择优取向纳米孪晶铜的塑性变形和断裂机理研究

IMR OpenIR

	择优取向纳米孪晶铜的塑性变形和断裂机理研究
	尤泽升
学位类型	博士
导师	卢柯 ; 卢磊
	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	材料学
关键词	纳米尺度孪晶变形机理加工硬化各向异性断裂机制温度效应 Nanoscale Twins Deformation Mechanism Work Hardening Anisotropy Fracture Mechanism Temperature Effect
摘要	" 纳米孪晶金属具有优异的力学性能，包括高强度、良好的塑性和加工硬化能力等，引起人们的广泛关注。目前大量实验和理论研究主要集中于孪晶片层厚度对纳米孪晶金属力学性能的影响，而关于孪晶界取向和晶粒尺寸等微观结构参数以及应变速率和温度等外部变形条件如何影响纳米孪晶金属的塑性变形和断裂过程的研究仍十分有限。本工作系统研究了直流电解沉积方法制备的具有择优取向纳米孪晶的柱状晶Cu的单向拉伸行为、不同方向压缩行为及其塑性变形和断裂机理，以及脉冲电解沉积方法制备的具有随机取向孪晶的等轴晶Cu的低温轧制变形行为。主要研究结果包括： 1、利用直流电解沉积技术在CuSO4溶液中制备出柱状晶纳米孪晶Cu样品。样品由沿沉积方向生长的柱状晶粒构成；晶粒内含有大致平行于沉积面的纳米尺度孪晶结构。通过改变电解沉积参数，可以实现晶粒尺寸和孪晶片层厚度的可控调节。 2、不同晶粒尺寸和孪晶片层厚度的柱状纳米孪晶Cu室温单向拉伸实验和微观结构表征表明，具有择优取向纳米孪晶的柱状晶Cu具有特殊的强化和塑性变形机制：柱状纳米孪晶Cu的屈服强度随孪晶片层厚度减小而增大，即孪晶界仍然起主要强化作用。但由于孪晶界平行于拉伸方向，贯穿位错在孪晶/基体片层内滑动主导塑性变形过程，其屈服强度与片层厚度的关系符合受限层滑模型。柱状纳米孪晶Cu的塑性变形具有明显的不均匀性。微观结构观察显示晶界附近区域承担更大塑性变形。晶界塑性变形导致其附近大量位错缠结以及位错亚结构或亚晶粒的形成。柱状纳米孪晶Cu的不均匀变形导致其塑性和加工硬化与晶粒尺寸密切相关。仅当晶粒尺寸超过3 mm时，晶内纳米孪晶加工硬化和晶界塑性变形共同作用使均匀拉伸塑性随晶粒尺寸增大而显著提高。 3、通过室温压缩实验研究相对于孪晶面的不同加载方向下纳米孪晶金属的强化和变形机理。研究结果表明：纳米孪晶Cu力学性能与加载方向密切相关。垂直和平行孪晶界压缩时，样品具有很高的屈服强度和有限的加工硬化能力；45°倾斜于孪晶界压缩时，样品在较低的强度下屈服但是屈服后表现出显著的加工硬化。考虑到滑移系与孪晶界的相对关系，孪晶或基体的12组滑移系可以分为三种类型：硬模式I，滑移面和滑移方向都倾斜于孪晶界；硬模式II，滑移面倾斜于孪晶界但滑移方向平行于孪晶界；软模式，滑移面和滑移方向都平行于孪晶界。Schmid因子和Taylor模型计算均表明，垂直加载和平行加载时主导的可动滑移系类型分别属于硬模式I和硬模式II，而45°加载时启动的主要滑移系属于软模式。不同位错机制主导不同方向的塑性变形。垂直孪晶界变形时，位错沿倾斜于孪晶界的滑移面向孪晶界运动，并塞集或穿过孪晶界，类似于传统的Hall-Petch强化机制；平行于孪晶界变形时，位错在相邻孪晶界的限制下平行于孪晶界运动；对于45°倾斜于孪晶界的变形，位错在孪晶界上滑移并导致孪晶界迁移。 4、通过室温和液氮温度下不同应变速率的拉伸实验，研究了柱状纳米孪晶Cu的拉伸塑性以及断裂过程随温度与应变速率的变化。结果表明：室温条件下，应变速率显著影响材料拉伸塑性。随应变速率提高，样品的均匀延伸率略微增大，但断裂延伸率和断裂真应变明显增大。低应变速率下样品发生沿晶断裂。随变形速率提高，晶间断裂逐渐被抑制。在液氮温度条件下，样品拉伸塑性受应变速率的影响很小。室温变形样品表面裂纹统计表明，低应变速率拉伸的裂纹形核率明显大于高应变速率拉伸。裂纹萌生于柱状晶界，并且在裂纹附近可以观察到晶界滑移迹象。晶界裂纹萌生可能是晶界滑移造成的不协调变形的直接结果。柱状纳米孪晶的室温晶界滑移可能与晶界位错运动有关。 5、通过液氮温度轧制和TEM微观结构观察研究等轴纳米孪晶Cu的低温塑性变形行为。研究结果表明，等轴纳米孪晶Cu室温轧制样品表现出明显加工硬化，而液氮温度轧制使显微硬度略微降低。TEM微观结构观察和统计显示液氮温度轧制导致部分孪晶片层变宽和孪晶界台阶形成。这是Shockley不全位错沿孪晶界运动的结果，说明纳米孪晶Cu的低温主导变形机制由室温全位错运动转变为Shockley不全位错运动。"
其他摘要	"Nanotwinned metals have attracted broad attention recently due to their high strength, enhanced ductility and work hardening ability. Numerous experimental and theoretical investigations have focused on the twin thickness dependence of the mechanical properties of nanotwinned metals. However, studies are rather limited concerning how microstructural parameters, such as twin-boundary orientation and grain size, and external loading conditions like strain rate and temperature, influence the plastic deformation and fracture process of nanotwinned metals. In this work, we investigated the uniaxial tensile behavior, mechanical anisotropy and tensile fracture mechanism of the columnar grained Cu with preferentially oriented nanoscale twins prepared by means of the direct-current electrodeposition. The cold rolling behavior at cryogenic temperatures of the equiaxed grained Cu with randomly oriented nanoscale twins prepared by pulse-electrodeposition was also studied. The main results are as follows: 1. Bulk nanotwinned Cu with preferentially oriented nanoscale twins was prepared by means of conventional direct-current electrodeposition. The nanotwinned Cu sample is composed of columnar grains along the deposition direction; inside the grains, there are high density nanoscale twins which are approximately parallel to the deposition plane. By adjusting the electroposition parameters, both grain size and twin thickness can be controllably changed. 2. The tensile behavior of columnar grained nanotwinned Cu with different grain sizes and twin thicknesses was investigated by uniaxial tensile tests at room temperature and systematic microstructural characterization. The results show a different strengthening mechanism and plastic deformation process in the columnar grained Cu with preferentially oriented nanoscale twins: (i) The yield strength of columnar grained nanotwinned Cu increases with decreasing twin thickness, revealing that twin boundaries (TBs) still play a key role in strengthening the metal. However, since the TBs are preferentially parallel to the tensile direction, the dominant plastic deformation mechanism has become threading dislocations gliding in the twin/matrix lamellae under the constraint of TBs. Therefore, the yield strength in this case depends on the average twin thickness, following the confined slip model. (ii) The plastic deformation in columnar-grained nanotwinned Cu is rather inhomogeneous. The SEM and TEM microstructural observations show that much larger plastic strain occurs in the grain boundary region than that in the grain interior. This concentrated grain boundary deformation induces the formation of high density dislocation tangles and dislocation substructure or subgrains near grain boundaries. (iii) The ductility and work hardening are intimately correlated with grain size due to the inhomogeneous deformation. Only when the grain size is increased up to 3 m, a rapid increase in the tensile ductility can be observed benefiting from the work hardening inside the nanoscale twins and suppressed grain boundary plastic deformation. 3. The influence of TB orientation on the strengthening and plastic deformation of nanotwinned Cu was investigated by compression tests under three different directions with respect to TBs at room temperature. The results are as follows: (i) The mechanical properties of nanotwinned Cu are closely related to the loading directions. The sample showed high yield strength and limited work hardening ability when compressed normal to or parallel to TBs, however, a relatively lower yield strength and a significant work hardening was observed under 45° compression. (ii) Considering the relative orientation of slip systems with respect to TB, the 12 slip systems can be classified into three categories: hard mode I, both slip plane and slip direction are inclined to TB; hard mode II, the slip plane are inclined to TB but the slip direction parallel to TB; soft mode, both slip plane and slip direction are parallel to TB. The Analysis of Schmid Factor and Taylor model calculation revealed that the dominant slip systems under 90°, 0°, 45° compression belong to hard mode I, hard mode II and soft mode, respectively. (iii) Different dislocation mechanism dominates the plastic deformation in different loading directions. For the case of 90° compression, dislocation slip in the inclined slip planes towards the TBs, and finally pile up or transfer the TBs, analogous to the conventional Hall-Petch model. When the sample is compressed parallel to TBs, threading dislocations glide inside the twin/matrix lamellae. In the case of 45° compression, dislocations move on the TBs leading to twin boundary migration. 4. The influences of strain rate and temperature on the tensile ductility and fracture of nanotwinned Cu were investigated through uniaxial tensile tests under different strain rates at room temperature and liquid nitrogen temperature. (i) The tensile ductility of nanotwinned Cu is strongly dependent on strain rates. With increasing strain rate, the uniform elongation increases slightly, but the elongation-to-failure and fracture true strain significantly elevate. At low strain rates, the sample fractured intergranularly. With increasing strain rate, the intergranular fracture was gradually suppressed. In contrary, the tensile ductility is insensitive to strain rate at liquid nitrogen temperature. (ii) The statistics of crack number and length on the sample surface tensioned at room temperature shows that the crack nucleation rate at low strain rates is obviously higher than at high strain rates. All cracks in wedge or penny shape are nucleated on the grain boundaries. Grain boundary sliding was observed near the cracks, revealing that the nucleation of grain boundary crack may be direct consequence of incompatible deformation caused by grain boundary sliding. The grain boundary sliding of columnar grained nanotwinned Cu at room temperature is correlated to the motion of grain boundary dislocations 5. The plastic deformation of equiaxed grained nanotwinned Cu at liquid nitrogen temperature was investigated by cold rolling at liquid nitrogen temperature and TEM microstructural characterization. The results show that the nanotwinned Cu exhibits obvious work hardening after rolling at room temperature while slight work softening upon rolling at liquid nitrogen temperature. TEM microstructural observation and statistics verified that liquid-nitrogen temperature rolling induced thickening of some twin lamellae and formation of ledges at twin boundaries. This could be consequence of Shockley partial dislocations moving along TBs, suggesting a transition of the dominant plastic deformation mechanism of nanotwinned Cu from full dislocation at room temperature to Shockley partial dislocations at liquid nitrogen temperature."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64474
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	尤泽升. 择优取向纳米孪晶铜的塑性变形和断裂机理研究[D]. 北京. 中国科学院金属研究所,2012.