Aging in the shear-transformation-zone theory of plastic deformation.

Aging phenomena in the plastic response of amorphous solids are studied within the theory of shear transformation zones (STZs), which describes the kinetic rearrangement of localized defects in response to external stress. To account for the slow, nonequilibrium dynamics after a quench below the glass transition temperature, two possible models are considered. In the first model, transition rates between the internal states of STZs decrease with time, while in the second model aging occurs due to the relaxation of an effective temperature that determines the number density of STZs and other out-of-equilibium degrees of freedom. It is shown that for reasonable choices of parameters, both models capture qualitatively typical aging features seen in computer simulations and experiments: (i) compliance curves measured for different waiting times t_{w} after the quench can be superimposed, when the observation times are rescaled with relaxation times proportional, variantt_{w};{mu} , 0<micro< or =1 , and (ii) stress-strain curves show a stationary plateau stress independent of t_{w} and a peak stress that increases logarithmically with both t_{w} and the strain rate. Trends of the aging behavior with the quench temperature are also discussed.