Abstract:Nanowires made of metallic glass have been actively pursued recently due to the superb and unique properties over those of the crystalline materials. The amorphous nanowires are synthesized either at high temperature or via mechanical disruption using focused ion beam. These processes have potential to cause significant changes in structure and chemical concentration, as well as formation of defect or imperfection, but little is known to date about the possibilities and mechanisms. Here, we report chemical seg… Show more
“…Similar Cu segregation at the top surface layer is also observed in Cu 64 Zr 36 SSAFs. Similar works for the segregation effect were recently reported 44 , 45 . Figure 3 The coordination number distributions of the total sample, core and shell components, ( a–c ) for both elements, ( d–f ) for only Cu-centered atoms, and ( g–i ) for only Zr-centered atoms, respectively.…”
The size effect on atomic structure of a Cu64Zr36 amorphous system, including zero-dimensional small-size amorphous particles (SSAPs) and two-dimensional small-size amorphous films (SSAFs) together with bulk sample was investigated by molecular dynamics simulations. We revealed that sample size strongly affects local atomic structure in both Cu64Zr36 SSAPs and SSAFs, which are composed of core and shell (surface) components. Compared with core component, the shell component of SSAPs has lower average coordination number and average bond length, higher degree of ordering, and lower packing density due to the segregation of Cu atoms on the shell of Cu64Zr36 SSAPs. These atomic structure differences in SSAPs with various sizes result in different glass transition temperatures, in which the glass transition temperature for the shell component is found to be 577 K, which is much lower than 910 K for the core component. We further extended the size effect on the structure and glasses transition temperature to Cu64Zr36 SSAFs, and revealed that the T
g decreases when SSAFs becomes thinner due to the following factors: different dynamic motion (mean square displacement), different density of core and surface and Cu segregation on the surface of SSAFs. The obtained results here are different from the results for the size effect on atomic structure of nanometer-sized crystalline metallic alloys.
“…Similar Cu segregation at the top surface layer is also observed in Cu 64 Zr 36 SSAFs. Similar works for the segregation effect were recently reported 44 , 45 . Figure 3 The coordination number distributions of the total sample, core and shell components, ( a–c ) for both elements, ( d–f ) for only Cu-centered atoms, and ( g–i ) for only Zr-centered atoms, respectively.…”
The size effect on atomic structure of a Cu64Zr36 amorphous system, including zero-dimensional small-size amorphous particles (SSAPs) and two-dimensional small-size amorphous films (SSAFs) together with bulk sample was investigated by molecular dynamics simulations. We revealed that sample size strongly affects local atomic structure in both Cu64Zr36 SSAPs and SSAFs, which are composed of core and shell (surface) components. Compared with core component, the shell component of SSAPs has lower average coordination number and average bond length, higher degree of ordering, and lower packing density due to the segregation of Cu atoms on the shell of Cu64Zr36 SSAPs. These atomic structure differences in SSAPs with various sizes result in different glass transition temperatures, in which the glass transition temperature for the shell component is found to be 577 K, which is much lower than 910 K for the core component. We further extended the size effect on the structure and glasses transition temperature to Cu64Zr36 SSAFs, and revealed that the T
g decreases when SSAFs becomes thinner due to the following factors: different dynamic motion (mean square displacement), different density of core and surface and Cu segregation on the surface of SSAFs. The obtained results here are different from the results for the size effect on atomic structure of nanometer-sized crystalline metallic alloys.
“…The rule of thumb in practice is to hold the samples at the temperature long enough to allow for fabrication without crystallization. 1-3 As we show below, however, subtle but substantial changes may still occur within this time window in chemical concentration, 13 atomic structure, and, consequently, the properties of metallic glass nanowires. These changes are hard to detect but play an important role in wires' properties.…”
mentioning
confidence: 93%
“…The details of the sample preparation methods are available in a recent publication. 13 Here, we only give a brief description of the procedure for modeling mechanical properties of the nanowires. In this work, all the nanowires have the diameter of about 12 nm and the respect ratio of about 3.…”
mentioning
confidence: 99%
“…One is the chemical segregation induced by both temperature and surface tension when the characteristic dimension of the devices becomes small. 13 Alloy element redistribution can introduce drastic changes in not only chemical or catalytic properties on the surface but also mechanical properties because segregated elements have different elastic modulus and yield behavior. Fig.…”
Compared to their crystalline counterparts, nanowires made of metallic glass have not only superb properties but also remarkable processing ability. They can be processed easily and cheaply like plastics via a wide range of methods. To date, the underlying mechanisms of how these different processing routes affect the wires' properties as well as the atomic structure remains largely unknown. Here, by using atomistic modeling, we show that different processing methods can greatly influence the mechanical properties. The nanowires made via focused ion beam milling and embossing exhibit higher strength but localized plastic deformation, whereas that made by casting from liquid shows excellent ductility with homogeneous deformation but reduced strength. The different responses are reflected sensitively in the underlying atomic structure and packing density, some of which have been observed experimentally. The presence of the gradient of alloy concentration and surface effect will be discussed.
“…For example, the local density gradient appears as shown in Fig. 2 (c) along with chemical segregation 31 and free volumes 19 . For comparison, in the insets of Fig.…”
Both strengthening and weakening trends with decreasing diameter have been observed for metallic glass nanowires, sometimes even in the samples with the same chemical composition. How to reconcile the results has reminded a puzzle. Since the detailed stress state and microstructure of metallic glass nanowires may differ from each other significantly depending on preparation, to discover the intrinsic size effect it is necessary to study metallic glass nanowires fabricated differently. Here we show the complex size effects from one such class of metallic glass nanowires prepared by casting using molecular dynamics simulations. As compared with the nanowires of the same composition prepared by other methods, the cast nanowires deform nearly homogeneously with much lower strength but better ductility; and also show strengthening in tension but weakening in compression with decreasing wire diameter. The subtle size dependence is shown to be related to the key factors including internal and surface stress state, atomic structure variation, and presence of various gradients. The complex interplay of these factors at decreasing size leads to the different deformation behaviors.
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