Abstract
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 Tg 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.
Highlights
The atomic structure of metallic glasses (MGs) is a long-standing unsolved issue in condensed matter community[1,2,3]
We resort to atomistic simulations to study the size-dependent atomic structure in small-size amorphous particles (SSAPs) and small-size amorphous thin films (SSAFs)
We report the results of atomic structures in a zero-dimensional Cu64Zr36 SSAPs, ranging from about 11 Å to 60 Å in diameter, and in a two-dimensional Cu64Zr36 SSAFs, ranging from 8.6 Å to 61.5 Å in thickness as a prototype model system in which the Cu-Zr potential was well developed and applied[27,28,29,30, 38,39,40,41], by molecular dynamics simulations
Summary
The intensity of the first peak increases with the size, while the g(r) for larger sized Cu64Zr36 SSAPs (4000 and 5000 atoms) becomes similar to that of bulk Cu64Zr36 MG. In the range of 4–6 Å, i.e., for the second and third neighbors, g(r) are different from 50 atoms to 700 atoms, while after 700 atoms, they are similar to the bulk sample. The partial pair distribution functions g(r) of Cu-Cu, Cu-Zr, and Zr-Zr pairs in Fig. 2 show the increase in intensity of the first peak with sample size while the positions of the peaks in partial g(r) remain almost unchanged, indicating the average atomic bonds are not sensitive to particle size. The first peak distance becomes larger as particle size increases. To dig out details of local atomic structure in particles, the core-and-shell model in the inset of Fig. 1b is applied, in which the thickness
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