Rejuvenation in Hot-Drawn Micrometer Metallic Glassy Wires

We report an enhanced rejuvenation in hot-drawn micrometer metallic glassy wires(MG wires)with the size reduction.Compared to metallic glasses(MGs)in bulk form,the modulus and hardness for the micro-scale MG wires,tested by nanoindentation methods,are much lower and decrease with the decreasing size,with a maximum decrease of〜26%in modulus and^17%in hardness.This pronounced rejuvenation is evidenced by the larger sub-Tg relaxation enthalpy of the MG wires.The pronounced rejuvenation is physically related to the higher energy state induced by a combined effect of severely thermomechanical shearing and freezing the shear How into a constrained small-volume region.Our results reveal that the internal states and properties of MGs can be dramatically changed by a proper modulation of temperature,flow stress and size.

Cr-Fe-Co-Ni-Zr高熵金属玻璃体系中的超大尺度反常相分离

金属玻璃中的反常相分离是一个长期未解决的问题,由于受到微观结构假象以及缺乏直接的大尺度(超过纳米尺度)证据的影响,自从1969年首次被提出以来一直备受争议.本研究综合应用无干扰可见光显微镜、原子力显微镜、扫描电子显微镜和双球差校正原子分辨分析电镜,在具有负混合热的Cr–Fe–Co–Ni–Zr高熵金属玻璃体系中确认了一种超大尺度上的反常玻璃相分离现象.分离的第二相具有数十微米的尺度并且可由冷却速率调控.引入类似偏晶前的液体不混融机制来解释反常相分离,这一机制不仅有利于理解金属玻璃中的反常相分离行为,而且也有助于理解其他领域的相分离行为,比如细胞中的液-液相分离行为.

通过原子制造制备高强高韧双相纳米结构Ta基金属玻璃

通过原子制造技术构建独特的纳米尺度结构可能是改善金属玻璃(MG)薄膜机械性能的有效途径.在此,我们使用脉冲激光沉积作为原子制造策略,制备了一种Ta基金属玻璃.TaNi MG具有小尺寸纳米晶体(1-4纳米)弥散在非晶基体上的双相纳米结构,因此在压缩过程中表现出7.99 GPa的超高屈服强度和15.87 GPa的高硬度,并伴有80%的大塑性.纳米晶体和MG基体之间的相互作用,导致了多个剪切带的形成,从而贡献了高塑性和强度.研究发现,原子制造方法有利于直接调控薄膜材料的微观结构和微观构型,并进一步优化其性能.这项工作为通过原子尺度的结构设计来打破金属材料的强度-塑性权衡提供了一个实用的方法.

Serrated plastic flow behavior and microstructure in a Zr-based bulk metallic glass processed by surface mechanical attrition treatment

The serrated plastic flow,microstructure and residual stress of a Zr_（55）Cu_（30）Ni_5Al_（10） bulk metallic glass（BMG）undergone surface mechanical attrition treatment（SMAT）have been investigated by a combination of compression tests with scanning electron microscopy（SEM）,high resolution transmission electron microscopy（HRTEM）and the incremental hole-drilling strain-gage method.It is found that SMAT leads to various microstructural modifications and residual stress distribution in the surface layers of the Zrbased BMG due to the mechanically-induced nanocrystallization and generation of shear bands.As a result,the BMG alloy exhibits a remarkable work-hardening like behavior and significant increase of plastic strain from less than 1%to 15%,and its plastic deformation dynamics yields a power-law distribution of shear avalanches.Based upon the analysis of the experimental results,it is indicated that this can be connected to the SMAT-induced microstructural modifications and the resulting residual compressive stress in the Zr-based BMG.

Effects of casting current on structure and properties of a nanostructured Zr-Cu-Fe-Al bulk metallic glass

The effects of casting currents on the thermophysical behaviors, atomic and nanoscale structure, and mechanical properties of two Zr-based-bulk metallic glasses, i.e., Zr59Cu33A18 and Zr59（Cuo.55Feo.45）33A18, were studied by using differential scanning calorimetry, wide-angle X-ray diffraction, and small-angle X-ray scattering, as well as compression tests. The casting currents can be tuned to change the casting initiative temperature. Results revealed that there is no anomalous structural change for the Zr59Cu33A18 molten liquid before crystallization during cooling with different casting currents. In contrast, liquid-state phase separation was suggested to occur in the Zr59（Cuo.55Feo.45）33A18 molten liquid prepared using lower casting current before crystallization during cooling. The position shift of the first sharp diffraction peak for the diffraction pattern of Zr59（Cuo.55Feo.45）33A18 shows that the density of the molten liquid may decrease upon cooling at different casting currents. The small-angle X-ray scattering results indicate that the heterogeneity of the Zr59（Cuo.55- Feo.gs）33A18 metallic glasses increases with decreasing the casting temperature. As a result, the metallic glasses with a liquid-state phase separation possess better mechanical properties, including higher-yielding stress and more significant compressive ductility. The increase in degree of heterogeneity formed by nanoscale liquid-state phase separation and their interactions with the shear bands for the Zr-Cu-Fe-Al bulk metallic glasses were suggested to be responsible for the enhanced mechanical properties.

Exceptionally shear-stable and ultra-strong Ir-Ni-Ta high-temperature metallic glasses at micro/nano scales

Ir-Ni-Ta metallic glasses(MGs)exhibit an array of superior high-temperature properties,making them attractive for applications at high temperatures or in harsh environments.However,Ir-Ni-Ta bulk MGs are quite brittle and often fracture catastrophically even before plastic yielding,significantly undercutting their high-strength advantage.Here,we show that the Ir-Ni-Ta MGs are not intrinsically brittle,but rather malleable when the feature size is reduced to micro/nano-scales.All tested Ir-Ni-Ta MG micropillars with a diameter ranging from~500 nm to~5μm display a large plastic strain above 25%(the maximum up to 35%),together with a yield strength up to 7 GPa,well exceeding the strength recorded in most metallic materials.The intrinsic shear stability of Ir-Ni-Ta MGs,as characterized by the normalized shear displacement during a shear event,is much larger than those malleable Zr-and Cu-based MGs.Our results suggest that Ir-Ni-Ta MGs are excellent candidates for micro/nanoscale structural applications used at high-temperature or extreme conditions.

Extracting compressive stress-strain curve based on stick-slip shear banding process in bulk metallic glasses

Based on the stick-slip process,an effective method to extract the stress-strain curve directly from the crosshead displacement-load raw data in compression of bulk metallic glasses was proposed.The method was tested in two bulk metallic glass samples with different plasticities and shear band morphologies.The extracted stress-strain curves were found to well resemble the stress-strain curve measured by a laser extensometer.In addition,the extracted curve could resolve fine structures of serrated flow much better than that measured by extensometer,thus facilitating the study of shear banding process.Results obtained by this method made the stick-slip dynamics of shear banding valid,and this method could be employed to obtain the real strain of small-sized metallic glass samples where extensometer cannot be applied.

Rate Dependence of Serrated Flow and Its Effect on Shear Stability of Bulk Metallic Glasses

The rate dependence of serrated flow and its effects on the stability of shear banding were systematically investigated in a prototypic bulk metallic glass.It was found that with the increase of external strain rate,the serrated flow is gradually suppressed and could completely disappear at a critical strain rate.The serration size,characterized by the mean stress drop amplitude,decreases inversely with the strain rate,while the waiting time for serration decreases with the strain rate in a power-law manner.The rate dependence of the serrated flow has important effects on the dynamics and stability of shear banding process,and leads to an optimal plasticity achieved around the critical strain rate for the disappearance of serrated flow.These results are discussed and interpreted in terms of the microscopic deformation theory and the stick-slip dynamics of shear banding for bulk metallic glasses.