体的相界面能有效地阻碍位错运动并增强电子散射,因此含有较多Ag析出相的Cu-Ag合金可达到更高的强度和更大的电阻率。通过铸造、预处理及冷拉拔等工艺制备了原位纤维复合Cu-12%Fe合金。观察了显微组织并测量了硬度。试验结果发现等轴铸态Cu及Fe晶粒在拉拔过程中均能演变成密集排列的纤维状.纤维组织轴向的择优取向与径向的择优取向不同。Fe纤维组织尺寸随拉拔应变程度增加呈指数形式下降。随拉拔应变程度增加到6.0,Fe晶粒应变程度呈线性增加,而当拉拔应变程度增加超过6.0后,则Fe晶粒应变程度的增加偏离线性关系。随拉拔过程中Fe纤维的长径比值增加,Cu/Fe相界面密度也呈指数增加。在硬度与纤维间距之间存在一种Hall-Petch关系。因拉拔应变造成的组织细化能够明显地提高硬度,尤其当拉拔应变程度较低时,这种组织细化提高硬度的作用更明显。采用熔铸法结合冷拉拔制备了Cu-6%Ag-6%Fe合金。采用SEM和TEM观察了不同拉拔变形量的合金微观组织以研究合金各相的结构演变。合金铸态组织由Cu基体、初生Fe粒子及共晶体区域等组织组成物组成。冷变形初期,共晶组织主要形态是均匀的直线状纤维束,而初生Fe枝晶演变为条带状纤维,在大变形条件下,两相均演变成细密的纤维丝形态。说明b.c.c.及f.c.c.两种晶体结构的合金相在拉拔应变的复合纤维组织中能够保持相同的应变行为或者两种相在由等轴组织变形流变成纤维组织的过程中应变过程能够保持基本同步。随着合金变形度的升高,位错缠结形成胞状结构,但在应变程度达到6.4以后,位错胞结构失稳逐渐转化为亚晶界,同时形变孪晶数量逐渐增加。合金中Ag相分为共晶体中的层叠状Ag相和Cu基体中的弥散次生Ag相。次生Ag相与Cu基体具有cube-on-cube位相关系,在变形过程中始终与基体保持协调一致,沿形变方向均匀变形,在应变程度达到8.6时,Cu/Ag界面部分转化为共格界面。变形过程中,有少量的Fe颗粒始终保持不变形,这些未变形的Fe颗粒容易导致位错塞积和裂纹萌生处。在界面应力推动下,部分Fe原子固溶进入Cu基体,随着变形度增加,数量增多。在三相纤维原位复合强化的Cu-6%Ag-6%Fe合金中,Cu基体纤维承担主要的电传导功能,Ag与Fe组元均危害电导率,尤其是Fe纤维对电导率的危害要高于Ag纤维。根据并联电路模型推导出了一个能够根据各组织组成物相对量及电导率预测这种原位复合纤维强化合金电导率的表达式。更多还原

【Abstract】 Cu-6wt.%Ag in situ filamentary composite was prepared by cold drawing. The filamentary microstructure, electrical resistivity and strain degree were investigated. With increasing the draw ratio, the equiaxed Cu-rich dendrites, eutectic colonies and Ag precipitates in the as-cast and homogenized structure developed into the fine filamentary structure and the electrical resistivity increased. There is a change of the electronic scattering effect of the dislocation, the phase interface between Ag precipitates and Cu grains and the interface between eutectic filamentary bundles and Cu matrix in the alloy under different strain condition. The change of the electrical resistivity with strain when the filamentary structure evolves into nanoscale at heavy draw ratios is generally in accord with the interfacial scattering model proposed in the presented investigation on the composites with higher Ag contents.The Cu-12wt.%Ag were prepared to investigate the role of Ag precipitates on the properties of the alloy. Two processes of heat treatment were performed to produce different amount of Ag precipitates in the alloy. The microstructure of Ag precipitates was systematically observed by optical microscopy, scanning electron microscope and transmission electron microscope. The interface between Cu matrix and Ag precipitates could significantly block dislocation movement and produce electron scattering. Therefore, the alloys containing more Ag precipitates would exhibit higher strength and electrical resistivity.Cu-12wt.%Fe in situ filamentary composite was prepared by casting and cold drawing. The microstructure was observed and the hardness was determined. The results show that the equiaxed Cu and Fe grains can develop into the crowded filamentary structure. There is a preferred orientation on the longitudinal direction different from that on the radial direction of the wire specimens. The reduction of fiber scale shows an exponential relationship with the drawing ratio. With the drwing ratio increasing up to6.0, the strain degree of Fe grains increases linearly. However, the increase in strain degree of Fe grains with the the drwing ratio deviates from the linear relationship once the drawing ratio is over6.0. With the increase in the ratio of the longness to the diameter of the filaments, the density of Cu/Fe interface increases exponentially. There is a Hall-Patch relation between hardness and filamentary space. The microstructure refinement from drawing strain can decrease hardness obviously. In special, the hardness reduction from the microstructure refinement is more significant.For the investigation on the microstructure evolution of Fe and Ag phases, the Cu-6wt.%Ag-6wt.%Fe alloys were prepared by casting and heavy cold drawing. The microstructure of the alloys at different drawing strains was studied by scanning electron microscopy and transmission electron microscopy. The microstructure of as-cast Cu-6wt.%Ag-6wt.%Fe is made up of Cu matrix, Fe dendrites and eutectic colonies. The microstructure components develop into filamentary bundles during cold drawing. The Ag-rich colonies show uniform line-like distribution while Fe dendrites show ribbon-like distribution at the heavy deformation state. Fe phase with b.c.c. structure in the composite can keep the similar strain behavior as Cu and Ag phases with f.c.c. structure during drawing deformation.Dislocations form the dislocation cell structure in Cu-6wt.%Ag-6wt.%Fe filamentary composites during cold deformation. After drawing ratio up to6.4dislocation cells lose stability and transform into subgrain boundaries. Meantime deformation twins in deformed Cu grains increase in number. Ag fibers result from eutectic colonies and precipitates. There is a cube-on-cube orientation relationship,{111}cu//{111} Ag and<110>cu//<111>Ag between Cu matrix and Ag precipitates, which ensures the co-defoemation and similar interface behavior. As drawn ratio increasing to8.6, Cu/Ag interface evolves into coherent interface partly. There still are some Fe particles among Cu filaments, which results in dislocation multiplication and crack generation. More Fe solute in Cu matrix can be found as strain increasing.Both Ag and Fe constituents can impair the conductivity in the composite. Fe filaments suppress more the electrical conduction in the microcomposite than Ag filaments. A formulation deduced from a parallel-circuit model for the resistivity of a microcomposite has been given for the evaluation of the resistivity.更多还原