文献标识码: 分类号 TB331 文章编号 1005-3093(2016)08-0575-06
收稿日期: 2016-03-23
网络出版日期: 2016-09-28
版权声明: 2016 《材料研究学报》编辑部 《材料研究学报》编辑部
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摘要
采用渗流铸造法制备W纤维/Zr基非晶复合材料, 研究高径比的变化对复合材料室温压缩力学性能的影响。结果表明, 复合材料的屈服强度随样品高径比的增大先降低, 高径比大于1时趋于平稳。高径比大于或等于1.25时, 复合材料的压缩塑性应变变化不大。高径比小于1.25时, 复合材料的压缩塑性应变均大于50%。压头与样品端部摩擦力的作用、W纤维之间非晶丝高径比的变化和W纤维与非晶基体之间变形的不匹配综合作用最终导致小高径比的复合材料样品具有更好的压缩力学性能。
关键词：
Abstract
The W fiber/Zr-based metallic glass composite was prepared by infiltration and rapid solidification. The effect of the ratios of length to diameter of fibers on the compressive properties of the composite was investigated in detail. The results show that the yield strength firstly decreases with the increase of the length to diameter ratio then reaches a stable value when the ratio is greater than 1. The plastic strain has no obvious change when the ratio is greater than or equal to 1.25, while the plastic strain is bigger than 50% when the ratio is smaller than 1.25. The reason for these phenomena is the comprehensive effect of the friction force between pressure head and the end of the compressive sample, the change of the length to diameter ratio of the metallic glass fibers between W fibers and the mismatching between metallic glass matrix and the W fiber during deformation.
Keywords：
非晶合金的变形和破坏机制与传统的晶态合金不同, 一直受到科研工作者的关注[1, 2]。一般来说, 非晶合金的室温塑性变形局限在很窄的剪切带内, 剪切带的快速扩展会导致样品的突然破坏[1-4]。但是, 大量研究结果表明, 无论在压缩载荷下还是拉伸载荷下, 非晶合金并不沿最大剪切应力面发生剪切断裂[4, 5]。样品尺寸和加载方式的改变也影响非晶合金的塑性变形能力, 一般情况下, 小尺寸非晶样品的塑性变形能力更好, 在压缩或者弯曲载荷下表现出一定的室温塑性变形能力[6-11]。这就带来一个问题, 剪切带的形成和扩展是如何控制非晶合金的塑性的呢?Conner等[10, 11]的研究结果发现室温下薄非晶条带在弯曲载荷下能够经历一定的塑性变形而不破坏, 但是厚非晶合金板却会发生突然破坏。Davis等[12]、Lewandowski等[13]和Lu等的[14]研究结果表明流体静压力对Zr基非晶的塑性流变以及断裂行为均有一定程度的影响, 非晶合金在有限制的情况下呈现出超过10%的塑性变形能力。Bruck等[15]研究了高径比分别为1: 2和2: 1的非晶合金样品的压缩性能, 结果表明小高径比的样品屈服强度略高且压缩塑性明显增大, 这说明非晶合金的塑性与样品的几何尺寸以及加载模式密切相关。
非晶合金在室温下塑性变形能力有限, 严重限制了非晶合金作为结构材料的应用, 因此改善非晶合金的室温塑性至关重要。而研究结果表明, 在非晶合金中引入第二相制备复合材料的方法能够有效地改善非晶合金的室温塑性[16, 17]。与纯非晶合金相比, 非晶复合材料的变形行为更加复杂, 除了受基体合金和增强相的影响外, 还与界面的结合状态以及加工过程中所产生的残余应力有关。既然非晶合金的性能与几何尺寸相关, 那么非晶复合材料的力学性能和变形过程也必然受几何尺寸的约束, 但是有关非晶合金复合材料的尺寸效应的报道还很少, 因此本文制备了不同高径比的W纤维/Zr基非晶复合材料, 详细探讨了高径比的变化对复合材料力学性能和变形行为的影响。
Zr41.2Ti13.8Ni10.0Cu12.5Be22.5母合金通过电弧熔炼制备。具体过程为: 选用纯度不低于99.8%(质量分数)的Zr、Ti、Cu、Ni、Be纯金属, 各种纯金属所需的质量按原子百分比配制。经过清洗干燥后将配制好的各种金属均匀混合置于电弧炉坩埚内进行熔炼。在熔炼合金前, 先将预置于炉内的吸收钛合金锭熔化以进一步降低电弧炉炉腔内的含氧量。为保证合金锭成分尽可能均匀, 每个合金锭至少翻转熔炼四次。Zr基非晶通过铜模浇注法制备。W纤维/Zr基非晶复合材料通过熔体浸渗法制备, 所用W纤维的直径为500 μm, 制备工艺为1173 K保温4 min后, 以0.3 MPa的外压将Zr基合金熔体吹入下方的模具中。实验所用压缩样品尺寸为: 非晶合金: ϕ5×10 mm、ϕ5×7.5 mm、ϕ5×5 mm和ϕ5×2.5 mm, 对应的高度和直径的比值(高径比)分别为2、1.5、1和0.5; W棒: ϕ2×4 mm、ϕ2×3 mm、ϕ2×2 mm和ϕ2×1.2 mm, 对应的高径比分别为2、1.5、1和0.6; W纤维/Zr基非晶复合材料: ϕ5×10 mm、ϕ5×8.75 mm、ϕ5×7.5 mm、ϕ5×6.25 mm、ϕ5×5 mm、ϕ5×3.75 mm和ϕ5×2.5 mm, 对应的高径比分别为2、1.75、1.5、1.25、1、0.75和0.5; 采用日本理学D/max-2500PC型 X射线衍射仪(Cu-Kα辐射, 波长λ=1.54056 nm)对样品进行物相分析。采用Instron5582型万能力学试验机对样品进行压缩试验, 应变速率为10-4 s-1。为保证数据的可靠性, 每组样品至少做3次重复试验, 取平均值。采用FEI Quanta 600型扫描电子显微镜观察变形或断裂后样品的侧表面剪切带形貌。
图1为Zr41.2Ti13.8Ni10.0Cu12.5Be22.5非晶合金及W纤维/Zr基非晶复合材料压缩样品横截面的XRD谱, 由图1a可以看出, Zr基合金的XRD曲线上只有象征非晶态结构的慢散射峰, 没有其他尖锐的衍射峰出现, 说明所制备的Zr基合金为完全的非晶态。由图1b可以看出, 复合材料的XRD曲线上只有对应于增强相W的衍射峰, 无其他的衍射峰出现, 说明复合材料的基体合金仍保持非晶态结构。图1b中的插图为复合材料原始形貌的SEM照片, 由图可见, 复合材料中W纤维为近似密排分布, 体积分数约为80%, 复合材料中无明显孔洞或裂纹等宏观缺陷。
图1 Zr基非晶和W纤维/Zr基非晶复合材料的XRD衍射谱
Fig.1 The XRD patterns of the Zr-based metallic glass (a) and the W fiber/Zr-based metallic glass composite (b)
图2为不同高径比的Zr基非晶合金、W棒以及W纤维/Zr基非晶复合材料的压缩应力应变曲线。图3为Zr基非晶合金、W棒和W纤维/Zr基非晶合金的屈服强度随样品的高径比的变化关系。可以看出Zr基非晶的屈服强度随样品高径比的增大而降低, 但高径比大于1时, 屈服强度增大的幅度降低。W棒的屈服强度随样品高径比的变化不大。W纤维/Zr基非晶复合材料的屈服强度随样品高径比的增大先降低, 高径比大于1时达到一个稳定值。由实验结果可以推断复合材料屈服强度的变化主要受非晶基体的影响。除了屈服强度外, 高径比的变化显著影响Zr基非晶和W纤维/Zr基非晶复合材料的压缩塑性, 对晶态W棒的影响不大。由图2a可以看出, 高径比为2和1.5时, Zr基非晶合金在室温下的塑性变形能力非常有限, 表现为弹性变形后突然发生的脆性断裂。但是当样品的高径比小于等于1时, Zr基非晶在压缩载荷下表现为弹塑性变形行为。高径比为0.5时, 塑性应变达到了30%而样品仍然没有发生破坏, 可见, 高径比的改变显著影响Zr基非晶的压缩变形能力。由图2c可以看出, 当样品高径比分别为2、1.75、1.5和1.25时, W纤维/Zr基非晶复合材料的压缩塑性应变分别为16%、18%、20%和20%, 变化不大。但当高径比小于1.25时, 复合材料的压缩塑性应变达到50%时仍然没有断裂。
图2 不同高径比的Zr基非晶合金、W棒以及W纤维/Zr基非晶合金复合材料的压缩应力应变曲线
Fig.2 Compressive stress-strain curves of the Zr-based metallic glass (a), tungsten (b) and W fiber/Zr-based metallic glass composites (c) with different aspect ratio
图3 压缩屈服强度随样品高径比的变化关系
Fig.3 Dependence of the Compressive yield strength on the aspect ratio. (a) Zr-based metallic glass, (b) tungsten stick and (c) W fiber/Zr-based metallic glass composites
Zr基非晶合金在压缩载荷下均发生剪切破坏, 破坏后的侧表面形貌如图4所示, 可见, 随高径比的减小, 样品侧表面的剪切带数量和密度明显增加, 剪切带之间的交互作用明显增强。高径比为0.5时, 样品侧表面出现了大量相互交错的剪切带, 且大部分剪切带的方向均偏离了45°方向, 样品端部靠近压头一侧尤为明显, 出现了大量近似垂直于加载轴向的剪切带。由于非晶独特的结构使之不能像晶态材料一样通过位错产生塑性变形, 而只能通过剪切带进行变形。而剪切带数量的多少和剪切带之间交互作用的程度往往决定着非晶材料的变形能力, 剪切带数量越多、密度越大, 则塑形变形能力越强; 相反, 剪切带数量越少、密度越小则塑形变形能力越差[11, 18]。本实验中, 随着样品高径比的减小, 变形后样品侧表面的剪切带数量和密度均明显增大, 且剪切带的交互作用明显增强, 这些现象均预示着样品的塑性变形能力也会随着高径比的减小而增强, 也就是说Zr基非晶的塑性应变随高径比的减小而增大。
图4 Zr基非晶合金变形后侧表面的剪切带形貌
Fig.4 Shear bands and cracks on the sample profiles of the W fiber/Zr-based metallic glass composite
图5为高径比分别为2、1.5、1和0.5时, 复合材料变形后的侧表面形貌图, 图5a和b分别为样品断裂前的侧表面形貌, 图5c和d为样品的塑性应变量分别为20%和35%时复合材料的侧表面形貌。可见, 复合材料在压缩载荷下中间部分墩粗, 而靠近压头部分截面直径变化较小, 变形后导致样品最终失效的宏观裂纹多出现在样品中间部位的W纤维中, 且裂纹多为纵向的裂纹。这是因为W纤维为拉拔加工而成, 晶粒沿轴向被拉长, 导致其纵向晶界强度较弱; 又因为纤维为近似密排分布, 多数纤维之间相互接触, 导致裂纹更容易沿互相接触的W纤维进行扩展。随高径比的变化, 样品中裂纹的数量和开裂程度也有区别, 高径比较大时, 宏观裂纹数量较少但开裂非常明显, 而高径比较小时, 复合材料中的宏观裂纹较多但开裂程度相对较小。压缩样品的侧表面上, 不同部位的剪切带方向也有较大区别, 样品中心部位剪切带大多与加载轴向成一定角度, 已有一部分剪切带垂直于加载轴向。而样品端部靠近压头一侧剪切带多与加载轴向垂直, 这与纯非晶侧表面观察到的剪切带方向近似。
图5 W纤维/Zr基非晶复合材料变形后侧表面的剪切带和裂纹形貌
Fig.5 Shear bands and cracks on the sample profiles of the W fiber/Zr-based metallic glass composite
由以上实验结果可以看出, 高径比的变化显著影响Zr基非晶和W纤维/Zr基非晶复合材料的室温压缩性能。压缩实验是一种有限制的加载过程, 样品的变形行为和力学性能受压头的影响作用。不同高径比的非晶合金压缩过程中剪切带扩展示意图如图6所示, 由于样品端部与压头摩擦力的作用, 会在样品端部产生一个由端部向中心逐渐减小的径向压缩应力, 使其处于双向压应力的受力状态, 而这种双向的压缩应力可以提高材料的强度并改变剪切带的方向[19]。随着高径比的减小, 样品处于双向压应力作用范围的比例也会随之增大, 因而更有助于样品强度和塑形的增大。假如初生剪切带与加载轴向成45°角并沿此方向进行扩展, 对于高径比为2的Φ5×10 mm的样品, 样品中心部位的剪切带要扩展出样品的距离为
图6 不同高径比的非晶合金压缩过程中剪切带扩展示意图
Fig.6 Schematic diagram of the spreading of the shear bands in metallic glass with different aspect ratio under compressive loading
1. W纤维/Zr基非晶复合材料屈服强度的变化主要受非晶基体的影响, 复合材料的屈服强度随样品高径比的增大先降低, 高径比大于1时达到一个稳定值。
2. 样品高径比大于或等于1.25时, W纤维/Zr基非晶复合材料的压缩塑性应变变化不大。高径比小于1.25时, 复合材料的压缩塑性应变大于50%。
3. 压头与样品端部摩擦力的作用、W纤维之间非晶丝高径比的变化和W纤维与非晶基体之间变形的不匹配综合作用最终导致小高径比的复合材料样品显示更好的压缩力学性能。
The authors have declared that no competing interests exist.
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The elastic properties, elastic models and elastic perspectives of metallic glasses,
Bulk metallic glass (BMG) provides plentiful precise knowledge of fundamental parameters of elastic moduli, which offer a benchmark reference point for understanding and applications of the glassy materials. This paper comprehensively reviews the current state of the art of the study of elastic properties, the establishments of correlations between elastic moduli and properties/features, and the elastic models and elastic perspectives of metallic glasses. The goal is to show the key roles of elastic moduli in study, formation, and understanding of metallic glasses, and to present a comprehensive elastic perspectives on the major fundamental issues from processing to structure to properties in the rapidly moving field. A plentiful of data and results involving in acoustic velocities, elastic constants and their response to aging, relaxation, applied press, pressure and temperature of the metallic glasses have been compiled. The thermodynamic and kinetic parameters, stability, mechanical and physical properties of various available metallic glasses especially BMGs have also been collected. A survey based on the plentiful experimental data reveals that the linear elastic constants have striking systematic correlations with the microstructural features, glass transition temperature, melting temperature, relaxation behavior, boson peak, strength, hardness, plastic yielding of the glass, and even rheological properties of the glass forming liquids. The elastic constants of BMGs also show a correlation with a weighted average of the elastic constants of the constituent elements. We show that the elastic moduli correlations can assist in selecting alloying components with suitable elastic moduli for controlling the elastic properties and glass-forming ability of the metallic glasses, and thus the results would enable the design, control and tuning of the formation and properties of metallic glasses.<br/>We demonstrate that the glass transition, the primary and secondary relaxations, plastic deformation and yield can be attributed to the free volume increase induced flow, and the flow can be modeled as the activated hopping between the inherent states in the potential energy landscape. We then propose an extended elastic model to understand flow in metallic glass and glass-forming supercooled liquid, and the model presents a simple and quantitative mathematic expression for flow activation energy of various glasses. The elastic perspectives, which consider all metallic glasses exhibit universal behavior based on a small number of readily measurable parameters of elastic moduli, are presented for understanding the nature and diverse properties of the metallic glasses. (C) 2011 Elsevier Ltd. All rights reserved.
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TiZr-base Bulk Metallic Glass with over 50 mm in Diameter,
Low-cost TiZr-base bulk metallic glasses (BMGs) (Ti(36.1)Zr(33.2)Ni(5.8)Be(24.9))(100-x)Cu(x) (x=5, 7 and 9) with a maximum size of over 50 mm in diameter were developed by optimizing the alloy composition. The idea is initiated by selecting a particular microstructure comprising primary beta-Ti dendrite and amorphous phase. Afterwards, based on this composition of amorphous phase, a class of TiZr-base bulk metallic glasses was designed step by step to reach the optimum composition range. The glass transition temperature (T(g)), initial crystallization temperature (T(x)) and width of supercooled region (Delta T) of (Ti(36.1)Zr(33.2)Ni(5.8)Be(24.9))(91)Cu(9) BMG are 611, 655 and 44 K, respectively. The (Ti(36.1)Zr(33.2)Ni(5.8)Be(24.9))(91)Cu(9) BMG exhibits low density of 5.541 g.cm(-3) and high compressive fracture strength of 1800 MPa, which promises the potential application as structural materials.
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Prediction of shear-band thickness in metallic glasses,
<p id="">We derive an explicit expression for predicting the thicknesses of shear bands in metallic glasses. The model demonstrates that the shear-band thickness is mainly dominated by the activation size of the shear transformation zone (STZ) and its activation free volume concentration. The predicted thicknesses agree well with the results of measurements and simulations. The underlying physics is attributed to the local topological instability of the activated STZ. The result is of significance in understanding the origin of inhomogeneous flow in metallic glasses.</p>
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Fracture mechanisms in bulk metallic glassy materials, |
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Difference in compressive and tensile fracture mechanisms of Zr59Cu20Al10Ni8Ti3 bulk metallic glass,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">The compressive and tensile deformation, as well as the fracture behavior of a Zr<sub>59</sub>Cu<sub>20</sub>Al<sub>10</sub>Ni<sub>8</sub>Ti<sub>3</sub> bulk metallic glass were investigated. It was found that under compressive loading, the metallic glass displays some plasticity before fracture. The fracture is mainly localized on one major shear band and the compressive fracture angle, <em>θ</em><sub>C</sub>, between the stress axis and the fracture plane is 43°. Under tensile loading, the material always displays brittle fracture without yielding. The tensile fracture stress, <em>σ</em><sub>F</sub><sup>T</sup>, is about 1.58 GPa, which is lower than the compressive fracture stress, <em>σ</em><sub>F</sub><sup>C</sup>(=1.69 GPa). The tensile fracture angle, <em>θ</em><sub>T</sub>, between the stress axis and the fracture plane is equal to 54°. Therefore, both <em>θ</em><sub>C</sub> and <em>θ</em><sub>T</sub> deviate from the maximum shear stress plane (45°), indicating that the fracture behavior of the metallic glass under compressive and tensile load does not follow the von Mises criterion. Scanning electron microscope observations reveal that the compressive fracture surfaces of the metallic glass mainly consist of a vein-like structure. A combined feature of veins and some radiate cores was observed on the tensile fracture surfaces. Based on these results, the fracture mechanisms of metallic glass are discussed by taking the effect of normal stress on the fracture process into account. It is proposed that tensile fracture first originates from the radiate cores induced by the normal stress, then propagates mainly driven by shear stress, leading to the formation of the combined fracture feature. In contrast, the compressive fracture of metallic glass is mainly controlled by the shear stress. It is suggested that the deviation of <em>θ</em><sub>C</sub> and <em>θ</em><sub>T</sub> from 45° can be attributed to a combined effect of the normal and shear stresses on the fracture plane.</p>
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Plastic heterogeneity in nanoscale metallic glass, Physica E: Low-dimensional Systems and
Elongation of CuZr metallic amorphous nanorods at a low strain rate has been examined by molecular dynamics methods. Results indicate that nanoscale metallic glasses (NMGs) show non-brittle fracture and plastic heterogeneity. In addition, the size effect has been carefully investigated via the NMG samples with different diameters. Meanwhile, the fluctuation of the stress and structure during the elongation provides reliable evidences to explain why plastic heterogeneity exists in the NMGs. Our results suggest that the mechanical properties of NMGs would be closely related to the size effect and heterogeneity in NMG samples.
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Size-dependent shear fracture and global tensile plasticity of metallic glasses,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">The tensile ductility or brittleness of metallic glasses is found to depend strongly on the critical shear offset. Based on experimental observations, the tensile shear fracture processes of metallic glasses can be divided into three stages: multiplication and coalescence of the free volume, formation of void and the final fast propagation of a shear crack. Accordingly, the size effect on the tensile shear deformation processes of metallic glass can be well understood: with decreasing specimen size smaller than the equivalent critical shear offset, the shear deformation of metallic glass is changed from unstable to stable, which leads to a transition from global brittleness on the macroscale to large global plasticity or even necking on the microscale. These results are fundamentally useful in understanding the physical nature of tensile shear deformation of various metallic glasses and even in the design of new metallic glass materials with good plasticity.</p>
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Size-dependent plasticity and fracture of a metallic glass in compression,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">A specimen size effect related to elastic spring-back and strain softening is reported on the deformation and fracture behavior of bulk metallic glass (BMG). Unlike in large specimens where unstable shear band propagation usually leads to catastrophic fracture, in small specimens yielding is followed by stable shear band propagation and extensive plastic deformation. Additionally, the fracture surfaces of the small specimens are smooth without the characteristic vein patterns seen in large specimens. The present results demonstrate that it is important to take specimen size into account when interpreting plasticity and fracture of BMGs, especially when considering the effects of composition on ductility.</p>
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| [9] |
Size-independent strength and deformation mode in compression of a Pd-based metallic glass,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">We present quasi-static, room temperature compression data for Pd<sub>40</sub>Ni<sub>40</sub>P<sub>20</sub> metallic glasses, with specimen sizes ranging from the submicron to several millimeters in diameter. We observe no change in deformation mode over this range. At all sizes, plastic flow is localized in shear bands, which are accompanied by sudden strain bursts. This metallic glass shows only a modest increase in strength in going from bulk to micrometer-sized specimens. We show that stress gradients in tapered specimens can complicate measurement of the yield strength of metallic glasses in microcompression. Estimates of yield strength based on the minimum cross-sectional area implicitly assume that yielding is controlled by a maximum effective shear stress criterion. An alternative is the shear plane yield criterion, in which the minimum shear stress on the shear band trajectory determines yield. Application of this criterion in tapered microspecimens reinforces the notion that metallic glasses possess relatively size-independent mechanical properties.</p>
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| [10] |
Shear bands and cracking of metallic glass plates in bending,
The thickness dependence of yielding and fracture of metallic glass plates subjected to bending is considered in terms of the shear band processes responsible for these properties. We argue that the shear band spacing (and length) scales with the thickness of the plate because of strain relaxation in the vicinity of the shear band at the surface. This is consistent with recent measurements of shear band spacing versus sample size. We also argue that the shear displacements in the shear band scale with the shear band length and plate thickness, thus causing cracks to be initiated in thicker plates at smaller bending strains. This leads to fracture bending strains that decrease markedly with increasing plate thickness, consistent with recent experiments. These results suggest that amorphous metals in the form of foams might have superior ductility and toughness.
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| [11] |
Shear band spacing under bending of Zr-based metallic glass plates,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Metallic glasses often exhibit marked ductility when subjected to compressive or bending loads as a result of multiple shear band formation. This observed ductility depends upon sample geometry; thin plates show ductility in bending while thicker plates of the same composition fracture under similar loading. The thickness dependence of yielding and fracture of metallic glass plates subjected to bending is considered in terms of the shear band processes responsible for these properties. Experimental results show that shear band spacing and length scale with the thickness of the plate at a ratio of 1:10. Both shear band offset and shear band spacing increase with increasing curvature; shear band offset as the square of the plate thickness. As bending is increased beyond yield, shear band spacing continues to increase until the strain is accommodated by a few long shear bands. Continued bending leads to crack formation and failure.</p>
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| [12] |
Flow and fracture of a Ni-Fe metallic glass,
When brittle failure modes are bypassed, sheet tensile specimens of Ni 49 Fe 29 P 14 B 6 Si 2 glass exhibit abrupt shear failures, coincident with yielding; at low to intermediate temperatures no reduction of area is evident in the narrow shear zone. At higher temperatures (up to 0.87 T g , where T g is the glass transition temperature) plastic flow at ordinary strain rates is different only in that it is less localized, i.e. yielding induces readily evident necking through the thickness of the sheet; failure still generally occurs by shear rupture through the neck. In the low temperature regime, the yield stress (σ y ) decreases slowly with increasing temperature; it varies from 3.04 to 1.96 GPa between 77 and 568 K ( T/T g 610.82). At higher temperatures, where necking is observed, σ y decreases rapidly, apparently approaching zero near T g (65694 K).49291462TgTgyT/TgyTgThe oblique shear zones which are generated by yielding apparently follow (or are close to) directions of zero extension (i.e. pure shear) in the sheet specimens. For Ni 49 Fe 29 P 14 B 6 Si 2 the angle between the normal to the shear “plane” and the tensile axis averages 6537°, only a few degrees larger than the expected value. This small variation may derive from the dynamics of the yielding process.49291462Oblique necking and shear through the neck occur during tensile deformation of crystalline material only when the sample is in sheet form; also the shear localization zone lies parallel to the thickness vector of the sheet. Metallic glasses are unique in that (1) the shear zone in low aspect ratio sheets is sometimes oblique to the thickness vector (as well as the width vector) and (2) shear yielding also occurs along directions of zero extension in radially symmetric (as opposed to sheet) gauge section specimens.
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| [13] |
Effects of hydrostatic pressure on the flow and fracture of a bulk amorphous metal,
The flow and fracture behaviour of a Zr-Ti-Ni-Cu-Be bulk amorphous metal have been determined in tension and compression at room temperature with levels of superimposed hydrostatic pressure ranging from 0.1 to 700 MPa. Metallographically polished cylindrical specimens tested in uniaxial tension and compression were utilized in the high pressure tests, while polished cylindrical torsion specimens were tested at 0.1 MPa (i.e. atmospheric pressure) in order to approach conditions of pure shear. All the tension and torsion tests, regardless of the level of superimposed pressure, exhibited linear elastic failure, as did the compression tests conducted with low levels (e.g. less than 450 MPa) of pressure. At the highest pressures (i.e. 450 MPa or higher), the compression tests exhibited elastic-perfectly plastic behaviour and an increase in the compressive elongation to fracture. The flow stress and fracture stress were not significantly affected by the superposition of pressure as failure occurred in shear, indicative of pressure-independent behaviour over the range tested. However, a change in fracture plane angle was detected. Tensile fracture surfaces were oriented at 50-59掳; compression fracture surfaces were oriented at 40掳. The flow and fracture behaviours were analysed in terms of a Mohr-Coulomb criterion of the form = 950 MPa- 0.038over the range of stress states examined. The results are discussed in the light of the various yield criteria and the flow and fracture theories provided for amorphous metallic systems.
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| [14] |
Deformation behavior of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass over a wide range of strain-rates and temperatures,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">The stress-strain relations for the Zr<sub>41.2</sub>Ti<sub>13.8</sub>Cu<sub>12.5</sub>Ni<sub>10</sub>Be<sub>22.5</sub> bulk metallic glass (Vitreloy 1) over a broad range of temperatures (room temperature to its supercooled liquid region) and strain rates (10<sup>−5</sup> to 10<sup>3</sup> s<sup>−1</sup>) were established in uniaxial compression using both quasi-static and dynamic Kolsky (split Hopkinson) pressure bar loading systems. Relaxation and jump in strain rate experiments were conducted to further understand the time dependent behavior of Vitreloy 1. The material exhibited superplastic flow above its glass transition temperature (623 K) and strain rates of up to 1 s<sup>−1</sup>. The viscosity in the homogeneous deformation regime was found to decrease dramatically with increasing strain rate. A fictive stress model was used to describe the basic deformation features of Vitreloy 1 under constant strain-rate loading as well as multiple strain-rate loading at high temperatures.</p>
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| [15] |
Quasi-static constitutive behavior of Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk amorphous alloys,
Mechanical tests have been performed on bulk amorphous metal alloys to determine their constitutive behavior. Based on the experimental results, it appears that amorphous metal alloys obey a Von Mises yield criterion. This result has implications in determining the micromechanisms of plastic deformation in these materials.
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| [16] |
Synthesis and properties of tungsten balls/Zr-base metallic glass composite,
The millimetre scale W balls reinforced Zr-base bulk metallic glass composite was prepared using infiltration technique. The compressive deformation behaviours of the composite at two strain rates of 1×10 614 s 611 and 1×10 612 s 611 were investigated. The enhanced plasticity is attributed to the effect of W balls, which constrain shear bands propagation and promote the formation of multiple shear bands. The properties of the composite depend evidently on the strain rates. The high strain rate leads to the higher strength and larger plasticity.
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| [17] |
Synthesis and characteristics of 80 vol.% tungsten (W) fibre/Zr based metallic glass composite, |
| [18] |
Ductile Bulk Metallic Glass, |
| [19] |
Rotation mechanism of shear fracture induced by high plasticity in Ti-based nano-structured composites containing ductile dendrites,
Ti-based nano-structured composites with ductile dendrites often fail under a shear fracture angle larger than 45degrees with the compression stress axis. This can be explained by a rotation mechanism of the shear plane and bending of the shear bands, indicating a good mechanical performance of the nano-structured composites. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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