【作者】 刘嘉斌；

【导师】 孟亮；

【作者基本信息】 浙江大学 ， 材料加工工程， 2009， 博士

【摘要】 通过冷拉拔结合热处理制备的纳米纤维复合强化Cu-Ag导线具有优异的强度和良好的电导率。已有研究初步阐明了Ag含量、热处理工艺和变形量对合金性能的影响规律,分析讨论了合金中各种强化因素并提出相应强度计算模型。为进一步深入研究冷变形过程合金组织演变及合金强化机制,本研究通过真空熔炼铜模浇铸Cu-6%Ag合金,对合金固溶时效处理后进行多道次冷拉拔制备了具有纳米纤维组织的Cu-6%Ag线材。采用金相显微技术和电子显微技术系统地观察了各制备阶段的合金组织。采用电子万能试验机测试了不同变形量的Cu-6%Ag合金强度,结合显微组织观察结果分析合金强化机制,初步建立了应变过程显微组织演化与材料宏观性能之间的关系。Cu-6%Ag合金固溶时效后组织包含大量弥散分布在Cu基体的纳米Ag析出相。Ag析出相通常为短棒状,惯习面为{111},惯习生长方向为<110>。Ag析出相与Cu基体具有cube-on-cube位向关系,<011>_Ag//<011>_Cu并且{111}_Ag//{111}_Cu。Cu/Ag相界面平直,界面上存在周期排列的错配位错,错配位错的平均间距为9（111）_Cu。冷变形过程中Ag析出相与Cu基体逐渐转动到平行拉拔方向并在大变形量下演变成纳米纤维结构。纤维直径和纤维间距都随变形量的增大而减小。在变形初期,组织内位错密度随变形量的增大而增大并逐渐形成位错胞。当变形量增大到η=6.0左右位错胞结构失稳,部分胞壁转变为亚晶界,位错密度有所下降且分布不均匀。同时原先粗大的Cu晶粒分裂为数个取向各异的小晶粒。在大变形量下合金组织出现形变孪晶,形变孪晶数量在η=5.0左右到达最大值,之后随变形量的增大而减少。形变孪晶主要通过界面发射的肖克莱位错束集以螺旋切变方式形成,贯穿Cu、Ag两相并中止于Cu/Ag相界面。Cu/Ag相界面在强应变过程中逐渐偏离{111}而转变为接近平行于{42（?）},变形前清晰的平直状界面也逐渐变得模糊弯折,两相界面的匹配度降低。Cu/Ag界面上错配位错的间距随变形量的增大逐渐偏离理论值(9（111）_Cu)。当Ag纤维直径减小至2nm左右,大部分的Cu/Ag界面上没有错配位错存在,Cu/Ag界面由半共格界面转变为共格界面。界面类型的转变与界面能随纤维尺寸的变化有关,当纤维直径小于2nm时共格界面具有相对低的界面能,而当纤维直径大于2nm时半共格界面具有相对低的界面能。Cu-6%Ag合金强度随变形量的增大而增大。在η＜3.0时的变形范围内,加工硬化主要来自于位错胞对位错的阻碍作用。在3.0＜η＜7.0的变形范围内,加工硬化主要来自于Cu/Ag相界面和孪晶界对位错的阻碍作用,硬化效果显著;在η＞7.0的变形范围内,高度纳米化的纤维组织内仅包含单个位错,加工硬化效果较弱,导致强度趋于饱和。合金组织中丰富的界面是合金在大变形量下仍具有高的加工硬化效果的主要原因。Cu、Ag两相在应变过程中保持一种动态协调变形。两相的实际变形量在变形过程总体同步而局部不同步,交替领先。两相相同的晶体点阵类型以及一致的晶体取向和相近的应变硬化特征使得两相能保持总体协调变形;两相点阵参数的不同以及相尺寸的差异导致两相局部变形的不协调。基于离散位错塞积的位错自由程强化模型很好地解释了Cu-6%Ag合金在强应变过程中的强化行为,根据该模型预测的强度与其他已报导的Cu-Ag二元合金试验结果相符。初步推断通过强变形获得高强度的Cu-Ag二元合金的强化机制都可通过位错自由程强化模型得以解释。相比于晶粒尺寸、位错密度、界面等其他强化因素,位错自由程是控制合金强化的更根本的因素。在实际合金组织中,位错自由程可以是位错胞尺寸、纤维平均间距或者层厚。更多还原

【Abstract】 Filamentary strengthening Cu-Ag nanocomposites prepared by cold deformation and heat treatment possess high strength and excellent electrical conductivity.Many studies were focused on the effects of Ag content,heat treatment and deformation on the properties.The strengthening components in the nanocomposites were analyzed and some models were proposed to predict the strength.In order to further study the microstructure evolution and strengthening mechanism during drawing strain,Cu-6%Ag nanocomposites were prepared by melting,casting,solution treatment,aging and then cold drawing.The microstructure at various preparation processes was observed by optical microscopy and electron microscopy.The tensile strength of the specimens at various drawing strains was tested by an electric universal test machine.The change of strength was related to the microstructure evolution.The relationship between the microstructure and properties of Cu-6%Ag nanocomposites was established.Plenty of Ag nano-rods are homogenously distributed in the Cu matrix in the aged Cu-6%Ag.The Ag precipitates have a habit plane of {111} and a growth direction of <110>. There is a cube-on-cube orientation relationship,<011>_Ag//<011>_Cu and {111}_Ag//{111}_Cu, between the Ag precipitates and Cu matrix.The Cu/Ag interface is planar and has misfit dislocations with an average interval of 9（111）_Cu.Cu and Ag phases gradually rotate to the drawing direction and evolve into filamentary structure at heavy drawing strain.The diameter and interval of the filaments are reduced with the increase in the drawing strain.At initial deformation,the dislocation density increases with the drawing strain and dislocation cells form.When the drawing strain reaches 6.0,the dislocation cells lose stability and some cell walls transform into subgrain boundaries.The dislocation density decreases and the distribution of dislocation becomes inhomogeneous, while a coarse Cu grain divides into several small grains with different orientation. Deformation twins appear at heavy drawing strain and the amount of deformation twins reaches the maximum at aboutη=5.0.Deformation twins are produced by emission of Shockley partials from the interface and always penetrate several filaments then terminate at the Cu/Ag interface.The Cu/Ag interface fails to keep parallel with {111} but tends to parallel with {422} during cold drawing.The planar interface changes to zigzag morphology and the interface matching decreases.The misfit dislocation interval departs from the theory value as the draw ratio increases.Most of the interface contains no misfit dislocation once the Ag filament diameter is below 2 nm and the interface changes from semi-coherent interface into coherent interface.The relationship between interface energy and filament scale is responsible for the change of the interface.The coherent interface has lower interface energy when the Ag filament diameter is below 2 nm and the semi-coherent interface has lower interface energy when the Ag filament diameter is over 2 nm.The strength of Cu-6%Ag increases with the increase in the drawing strain.The work hardening atη＜3.0 benefits from the dislocation pinning by dislocation cell.The significant work hardening at 3.0＜η＜7.0 benefits from the dislocation pinning by Cu/Ag interface and twin boundary.The neglectable work hardening atη＞7.0 is due to the refinement of filamentary structure which only contains single dislocation.The plenty of interface is responsible for the notable work hardening even at high drawing strain.Cu and Ag phases keep a dynamic synchronization deformation during cold drawing.The real drawing strains of both phases keep the same as a whole but have different in local deformation.The same crystal type and orientation,and similar work hardening behavior lead to the co-deformation of both phases.The different lattice parameter and phase scale result in a mis-matching of deformation in local.The mean free path of dislocation strengthening model well explains the strengthening behavior of Cu-6%Ag during cold drawing.The predicted strength by the model is also consistent with the experimental results of other Cu-Ag alloys.Therefore,the strengthening mechanism in most of the Cu-Ag binary alloys could be explained by that model.Compared with the crystal size,dislocation density and interface,the mean free path of dislocation is the essential strengthening factor.The mean path of dislocation could be dislocation cell size, filamentary interval or layer thickness in practical microstructure.更多还原