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Comparing Glass Formability of Cu-Zr and Ni-Zr Systems by Molecular Dynamics Simulation

Ghaemi, Milad | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49990 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Tavakoli, Rohollah
  7. Abstract:
  8. Metallic glasses (MG) are a group of materials that have unique properties such as high yielding strength and high elastic deformation. Because of limitation in the production of these materials, a few industrial materials of MG have been made so far. A few of binary alloys, like binary alloys made of Cu-Zr, have high Glass Forming Ability (GFA) in medium cooling rates. Between two binary systems of Cu-Zr and Ni-Zr, although constituent elements in aspect of atomic radius, electronic structure, physical properties, phase diagrams and intermetallic compounds are so similar to each other, but GFA between them are very different. For example, in Cu-Zr system there are compounds, such as Cu50Zr50 and Cu64Zr36, that have high GFA and also capability produce Bulk Metallic Glass (BMG) by large scale (to centimeters) in casting mold. On the other hand, GFA in Ni-Zr system is so low that BMG can’t be produced in this system. The reason for this different GFA between these two systems haven’t been explained and it is as a question yet to be answered. The aim of this thesis is to study these two systems in three different components and also evaluation on difference in their structure by using simulation in molecular dynamic method. Lammps software is used for simulation in this study. By selecting proper interatomic potential, using NPT ensemble and periodic boundary condition this study is conducted. Crystal and glass structures, icosahedrons fractions, longest interconnected icosahedron cluster and pair distribution function (PDF) are studied. Mass density and ten indexes that have the most fraction between among all are calculated. According to type connection of icosahedrons, medium range order (MRO) is surveyed. Type of connections are vertex-sharing, edge-sharing, face-sharing and interconnected. Number of each type of connections and also number of isolated icosahedron calculated and are compared between two systems. It is observed that by increasing Cu/Ni in both systems, the plot of PDF goes to the left side and mass density as well as fractions of Cu/Ni-center polyhedrals which have index of <0 0 12 0>, <0 2 8 2>, <0 3 6 4> and <0 1 10 2> are increased. During studying types of connections, it is observed that by increasing Cu/Ni, number of interconnected icosahedrons are increased and this enhancement for Cu64Zr36 is much more compared to others. In addition, the fraction of the local structure of icosahedron in Cu-Zr system is more than Ni-Zr. According to these results, it seems that the fraction of the local structure of icosahedron and connection among them is a sufficient condition for increasing of GFA
  9. Keywords:
  10. Molecular Dynamics ; Zirconium-Copper Alloy ; Glass Forming Ability ; Nickel-Zirconium Alloy ; Bulk Metallic Glass

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