Molecular Dynamics Simulations of Supercooled Liquid Metals and Glasses
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Molecular dynamics simulations of supercooled liquid metals and glasses Hyon-Jee Leea,*, Yue Qib, Alejandro Strachanb, Tahir Caginb, William A. Goddard IIIb, and William L. Johnsona a) Materials Science Department, 138-78 California Institute of Technology, Pasadena, CA 91125, U.S.A. b) Materials and Process Simulation Center, 139-74 California Institute of Technology, Pasadena, CA 91125, U.S.A. Abstract The thermodynamic, transport and structural properties of a binary metallic glass former in solid, liquid, and glass phases were studied using molecular dynamics simulation. We used a model binary alloy system with a sufficient atomic size mismatch and observed a glass transition in a quenching process. The diffusivity and viscosity were calculated in the liquid state and the super-cooled liquid state. The smaller atom showed higher diffusivity and more configurational randomness compared to the larger atom. The viscosity increased abruptly around the glass transition temperature. The solvent/solute concentration effect on the glass transition was examined in terms of a packing fraction. We find that the glass forming ability increases with the packing fraction in the liquid state because the densely-packed material requires more time to rearrange and crystallize. Introduction In understanding the physics of the glass transition of metallic alloys, Molecular Dynamics (MD) simulations can provide important insights by allowing one to determine quantities which are difficult to access in real experiments or hard to obtain with reasonable precision. To precisely describe the interactions in metals and metallic alloy system, we adopted the empirical many body potential developed by Sutton and Chen [1]. Force-field parameters were optimized for face centered cubic (FCC) transition metals by fitting to such experimental properties as density, cohesive energy, moduli, and phonon frequencies [2,3]. It has been previously established that the atomic size mismatch plays a major role in the glass transition [4,5]. Therefore, the generalized binary alloy system with sufficient atomic size mismatch was chosen as a model glass former. Among possible candidates, we used Cu as the base material. For the second component, we introduced an artificial Cu-like atom, Cu*, which has same force-field parameters except its size parameter, i.e. its atomic radius. By introducing Cu*, we could characterize the atomic size mismatch effect on the glass transition more easily without considering other factors such as a mismatch in chemical bonding character. The atomic size ratio of Cu* and Cu was set 1.13, corresponding to the Ag-Cu alloy system which is known to be a good glass former. The simulation was performed at constant temperature, constant thermodynamic tension (TtN) MD conditions [6]. The TtN MD was started from an FCC random solid L2.3.1
solution with 500 atoms in a simulation cell subject to periodic boundary conditions. The heating experiment was carried out by increasing the temperature form 300 K to 1600 K in 100 K increments.
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