Relaxation processes in Cu-Zr metallic glasses by molecular dynamics simulations (Doctoral thesis)

Palomino Rico, Pablo Antonio

We present Molecular Dynamics simulations results referring to relaxation processes, occurring in Cu65Zr35 computer glass at the quiescent state, during tensile solicitation, as well as the stress releasing effect that takes place when this material is under constant strain. The study comprises a thorough analysis of the thermodynamics, structural and dynamical properties of this system. We found a rattling mode of atomic motions that takes place in highly localized regions characterized by their high mobility and low local density and composed mostly by loosely packed clusters of atoms. The rattling mode has a frequency that is essentially independent from both temperature and applied strain. However, the regions where the rattling atoms are localized, and thus the fractions of atoms undergoing rattling, increase with temperature and applied strain. It is argued that the rattling motions constitute the ‘fast processes’ that lie between the β relaxation and the Boson peak in the characteristic spectrum of dynamic modes in glasses. In addition, we found that our glass exhibited a characteristic endothermic response upon straining, which has not been reported before. The study of this response and its evolution upon straining indicates the presence of aging and rejuvenation processes and delimits the different stages at which each one of these processes is dominant in our glass. It is concluded that the fast processes are precursors of the β relaxation mode, and could be considered as predictors of the spatial origin of the shear transformation zones, governing the onset of plastic flow. Furthermore, during the stress relaxation process occurring when the glass is subjected to a fixed strain, the location of the rattling atoms coincides with the heterogeneous regions where most of the stress and energy of the system is being released, thus they act as the stress mediators of the metallic glasses. These results could be useful for the understanding of the complex relaxation spectrum of the metallic glasses and provide a better insight of the intricate relaxation dynamics that characterizes these materials, especially at the higher frequency region. Moreover, the results presented here have important consequences in the macroscopic properties of metallic glasses, like ductility, brittleness and toughness fracture. It turns out, therefore, that understanding the structural and dynamical behaviour of the MGs during these relaxation processes is crucial for eventual control and improvement of their properties
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Φυσικής
Subject classification: Alloys
Keywords: Amorphous alloy,Dynamic modes,Metallic glass,Molecular dynamics,Structural relaxation,Mechanical properties,Shear transformation zones,Aging/Rejuvenation in metallic glasses,Άμορφα κράματα,Τρόποι δόνησης,Μεταλλικές ύαλοι,Προσομοιώσεις μοριακής δυναμικής,Δομική χαλάρωση,Μηχανικές ιδιότητες,Ζώνες μετασχηματισμού διάτμησης,Γήρανση / αναζωογόνηση μεταλλικών υάλων
Appears in Collections:Διδακτορικές Διατριβές

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