On Relaxation Kinetics in Liquid and Glassy Ag-Cu Metallic Alloy

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On Relaxation Kinetics in Liquid and Glassy Ag-Cu Metallic Alloy Alexander S. Bakai, Nikolai P. Lazarev and Kia L. Ngai1 National Science Center Kharkov Institute of Physics and Technology 61108 Kharkov, Ukraine 1 Naval Research Laboratory Washington, DC 20375-5320, USA ABSTRACT Molecular dynamics simulations of structure, thermodynamic and kinetic properties of model metallic AgCu alloy are performed to elucidate its behavior at glass transition. In spite of small variations of inherent structure of the alloy the relaxation kinetics undergo dramatic changes at the glass transition. The time dependences of the mean square displacements and the non-Gaussianity parameter show the signatures of anomalous diffusion in an intermediate time region. Analysis of time evolution of van Hove correlation function indicates the existence both jump displacements and short-range cooperative atomic rearrangements. Below Tg these cooperative rearrangements do not contribute to long-range diffusion but they still dominate the relaxation at short time. INTRODUCTION Glass transition in metallic alloys is still poorly understood. Evolution of the relaxation kinetics with temperature near glass transition is a crucial property of the system because the glass transition is a kinetic process [1]. Cooperative rearrangements are necessary in the liquid due to either molecular coupling or configurational entropy considerations. In crystalline solids, single vacancy jumps control the diffusional relaxation. In a glass or “a frozen liquid” the configurational entropy (and complexity) is much lager than in crystals. A pertinent question to ask is: how important are the cooperative rearrangements for the relaxation kinetics of glass and what is the nature of the diffusional kinetics in glasses? An important feature of the diffusional relaxation kinetics is the time dependence of diffusion coefficient determined from the time dependence of the mean square displacement r < r 2 (t ) > . Molecular dynamics (MD) simulations on ionic conducting glasses show that there are several stages in the dynamics of diffusion [2]. Accordingly to the coupling model [3] interpretation [2], at short times the dynamics is stochastic and non-cooperative, involving local short-range displacements of the atoms. The cages formed by neighboring atoms decay slowly at this stage. At longer times the dynamics cross over to cooperative hopping of the atoms resulting in larger displacements. The crossover can be seen from the pair correlation function, which exhibits a substantially more rapid decrease after the onset of the second stage. The crossover time from local to longer-range motion turns out to be close to the independent (i.e., without the effect due to coupling or cooperativity between the atoms) relaxation time of the coupling model. Moreover, they have about the same temperature dependence. In this work we investigate the structure of and relaxation processes in liquid and glassy state of 60Ag-40Cu alloy at temperatures down to 100K below glass temperatu