| 面心立方晶体循环形变行为研究 | |
| Alternative Title | Investigation on the cyclic deformation behaviors of face-centered cubic crystals |
| 李鹏 | |
| Subtype | 博士 |
| Thesis Advisor | 张哲峰 |
| 2009-09-21 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料物理与化学 |
| Keyword | 面心立方晶体 铜单晶体 银单晶体 Css曲线 位错组态 驻留滑移带 形变带 层错能 取向 层错偶极子 扩展位错 |
| Abstract | 众所周知，fcc晶体循环形变行为的广泛研究起始于1956年Thompson对铜晶体中滑移带“驻留”（PSB）现象的发现。在上世纪50年代到70年代，大量研究工作集中于揭示驻留滑移带的力学及结构特性，包括硬度、位错分布、体积分数、两相模型等等。基于这些早期的研究工作，Mughrabi于1978年建立了著名的单滑移取向铜单晶体的循环应力-应变（CSS）曲线，这是第一次定量化地将fcc晶体的宏观形变行为与微观位错结构相联系。进入80年代，Bretschneider和Holste领导的研究小组从温度和取向这两个角度入手完整地阐述了镍单晶体循环形变行为的主要特征。与此同时，各种其他fcc晶体，包括铝、铜铝合金、铜锌合金单晶体疲劳行为的研究也逐渐展开。综合分析前人的大量研究成果，我们发现影响fcc晶体循环形变行为的主要因素可以列举如下：温度、频率、取向和层错能。其中，取向和层错能的影响尤为重要。因此，在系统地分析和比较不同fcc晶体循环形变行为的基础上，提出了关于各种规则位错组态的形成机理以及fcc晶体循环形变行为的物理本质。 温度和频率可以看作是影响fcc单晶体循环形变行为的外在因素。对于铜单晶体而言，随加载温度的升高，其CSS曲线的平台行为逐渐消失，饱和分切应力值在低应变幅下明显降低。与此同时，铜单晶体中PSB的梯间距逐渐增大，而这进一步引起了梯棱在PSB中所占体积分数的减小，体积分数的改变最终导致了饱和应力值的降低。随着加载频率的升高，平台行为始终存在，饱和应力值略有增加。这是由于较高的频率使得刃型位错相互作用的几率增大，进而使得位错交互作用时的阻力增加。而这种阻力的增加会进一步导致梯棱中局部流变应力增大，并最终引起了饱和应力值的增加。 不同取向银单晶体的CSS曲线在γpl=8.0×10-5-6.0×10-3范围内表现出不同程度的平台行为，而平台饱和应力根据取向的不同分为18~21MPa和25~26MPa两个取值段。随后，不同取向银单晶体的饱和位错组态表现出如下特征：1) 单滑移取向银单晶体(包括[-239]、[-459]以及[-1 8 18])以PSB和脉络两相结构为主；2)[011]银单晶体中以位错墙结构组成的发达形变带为典型特征；3)[-1 4 14]银单晶体主要由两组交互作用的PSB梯结构组成；4)[-233]取向银单晶体更多的表现为位错胞结构。同时，需要指出的是[-459]取向与共轭双滑移取向相近，[-1 8 18]和[-1 4 14]则与临界双滑移取向相仿，而在高应应变幅下，[-1 8 18]中有迷宫结构形成，在低应变幅下，[-233]中有脉络结构出现。 取向对铜、镍及银单晶体循环形变行为的影响遵循着某种共同的规律。根据不同取向三种晶体中位错组态的分布特征，可以将取向三角形化分为三个区域，其中包括：1) 以单滑移取向和[011]多滑移取向的梯墙结构为代表的“中心区”；2) 以迷宫结构为代表“[001]区”； 3) 以胞结构为代表的“[-111]区”。基于三种典型多滑移取向的研究结果，可以发现复杂位错组态的形成取决于优先开动的次滑移系统，临界次滑移系的开动决定了迷宫结构的形成；共面次滑移系的开动决定了胞结构的形成；而如果没有次滑移系及时开动，更高的塑性形变将通过位错墙构成的DBII所承担。 铝单晶体的循环饱和应力值远远低于铜、镍等单晶体的平台饱和应力，其位错结构以位错团簇构成的胞结构为主，这些胞结构的中央相对来说位错密度较低，位错主要集中在胞壁处。铜铝合金的CSS曲线则随着铝含量的增加平台区有所缩短，对应的位错组态也由典型的PSB结构向PLB结构转变。对Cu-16at%Al合金中位错结构的研究表明，PLB主要以偶极子列或层错列为主体，再通过与其他次级位错交互作用最终形成。 不同fcc单晶体的循环形变行为，特别是位错组态随着层错能的变化也表现出明显的规律性。结合前人的理论模型以及自身对位错组态形成的理解，我们提出了决定位错组态演化的两个基本参量：G/γsf和τs/G，并且发现它们各自表征了单元位错结构中的扩展位错宽度和偶极子捕获距离。在位错组态形成过程中，其宏观演化过程为：偶极子段→脉络→PSB梯；相应的微观位错组态演化过程为：主刃型偶极子→主刃型偶极子+层错偶极子→层错偶极子。在此过程中，比值dextend/dtrap起到了决定性的作用，将这一比值带入到演化过程中，得到了fcc晶体中规则PSB梯状结构形成的基本判别公式。 以上述判据为出发点，分别验证了由此得出的三点推论，包括金单晶体中PSB梯状结构的形成；银单晶体中层错列与位错墙的交互作用以及铜锌合金中层错列与偶极子列的内在关联。综合考虑上述实验结果，以派-纳力和孪晶驱动力为判据，通过扩展位错宽度的变化系统揭示了关于fcc晶体两种基本变形方式—滑移与孪生之间竞争关系的物理本质。 |
| Other Abstract | It is well known that cyclic deformation behaviors of fcc single crystals have been widely studied since “persistent slip band” (PSBs) was defined as the common presence in fatigued crystals by Thompson et al. in 1956. In 1950’s – 1970’s, a large number of researchers (Broom and Ham (1959), Helgeland (1965), Laufer and Roberts (1966), Woods (1973), Winter (1974), Finney and Laird (1975)) focused on various PSBs’ mechanical and structural characteristics, including hardness, distribution, volume fraction, two-phase model and so on. Based on these early works, Mughrabi (1978) established the famous cyclic stress-strain (CSS) curve of single-slip-oriented Cu single crystal, which is the first quantitative determination of the relationship between the macro-scale deformation behaviors and micro-scale dislocation structures. Since 1980’s，Prof. Bretschneider and Prof. Holste have devoted to the cyclic deformation behaviors of Ni single crystals based on the temperature and orientation effects all along. During the same period, various other fcc single crystals, including Al, Cu-αAl, Cu-αZn and so on, were paid much more attentions on the fatigue behaviors. Combined with plenty of previous research results, we found that the influencing factors of cyclic deformation behaviors can be listed as follows: temperature, frequency, orientations and stacking fault energy (SFE). Among them, the orientation and SFE are the core factors. Then we proposed a new criterion to judge the cyclic deformation behaviors of different fcc crystals and explained the physical nature of formation mechanism of PSBs in various fcc crystals according to the criterion. Temperature and frequency can be regarded as the extrinsic factors on cyclic deformation behaviors of fcc single crystals. As testing temperature increases, the plateau of CSS curves gradually disappears and the saturation resolved shear stress decreases distinctly at low strain amplitudes. At the same time, the space between PSB ladders in Cu single crystals is gradually enlarged, which causes the decrease in the volume fraction of the rungs in PSBs and thus leads to a lower of the saturation stress. On the other hand, the increase of frequency does not affect the appearance of the plateau behavior, but the corresponding plateau stress slightly increases. This is because a higher frequency increases the probability of interaction between edge dislocations, which further leads to the increase in the local flow stress in the rungs, and eventually causes the increment of saturation stress. The CSS curves of silver single crystals of different orientations show a clear plateau region over a strain range of γpl=8.0×10-5-6.0×10-3 with two saturation shear stresses of about 18~21MPa or 25-26MPa, respectively. After that, the dislocation configurations from differently oriented silver single crystals are well summarized as follows: 1) PSB ladders or walls appear in [-239],[011]and [-459]silver single crystals; 2) labyrinth is the main dislocation structure in the [-1 8 18]silver single crystal at high strain amplitudes; 3) vein and cell structures form in the [-233] silver single crystal at low and high strain amplitudes, respectively; 4) the interaction between the primary and secondary PSBs arises in the [-1 4 14] silver single crystal. Combined with the results of copper, nickel and silver single crystals, it can be concluded that the effects of the orientations on the cyclic deformation behaviors follow a general principle: the orientation-dependent dislocation configurations can be divided into three regions in the stereographic triangle, including the central region,[011] and [-111] regions. Based on further research about the three multiple-slip orientations, it can be found that the formation of more complicated labyrinth, cell or wall structures depends on which slip system is priority to actuate. And the choice of slip system shows that the physical nature of the orientation effect is strongly associated with fcc crystal structure. The saturation stress of cyclic saturated Al single crystal is far lower than that of Cu single crystal. In fatigued Al single crystal, the cell structure is the most classical dislocation configuration. These cells are mainly composed of loose clusters of dislocations, so dislocations in the cells move much freely. The CSS curves of Cu-Al alloys have a shorter plateau with Al content increase. Meanwhile, the dislocation configurations change from PSB ladders in Cu-Al crystal with low Al content (≤5at%Al) to persistent Lüder’s bands (PLBs) in Cu-Al crystal with medium, even high Al content (≥8at%Al). Studies show that PLBs mainly consist of the dipole array or stacking faults (SFs) by dislocation reactions and interactions with other secondary dislocations. The cyclic deformation behaviors, especially dislocation configurations from different fcc single crystals show obvious regularity with the change of SFE. Combined our understanding on the formation of dislocation configurations with previous theoretical models, it is summarized that characterizing the width of extended dislocation and the trap distance of dipole, respectively, G/γsf and τs/G are regarded as two important parameters to influence the formation of PSB ladders or not. In view of the formation of PSB ladders, the evolution processes of the macroscopic dislocation configurations can be shown as below: dipole segment → vein → PSB ladder; the corresponding microscopic dislocation configurations go through the course: primary edge dipole → primary edge dipole + faulted dipole → faulted dipole. Further researches show that in this process the ratio dextend/dtrap plays a vital role. Based on this criterion, the physical fundamental on the cyclic deformation behaviors of fcc crystals is established. According to the width of extended dislocation, the relationships between slip and twinning, dislocation and stacking fault are shown. With the increment of the width, the slip mode of fcc crystals experiences the wavy-to-planar-slip transition and the formation of the SFs becomes easier, which leads to the transition of deformation mode from slip to twinning. |
| Pages | 260 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17136 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 李鹏. 面心立方晶体循环形变行为研究[D]. 金属研究所. 中国科学院金属研究所,2009. |
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