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探索非晶合金塑性流动的结构起源
Alternative TitleExploring the structural origin of plastic flow in amorphous alloys
孙星
Thesis Advisor蒋敏强
2019-05-26
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline固体力学
Keyword非晶合金 塑性流动 剪切转变 自由体积 弛豫动力学 本构模型 结构非均匀
Abstract

    非晶合金短程有序、长程无序的拓扑结构极大地挑战了基于位错、孪晶等的传统塑性机制。一般认为,非晶合金塑性流动的基本过程是局部原子以集团模式的“剪切转变”运动。但是,这种熵焓耦合驱动的热激活事件是否具有明确的结构起源仍然存在很大争议。本论文针对这一争议,通过开展系统的同步辐射X射线衍射和小角散射实验、动态力学分析以及连续介质尺度理论分析,探究了非晶合金无序结构与其塑性流动的时空关联。本论文的主要研究内容以及取得研究成果如下:
    通过开展同步辐射X射线小角散射实验,得到了一种典型非晶合金在弹性阶段的纳米尺度结构非均匀图像。基于小角散射强度曲线的Porod分析,揭示出非晶合金属于非理想两相散射体系,两相界面弥散且任一相内都存在电子密度涨落。基于散射曲线的Guinier定律分析,进一步发现非晶合金中散射体形状远偏离球形,其特征尺度主要分布在0.8-1.6 nm之间,且在弹性变形阶段几乎不变。最后,通过Debye相关函数分析,发现这些纳米尺度散射体仅在1 nm之内存在强关联,符合非晶合金短程有序、长程无序的结构特征。
    通过开展同步辐射X射线衍射和小角散射实验,研究了非晶合金在弹塑性变形过程中从原子尺度到纳米尺度的结构演化信息。以局部体胀信号为线索,揭示了非晶塑性流动的基本原子图像。基于X射线衍射分析,发现塑性屈服以及软化流动过程中伴随着固有的局部体胀信号。这种局部体胀由亚纳米尺度的第二近邻右劈裂峰原子承担,介导了从第二近邻到纳米尺度第四近邻的原子团簇重排。X射线小角散射分析,进一步从纳米尺度结构非均匀角度证实了上述非晶塑性伴随的局部体胀行为。
    基于非晶合金的动态力学分析,以弛豫动力学非均匀为桥梁,澄清了非晶合金塑性变形的结构起源。不同变形阶段的等频温度谱表明,变形加剧了局域原子的平移运动和长程扩散运动,从而使非晶合金易于在较低温度发生β弛豫和α弛豫。不同变形阶段的等温频率谱表明,β弛豫与非晶塑性基本“剪切转变”事件具有相同的激活能,从而能够有效指示非晶合金的屈服、软化和稳态流动行为,意味着两者具有相同的结构起源。
    将基于剪切转变和自由体积(局部体胀)相互作用的非晶塑性本构模型进一步推广到三维应力状态,并考虑了静水压力、自由体积扩散和温度效应。通过有限元实现,该推广模型能够正确预测应力过冲和应变软化现象,重现非晶塑性流动的典型行为。基于该模型还研究了初始结构非均匀、固有体胀效应、加载应变率、环境温度等因素对非晶塑性流动行为的影响,进一步证实了非晶塑性的基本载体是剪切转变运动,而自由体积通过影响剪切转变主控塑性屈服后的流动行为。
 

Other Abstract

    Amorphous alloys display topological structures with short-range order and long-range disorder. This poses a great challenge to the traditional mechanism of crystalline plasticity based on dislocations, twining, etc. It has been generally accepted that the unit process of amorphous plasticity is the “shear transformation (ST)” of local atomic groups. However, whether this enthalpy-entropy driven, thermally activated event has a clear structural origin or not still remains much controversies. The present thesis aims at this controversy, and explores the spatiotemporal correlation between amorphous structures and resulting plastic flow, by combining synchrotron radiation X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS), dynamic thermomechanical analysis (DMA) with continuum mechanics modeling. The main results we obtained are as follows.
    Using the synchrotron radiation SAXS experiments, the structural heterogeneity of the Vitreloy 1 amorphous alloy in the elastic deformation stage is obtained. The results indicate that the scattering intensity curve of the Vitreloy 1 amorphous alloy exhibits the positive deviation of Porod law. According to the Porod's law, it is revealed that the diffuse interface exists between the two phases, associated with the density fluctuations in either of phases. Furthermore, we demonstrate that the shape of scatterer is far from a sphere and their characteristic sizes measured by the radius of gyration are mainly distributed between 0.8 nm and 1.6 nm. The radius of gyration is almost not changed in the elastic deformation stage of the amorphous alloy. Finally, based on the correlation function defined by Debye, we analyze the correlation of electron density fluctuation between two arbitrary scatterers. The result indicates that the nanoscale scatterers in the amorphous alloy are strongly correlated only within a range of about 1 nm, which is consistent with the short-range ordered and long-range disordered structural features of the amorphous alloy. 
    Using the synchrotron radiation XRD and SAXS experiments, the atomic-scale-to-nanoscale structural evolution of the amorphous alloy is studied in its elastoplastic deformation stage. The basic image of amorphous plastic flow is revealed by the clue of local dilatation. Based on XRD analysis, it is found that there are inherent local dilatation in the plastic yielding and softening stage. Mediated by the local dilatation, local atomic rearrangements can operate at the left-subpeak of the second shell and/or at the fourth shell with a characteristic lengthscale of about 1 nm, pointing to the nanoscale ST operation. The obtained dilatancy signatures of amorphous plasticity are further confirmed by SAXS in terms of the nano-scale structural heterogeneity.
    With the dynamic thermal analysis, the relaxation kinetics serves as a bridge to link the structural signature and the plastic deformation of amorphous alloys. The temperature dependence of dynamic modulus at different deformation stages show that the deformation increases the local translational motion at low temperatures and the large-scale diffusion motion at high temperatures, which makes it easier to relaxation. Therefore, both  β relaxation and α relaxation occur at lower temperatures. It is found from the frequency dependence of dynamic modulus that, the β relaxation has the same activation energy as that of basic ST events, which can effectively indicate the yielding, softening and steady-state flow behavior of amorphous alloys, implying that they have the same structural origin.
    The amorphous plastic constitutive model based on shear transformation and free volume (local dilatation) interaction is extended to the three-dimensional stress state, further taking hydrostatic pressure, free volume diffusion and temperature effects into account. The extended constitutive model is realized by finite element simulation, which can correctly predict stress overshoot and strain softening, and reproduce the typical behavior of amorphous plastic flow. Based on the model, the effects of initial structural heterogeneity, inherent local dialation effect, loading strain rate and ambient temperature on the amorphous plastic flow behavior is also studied. These results confirm that the basic carrier of amorphous plasticity is STs, whereas the free volume dominates the mechanical behavior after plastic yielding. 
 

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/79096
Collection非线性力学国家重点实验室
Recommended Citation
GB/T 7714
孙星. 探索非晶合金塑性流动的结构起源[D]. 北京. 中国科学院大学,2019.
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