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Aging and memory in amorphous solids Warren, Mya


Non-equilibrium dynamics in the glassy state lead to interesting aging and memory effects. In this dissertation, extensive computer simulations are performed in order to determine the microscopic origin of these phenomena. Molecular dynamics simulations show all of the qualitative characteristics of real glasses and additionally provide microscopic information that is not typically available to experiments. After a rapid quench to the glassy state, particle correlation functions exhibit dynamical rescaling: all of the relaxation times increase identically with the age of the sample. To investigate the microscopic origins of this behaviour, a new numerical analysis technique is developed to identify structural relaxations on the single particle level. The full distribution of relaxation times and displacements is obtained and used to parametrize a continuous time random walk, which reproduces all features of the dynamics, including dynamical rescaling. These results demonstrate that aging is primarily a kinetic phenomenon, due to the wide distribution of relaxation times. So far, neither the average nor the local structural order can explain the aging dynamics. Variations in temperature and deformation can modify the aging dynamics, causing both rejuvenation and overaging (an apparent increase/decrease in the dynamics compared to simple aging). Non-linear creep is shown to accelerate the dynamics and cause an apparent reversal of aging, whereas a temperature step has complex effects on the relaxation times that are impossible to describe as simple rejuvenation or overaging. The effects of parameters such as the temperature, stress, strain, strain rate, and quench history on the apparent age of the sample are investigated through stochastic simulation of the soft glassy rheology model. In this model, rejuvenation due to load predominates, and overaging is observed only under specific conditions of low temperatures, small strains, and high initial energies. Comparison with molecular dynamics simulations shows qualitative agreement, but also identifies several limitations to the model. Investigating the single particle relaxation dynamics under deformation and at different temperatures may enable further improvements of models of plasticity in amorphous solids.

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