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The atomic arrangement of metallic glasses lacks long-range periodicity, and exhibits structural characteristics of an amorphous state. Their unique structural features lead to research methods that differ traditional metallic crystalline materials, focusing mainly on two scales: one is a macroscopic scale, on which glass-forming ability and mechanical behavior are investigated through alloy design, thermodynamic parameters, and other means; the other is an atomic scale, on which medium- to short-range orders of metallic glass are studied through computational simulations and diffraction techniques. There is a difference of over seven-orders of magnitude between the scales of these two methods, which makes it difficult to establish a direct quantitative relationship between them. Therefore, a structural feature is needed that can connect atomic configurations with macroscopic properties on a mesoscopic scale. With the development of amorphous structure characterization technique, it has been found that metallic glasses exhibit spatial heterogeneity at the nanometer and micrometer levels above a medium-to-short range, with their scales ranging between macroscopic and atomic scales. This article introduces experimental characterization methods for spatial heterogeneity, focuses on the electron microscopic characterization methods of spatial heterogeneity and local atomic orders, and discusses their intrinsic correlations with macroscopic properties such as β-relaxation behavior, mechanical behavior, thermodynamic stability, and glass-forming capability. Spatial heterogeneity, as a structural characteristic of metallic glasses on a mesoscopic scale, can serve as a link between short-range order and macroscopic properties of atoms.
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Keywords:
- metallic glass /
- glass structure /
- glass property /
- spatial heterogeneity
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图 3 不同制备条件和测量方法下超快冷却Zr-Cu-Al非晶合金的微观结构 (a) 高分辨透射电子显微图; (b), (c) 离子减薄(b)以及电解双喷(c)制备条件下样品的高角环形暗场成像(HAADF)模式的扫描透射电子显微(STEM)图片; (d) 调幅动态原子力显微镜的相位偏移图以及对应的放大图片, 图(b)—(d)中的空间非均匀性的特征关联长度都在6 nm左右[18,19]
Figure 3. Microstructure of ultraquenched Zr-Cu-Al metallic glass prepared under different conditions and measured by different methods: (a) High-resolution transmission electron micrograph; (b), (c) scanning transmission electron micrographs of samples with ion milling (b) and twin-jet polishing (c) preparation conditions in high-angle annular dark-field imaging modes; (d) phase-shift image of amplitude-modulation dynamic AFM and the corresponding enlarged images, the characteristic correlation lengths of spatial heterogeneity in (b)–(d) are all about 6 nm[18,19].
图 4 通过离子减薄制备的超快冷Zr-Cu-Al非晶合金的密度和厚度分析 (a) HAADF-STEM 图像的强度分布, 可以用高斯函数拟合; (b) 从HAADF-STEM 图像中标记的区域提取的强度曲线; (c) 根据电子能量损失谱零损失峰(EELS zero-loss peak)绘制的厚度图, 色标显示t/l的值, 其中t为样品厚度, l为非弹性平均自由路径; (d) 从厚度图中相应区域提取的强度曲线[19]
Figure 4. Density and thickness analysis of a hyper-quenched Zr-Cu-Al metallic glass prepared by ion milling: (a) The intensity distribution of HAADF-STEM images which can be well fitted by the Guassian function; (b) the intensity profile taken from the regions marked in the HAADF-STEM image; (c) a thickness mapping derived from the EELS zero-loss peak with a color scale showing the values of t/l where t is the thickness of sample and l is the inelastic mean free path; (d) the intensity profile taken from the corresponding regions in the thickness mapping[19].
图 6 (a) 扫描埃尺度相干电子衍射的示意图; (b) 扫描ABED衍射花样与二维位置信息一起存储在四维数据库示意, 非晶合金亮区和暗区的代表性 ABED 图样; (c)亮区和(f)暗区提取的单个ABED花样; (d), (g) 衍射矢量之间的角度标注; (e) 完全非晶的样品和(h) 部分晶化的样品的选区衍射(SAED)图样[19]
Figure 6. (a) Schematic illustration of scanning Angstrom-beam electron diffraction; (b) the scanning ABED patterns are stored with the two-dimensional positioning information as a 4D database, representative ABED patterns from bright and dark regions of the hyper-quenched metallic glasses; individual ABED patterns taken from (c) bright regions and (f) dark regions; (d), (g) the angles between diffraction vectors are marked; SAED patterns taken from (e) the as-prepared hyper-quenched metallic glass; and (h) the partially devitrified metallic glass[19].
图 7 超快冷Zr-Cu-Al非晶合金 (a) 退火前、在553 K温度下退火 (b) 5 min和(c) 720 min的调幅动态原子力显微图片; (d) 553 K下不同退火时间的相偏移相关方程曲线; (e) 根据空间非均匀性体积的特征弛豫时间与退火温度的倒数关系推导出的演化激活能[18]
Figure 7. Amplitude-modulation dynamic AFM of ultraquenched Zr-Cu-Al metallic glasses (a) before annealing and after annealing at 553 K for (b) 5 min and (c) 720 min; (d) phase-shift correlation function curves for samples annealed at 553 K for different times; (e) the evolution activation energy derived from the dependence of characteristic relaxation time of spatial heterogeneity volumes on the reciprocal annealing temperature [18].
图 8 (a) 超快冷却Zr-Cu-Al非晶合金的高分辨率图像, 插图为对应的选区电子衍射图; (b) 超快冷却、中速冷却和慢速冷却样品的高角度环形暗场扫描图像; (c) 纳米压痕硬度和模量与空间非均匀性的特征长度的关系图, 实心曲线是对各点的线性拟合; (d) 超快冷却和慢速冷却的微米柱在单轴压缩下的工程应力应变线[20]
Figure 8. (a) High-resolution TEM (HRTEM) image of the hyper-quenched Zr-Cu-Al metallic glass, the inset is the corresponding selected area electron diffraction pattern; (b) high-angle annular dark-field scanning TEM (HAADF-STEM) images of the hyper-quenched, the intermediate and the highly relaxed samples; (c) nanoindentation hardness and modulus plotted with the characteristic length of spatial heterogeneity, the solid curve is a linear fitting for the points; (d) micro-pillar compression testing, engineering stress-stain curves of the hyper-quenched and the highly relaxed samples subjected to the uniaxial micro-pillar compression[20].
图 9 应变弛豫测试 (a) 50 mN的恒定加载条件下, 压头位移h与保持时间th的应变弛豫测试曲线; (b) 拉伸指数β与特征长度ξ的关系图, 通过应变弛豫测量的β实验值与混合物模型的理论值完全一致, 左上插图是亮区的代表性埃尺度相关电子衍射(ABED)图案, 右下插图是暗区的代表性ABED图案[20]
Figure 9. Strain relaxation measurements: (a) Indenter displacement h as a function of the holding time th at a constant loading of 50 mN; (b) the stretching exponent β plotted with the characteristic length ξ, the experimental values of β measured by the strain relaxation are well consistent with the predictions of the mixture model, the top left inset is a representative ABED pattern for the bright regions, and the bottom right inset for the dark regions[20].
图 11 (a) Cu45Zr45Ag10非晶合金的透射电子显微镜(TEM)图像, 插图是对应的选区电子衍射图; (b) Cu45Zr45Ag10非晶合金的高分辨电子显微图像和(c) 高角环形暗场成像模式的扫描透射电子显微图像; (d) 暗场像及对应的混合(MIX), Zr, Cu和Ag元素的能谱(EDS)扫描图; (e) 势能随有效冷却速率变化的关系图; (f) Zr原子(红色)、Cu原子(紫色)和 Ag原子(黄色)在不同冷却速率下的模型玻璃中的化学分布[22]
Figure 11. (a) TEM image of a Cu45Zr45Ag10 metallic glass, inset is the selected area electron diffraction; (b) high-resolution TEM image and (c) HAADF-STEM image of Cu45Zr45Ag10 metallic glass; (d) ADF image and corresponds EDS mappings for mixed, Zr, Cu and Ag elements; (e) plot of potential energy per atom as a function of cooling rate; (f) chemical distributions of Zr atoms (red), Cu atoms (purple) and Ag atoms (yellow) in model glasses of different cooling rates[22].
图 12 (a)—(c) Zr原子(红色)、Cu 原子(紫色)和Ag 原子(黄色)在冷却速率约为106 K/s的(a) Cu50Zr50, (b) Cu45Zr45Ag10和(c) Cu40Zr40Ag20的化学分布图; (d)—(f) 二十面体团簇(青)和 Ag元素(黄)的分布; (g) 冷却速率为106 K/s的Cu-Zr-Ag非晶合金模型中Ag富集区域原子的质量分布与径向距离的关系曲线; (h) 非均匀(MCMD)与均匀(MD)非晶合金的二十面体团簇含量、Ag的维度与Ag含量的关系曲线, 误差条表示标准偏差[22]
Figure 12. Chemical distributions of Zr atoms (red), Cu atoms (purple) and Ag atoms (yellow) in (a) Cu50Zr50, (b) Cu45Zr45Ag10, and (c) Cu40Zr40Ag20 with a cooling rate of ~106 K/s. (d)—(f) Icosahedral-like clusters (cyan polyhedron) and Ag (yellow particle) distributions; (g) mass distribution of atoms in Ag-rich regions as a function of radial distance for MCMD models; (h) the fractions of icosahedral-like clusters of MCMD and MD models as a function of Ag content, the dimensionality of Ag distribution is also plotted with the Ag content. The error bars indicate standard deviation[22].
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