严重塑性变形技术制备双相铜银合金的组织和力学性能研究

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	严重塑性变形技术制备双相铜银合金的组织和力学性能研究
	田艳中
学位类型	博士
导师	张哲峰
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	铜银合金高压扭转等通道转角挤压强度硬度 Cu-ag Alloys High-pressure Torsion (Hpt) Equal-channel Angular Pressing (Ecap) Strength Hardness
摘要	"本论文以双相Cu-Ag合金作为研究对象，采用等通道转角挤压(equal-channel angular pressing: ECAP)和高压扭转(high-pressure torsion: HPT)两种严重塑性变形(severe plastic deformation: SPD)技术，分别研究了Cu-Ag合金在ECAP和HPT过程中的剪切变形机制、晶粒细化机制，ECAP路径对其组织演变与力学性能的影响及Cu-Ag合金的强化机制等问题。本文试图通过对上述实验结果的分析与讨论，为制备高强度双相复合材料提供设计思路。通过观察具有标识性的Cu-16wt.%Ag合金共晶组织的剪切变形特征，发现在ECAP过程中，剪切变形在ECAP模具的扇形区发生。通过统计扇形区和变形完成区的剪切带的角度，发现这些剪切带的角度主要分布在两个区间。根据双剪切变形条件进行计算，发现计算出的剪切、角度结果与实验结果基本一致，因此，进一步证实了ECAP变形过程中存在着双剪切变形，即除了沿对角面发生剪切之外，沿垂直于对角面方向也发生了剪切变形。 ECAP路径（A、BA、C、BC）对双相Cu-Ag合金的微观组织和力学性能具有明显的影响。电子背散射衍射(electron backscattering diffraction技术结果表明采用A路径可以最有效地细化组织和强化材料。对于A路径来说，经过四道次后的样品晶粒尺寸最细小，大角晶界含量最高。组织的强烈细化可以归结为位错主导的晶粒分割过程，原始晶界面积的增加和剪切带的大量发展。对于BC路径来说，即使经过四道次变形之后，获得的组织依然很不均匀，同时，材料的强度也相对较低。因此，对于双相Cu-Ag合金复合材料来说，采用A路径可以获得相对细小均匀的组织结构和较高的强度。 :EBSD) 微观组织观察结果表明：双相Cu-28wt.%Ag合金在HPT变形过程中的组织演变过程相对缓慢，并且组织会不断细化至纳米级。在变形量较小时，共晶组织内部两相由于剪切变形呈现出纤维状，而在Cu基体内部出现带状亚结构。随应变量增加，亚结构的宽度不断减小，且共晶组织间距也不断减小，同时共晶组织内部微观结构会比较快地达到饱和状态。当变形量极大时，共晶组织间距会小到一个极限，即Cu基体内部的亚结构消失。此时，进一步塑性变形会通过发展大量剪切带来实现。该合金的饱和硬度对应的应变量为~330，其持续硬化能力是由于微观组织不断细化到纳米级尺度造成的。为了揭示Cu-Ag合金体系的强化机制和强化能力，从而为设计高强度Cu-Ag合金提供依据，通过对比共晶Cu-71.9wt.%Ag合金与亚共晶Cu-28wt.%Ag合金在HPT变形后的组织和力学性能，对不同成分Cu-Ag合金的强化能力进行了总结，认为Cu基体内部的Ag沉淀物在强化Cu-Ag合金的过程中发挥了重要的作用。当Cu-Ag合金的Ag含量在10-20wt.%时，同时，在其Cu基体上分布着细小连续的Ag沉淀物时，可以获得最高的强度。 HPT变形可以剧烈地细化Cu-Ag合金，同时也可以改变其拉伸断裂方式。随着HPT转数的增加，Cu-Ag合金的断裂方式发生了由颈缩到剪切断裂的转变。当转数比较低时，剪切与颈缩同时出现。随应变量增加，材料最终发生完全剪切断裂，并且剪切断裂角大于。同时，在样品的拉伸断口靠近表面的地方可以观察到剪切台阶，并且剪切台阶的大小随HPT转数的增加而不断减小，表明材料的抗剪切变形能力在逐步变弱。这可以归结为在HPT变形过程中，材料内部的缺陷密度大量增加的原因。采用HPT技术制备出了直径为30 mm的大块、高强度且组织比较均匀的Cu-Ag-Zr合金。拉伸、压缩实验结果表明：该合金存在一定程度的拉压不对称性。硬度结果表明：随着应变量的增加该合金硬度会先快速增加，之后达到饱和，并且在应变量极大时稍微下降，这是因为在变形量比较小时组织发生细化，而在变形量极大时，已经细化了的组织又发生了晶粒长大的原因。"
其他摘要	"In order to get better understandings on the shear deformation and grain division mechanisms during equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), the effects of ECAP route on the microstructural evolution, mechanical properties and the strengthening mechanisms of Cu-Ag alloys, investigations are conducted by using ECAP and HPT techniques to process two-phase Cu-Ag alloys. Based on these experimental analysis and discussions, it is aimed to give some instructions on processing high-strength two-phase composites. It is found that shear deformation occurs in the fan region of the ECAP die by observing the deformation characteristics of the decorative two-phase structure of a Cu-16wt.%Ag alloy. Two kinds of shear bands are found in both the fan region and the exit region of the ECAP die. Assuming that shear deformation occurs along the intersection of the ECAP die and perpendicular to the intersection plane, the theoretical shear angle ranges are predicted, and they match well with the experimental results, indicating that two shear deformation modes exist during ECAP. ECAP routes (A, BA, C, BC) play a key role in the microstructures and mechanical properties of a Cu-8wt.%Ag alloy. Electron backscattering diffraction (EBSD) results indicate that route A is most effective in refining and strengthening the material. After four passes, route A produces smallest grain size and largest amount of high angle boundaries among the four routes, which are resulted from the dislocation-mediated grain division process, the increase of the original grain boundaries and the shear banding process. When processing by route BC for four passes, the material is still inhomogeneous and its strength is very low. It is found that the microstructure of the Cu-28wt.%Ag alloy can be refined to be nanoscale during HPT. When the shear strain is small, the eutectic becomes fibrous, and banded substructures form in the Cu matrix. With increasing the shear strain, the band width and the eutectic spacing decrease, and the eutectic saturates quickly. When the shear strain is large, the substructures can disappear in the Cu matrix. Further shear deformation is accommodated by shear banding. The equivalent strain for the saturation hardness is ~330, indicating a strong strain hardening capability, which is related to the continuous refinement of the microstructure. It is aimed to reveal the strengthening mechanism and strengthening capability of Cu-Ag system, thus fabricating high-strength Cu-Ag alloys, microstructures and mechanical properties of the eutectic Cu-71.9wt.%Ag and the Cu-28wt.%Ag alloy are compared after HPT, and the strengthening capabilities of various Cu-Ag alloys are summarized. It is suggested that Ag precipitates in the Cu matrix play an important role in strengthening Cu-Ag alloys. When the Ag content falls in 10-20wt.%, and the Cu matrix contains fine continuous Ag precipitates, Cu-Ag alloy with the highest strength can be obtained. Cu-Ag alloys can be significantly refined by HPT, and the tensile fracture modes of the materials processed by HPT also change. It is found that the tensile fracture modes of the Cu-Ag alloys change with increasing the numbers of HPT revolutions. Shearing and necking occur at small HPT number. With increasing the numbers of revolutions, the materials fracture with pure shearing, and the shear angle is larger than. Some shear offsets can be found on the fracture surface, and they decrease with increasing the HPT revolutions, indicating a deterioration of the shearing resistance. This can be attributed to the highly increased defect density. Bulk, high-strength and homogenous Cu-Ag-Zr alloy with a diameter of 30 mm was fabricated by HPT. Tension-compression asymmetry was observed. With increasing the shear strain, it is found that the microhardness increases sharply initially due to the grain refinement, and then it becomes saturated; when the shear strain is extremely large, the microhardness decreases slightly, which can be attributed to the grain growth."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64490
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	田艳中. 严重塑性变形技术制备双相铜银合金的组织和力学性能研究[D]. 北京. 中国科学院金属研究所,2012.