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Molecular Dynamic Study of Short-and Medium-Range order Structures in Bulk Metallic Glasses

Foroughi, Alireza | 2018

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 50978 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Aashuri, Hosien; Tavakoli, Roholah
  7. Abstract:
  8. In this work, structures of Cu-Zr bulk metallic glasses at atomic scale were studied by molecular dynamics simulation. Bulk metallic glasses have high glass form ability, which makes it possible to more effectively examine the relationship between structure and properties in glassy materials. Due to this reason, this family of materials has been selected in this research. Voronoi tessellation method, coordination number analysis, short-range order examination, glass transition temperature and pair distribution function have been selected to investigate the structure in atomic scale. Results show that full icosahedron (with the highest five-fold symmetry) and some distorted icosahedra have great effect in reducing the dynamics of the glass. Additionally, full icosahedron has the highest tendency, among all other locally ordered structures, to connect each other and form the so-called medium-range ordered structures. Medium-range order could develop to a few nano-meters inside the glassy structure. It was found that glass form ability of metallic glasses is not directly related to the fraction of icosahedra and chemical composition of icosahedra plays an important role in enhancing the stability of these local structures against crystallization. Based on this observation, a criterion was proposed to evaluate the glass form ability considering both fraction and chemical composition of local ordered structures. Additionally, it was found that cooling rate is another important factor, which influence the structure of metallic glasses. Because, the cooling rates in molecular dynamics simulation is very high (comparing to cooling rates utilized in practice), pressurized sub-Tg annealing has been proposed in this study to effectively reduce the equivalent cooling rates in MD simulation. After pressurized sub-Tg annealing, the equivalent cooling rates in the simulation reach 3.3 x 106 K/s and 3.7 x 107 K/s in Cu50Zr50 and Cu64Zr36 MGs that are comparable to cooling rates in melt spinning experiment. MD simulation showed that the mechanism of structural relaxation during sub-Tg annealing depends to the value of applied hydrostatic pressure. At zero external pressure, atomic diffusion trough open cannels is the dominant mechanism. By increasing hydrostatic pressure up to 2 GP, local atomic rearrangement become the main mechanism. Local atomic rearrangement is concomitant with annihilation of free and anti-free volumes in the glass. Increasing hydrostatic pressure higher than 2 GPa causes the center of atoms become too close to each other therefore retarding relaxation phenomena. Furthermore, with increasing the degree of relaxation, fraction of icosahedra markedly increases. High fraction of icosahedra leads to formation of a connected network of them throughout the structure that acts as a structural backbone of the glass. Moreover, the morphology of such a backbone depends on the equivalent cooling rate (and thus the degree of structural relaxation). With decreasing cooling rate, the morphology of the network changes from string-like to aggregate-like that is associated with higher compactness of the network. Higher compactness of the network is the main reason of higher strength of the glass
  9. Keywords:
  10. Bulk Metallic Glass ; Molecular Dynamic Simulation ; Short-Range Order ; Medium-Range Order

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