金属玻璃的剪切变形与断裂行为研究

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	金属玻璃的剪切变形与断裂行为研究
	屈瑞涛
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	金属玻璃拉伸压缩剪切带断裂强度塑性韧性缺口 Metallic Glass Tension Compression Shear Band Fracture Strength Plasticity Toughness Notch
摘要	" 尽管金属玻璃具有极高的强度，然而其灾难性的脆性断裂行为一直是金属玻璃作为结构材料应用的主要障碍。本文主要以几种金属玻璃材料为研究对象，通过设计开展一系列的实验并进行理论上的分析讨论，研究了金属玻璃的剪切变形与断裂行为，以理解其脆性断裂的物理本质并为防脆断设计提供建议。为深入理解金属玻璃的变形机制，系统地研究了压缩与弯曲两种加载方式下的剪切带行为；实验验证了椭圆准则对预测金属玻璃拉伸断裂行为的有效性，并进一步提出了普适性断裂准则，为预防脆性断裂提供了指导；通过研究缺口拉伸行为探讨了金属玻璃的韧性问题，并对改善拉伸塑性变形能力的新方法进行了探索。在压缩时，当外应力达到金属玻璃的弹性极限时，可观察到剪切带的萌生。未贯穿样品的剪切带扩展以“渐进剪切”方式进行，对应于应力-应变曲线的过渡段，在曲线上表现出类似“加工硬化”的现象，其硬化原因与剪切带前沿的压缩正应力集中及剪切带的交互作用有关。当外应力达到宏观屈服点时，剪切带贯穿样品；剪切带变形具有不连续特征，因而贯穿后不会立即引起失稳断裂，而会以“同步剪切”方式继续变形。在存在应力梯度的弯曲样品中，剪切带的萌生应力比压缩时要大，剪切带扩展只在前方扩展路径上的局部应力均达到屈服应力时发生。剪切带的失效原因并非热的不断积累，而是热软化与结构损伤共同作用的结果；剪切带交互作用引起微裂纹的产生也是失效的一种方式。塑性较好的金属玻璃中剪切带萌生更易、扩展更难，并有较高的裂纹萌生抗力。通过设计开展倾斜缺口拉伸实验，直接确认了剪切断裂面上的正应力对金属玻璃断裂行为的影响。该实验结果证明：椭圆准则能够精确有效地对这种正应力效应进行描述，可以对金属玻璃的拉伸剪切断裂行为进行定量预测；而Mohr- Coulomb准则虽然可以描述金属玻璃断裂的正应力效应，但在定量预测断裂行为时不够精确。基于椭圆准则，并对大量金属玻璃的断裂行为进行分析，提出了适用范围更广的普适断裂准则。该准则给出了在所有应力状态下的临界断裂应力条件，因此可对金属玻璃在拉伸、压缩和纯剪切下的断裂行为进行有效预测。该准则继承了椭圆准则的统一性特点，同时又有普适性特点，可以对多种材料在不同应力条件下的断裂行为进行合理的解释。剪切断裂面应力状态对金属玻璃的断口形貌有重要影响。提出了四个参数来定量表征拉伸剪切断口特征，实验发现应力状态主要影响断裂面上的脉络核密度与光滑区尺寸。拉伸与压缩剪切断口形貌有很大差异，反映了其断裂机理的不同。拉伸剪切断口的放射状脉络花样是在拉伸正应力与切应力共同作用下沿微裂纹张开粘流态剪切带时发生Taylor失稳而形成；而压缩剪切断口上的剪切脉络花样则主要在切应力的作用下，微裂纹附近粘性材料发生剪切断裂与塑性流变后形成。拉伸正应力有助于裂纹张开，而压缩正应力则会促进裂纹闭合，所以拉伸剪切断裂应是I+II混合型断裂，而压缩断裂则主要是II型断裂。与传统脆性材料不同，实验发现Zr基金属玻璃的强度对缺口并不敏感，较钝的缺口甚至会提高名义拉伸强度，出现“缺口强化”效应。同时，Zr基金属玻璃不仅不会出现金属晶体材料中的“缺口脆化”效应，反而会有“缺口韧化”现象：即相比光滑样品，缺口样品能表现出更大的拉伸“塑性”。缺口尖端的应力集中程度对金属玻璃的缺口行为有一定的影响：在拉伸样品中引入钝缺口不仅可使名义强度提高，又能改善塑性变形能力；而尖锐缺口则会对强度与韧性都构成损害。金属玻璃在缺口根部具有一定量的塑性变形对其表现出“缺口强化”非常重要；而缺口根部的不均匀应力状态、以及高度应变局域化的剪切带变形机制，使得金属玻璃出现独特的缺口韧化行为与较高的韧性。控制剪切带的失稳扩展对改善金属玻璃的拉伸塑性至关重要。提出了引入设计分布的人为缺陷来稳定剪切带扩展、诱发剪切带萌生从而改善金属玻璃拉伸塑性变形能力的新策略。通过在压缩塑性不到1%的普通Zr基块体金属玻璃中引入设计分布的人为缺陷，制备出了具有一定拉伸塑性的“金属玻璃+设计的人为缺陷”混合材料，验证了这种新方法的有效性。这种方法包括两个步骤：（1）引入人为缺陷，这有利于出现多重、稳定的剪切带；（2）设计缺陷的分布来阻碍金属玻璃的本征剪切断裂，从而最优化综合力学性能。通过合理设计，这种新型的混合材料可达到强度与塑性的同步增加。
其他摘要	"Although metallic glasses (MGs) have extremely high strength, the catastrophic brittle fracture behavior often limits their applications as structural materials. In the present thesis, in order to explore the physical nature of the brittle fracture of MGs and to find solutions to prevent or delay the brittle fracture, the shear deformation and fracture mechanism of MGs are investigated by designing and conducting a series of experiments and theoretic discussions on the results. The shear banding behaviors of MGs under compression and bending are systematically investigated to understand the deformation mechanisms. The fracture criterion of MGs is also studied to provide a theoretical instruction for the engineering design to prevent the brittle fracture. The Ellipse criterion is experimentally verified to be capable to effectively and precisely predict the tensile fracture behaviors of MGs, and a universal fracture criterion is proposed. The notch tensile behavior of MGs is investigated and some issues on the toughness of MGs are discussed. At last, we provide a new strategy to improve the tensile plastic deformation ability of MGs. Under compression, the initiation of shear bands (SBs) in MGs can be observed when the applied stress approaches to the elastic limit. The SB propagation prior to the penetration of the sample occurs in a “progressive shearing” mode, and corresponds to the transition stage of the stress-strain curve, which often exhibits a short “work-hardening” phenomenon. The reason for the “hardening” could be the concentrated compressive normal stress in the propagating SB front and the SB interactions. When the applied stress approaches to the macroscopic yield point, the major SB penetrates the sample and the deformation mode becomes a “simultaneous shearing” manner. The discrete behavior of shear banding deformation can explain why the compressive sample is still safe from fracture after the major SB has penetrated the sample and carried the entire external load. However, the bending sample, which has inherent stress gradients, requires a higher stress for SB initiation than that under compression. An SB propagates only when the yield strength has been exceeded along the entire length of the viable shear path. The failure of SB may not result from the accumulation of heat, but from the combined effect of the thermal softening and the structural damaging. Cracking induced by SB interaction is also one reason for the shear failure. MGs with larger plasticity often exhibit lower resistance to SB initiation but higher resistance to SB propagation and cracking. The normal stress effect on the fracture behaviors of MGs is directly confirmed by designing and conducting a series of inclined notch tensile experiments. The experimental results also verify that the Ellipse criterion can precisely and effectively describe the normal stress effect and quantitatively predict the tensile fracture behaviors of the MGs. The Mohr-Coulomb criterion is able to qualitatively describe the normal stress effect but fail to quantitatively predict the fracture behaviors. Based on the experimental results and the Ellipse criterion, a universal fracture criterion is proposed. Since the proposed criterion gives the critical stress conditions for fracture in all stress states, the fracture behaviors of MGs under tension, compression and pure shear tests can be well described with good accuracy. The new criterion inherits the characteristic of unification from the Ellipse criterion and also has the characteristic of universality, which means the proposed criterion has the ability to describe the fracture behaviors of various different materials under various stress conditions. It is found that the stress states on the shear fracture surface have a crucial effect on the fracture morphologies of MGs. We propose four parameters to characteristic their tensile shear fracture morphologies. The results indicate that the core density and the size of smooth region can be significantly influenced by the stress states on shear fracture surface. The tensile fracture morphology is distinct from the compressive one, which indicates the differences in the fracture mechanism. The radiating veins in the tensile shear fracture surface are formed due to the Taylor instability which happens when opening the viscous fluid around the micro-cracks in the major SB with the applied shear stress and tensile normal stress. However, the shear veins on the compressive fracture surface result from the shear fracture and flow of the viscous materials around the micro-cracks with the applied shear stress. The tensile normal stress is helpful for crack opening; while the compressive one tends to closure of cracks. Therefore, the tensile shear fracture should be a mixed I+II mode fracture，while the compressive fracture is mainly a mode II fracture. Distinct from the conventional brittle materials, the strength of Zr-based MGs is insensitive to notch. MG sample with blunt notch often has an increased nominal strength compared to the smooth one, showing a “notch strengthening” effect. Meanwhile, notch does not induce the embrittlement for Zr-based MGs as that happens for conventional metallic crystalline materials; rather, nortch tensile MG sample often exhibits larger “plasticity” than smooth one, showing a “notch toughening” effect. The stress concentration around notch tip influences the notch effect of MGs. MG specimens with blunt notches show not only increased nominal strength but also enhanced plastic deformation ability, where sharp notch is detrimental for both the strength and the toughness of MGs. A certain amount of plastic deformation in the front of the notch tip is necessary for MGs to show “notch strengthening” effect. The inhomogeneous stress filed ahead of notch tip and the shear banding mechanism should be responsible for the unique “notch toughening” effect and the high toughness of MGs. How to control the SB from unstable propagation is of importance for the improvement of the tensile plasticity of MGs. We propose a new strategy through introducing designed artificial defects to stabilize SB propagation and to promote SB initiation. According to this strategy, “MG + designed artificial defects” hybrid materials with appreciated tensile plasticity are successfully fabricated with common Zr-based MGs which possess the compressive plasticities less than 1%. The principles of the new method include: (1) introducing artificial defects to produce multiple and stable SBs to contribute plasticity; and (2) designing the distribution of defects to inhibit the intrinsic shear fracture of BMGs to optimize the comprehensive mechanical performance. Through suitable designing, the new hybrid materials will have simultaneously enhanced strength and plasticity."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64464
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	屈瑞涛. 金属玻璃的剪切变形与断裂行为研究[D]. 北京. 中国科学院金属研究所,2012.