ANISOTROPIC CREEP AND GROWTH OF AMORPHOUS SOLIDS UNDER SWIFT HEAVY-ION BOMBARDMENT - AN ASYMPTOTIC THERMAL SPIKE APPROACH

Under swift heavy ion bombardment, stressed and stress-free amorphous solids show substantial anisotropic creep and growth, respectively. In the framework of a visco-elastic model suggested recently, these phen omena are attributed to shear stress relaxation in ion induced thermal spikes, followed by the freezing-in of the associated strain incremen t upon cooling down. In the case of creep, the local shear stress is d efined by the externally applied stress, in the case of growth it is d ue to the thermal expansion in cylindrical thermal spikes. According t o this picture, each frozen track represents a mesoscopic defect in th e form of a thermo-elastic inclusion. Concerning creep, amorphous soli ds under swift heavy ion bombardment are shown to behave like nematic fluids. For high electronic stopping power and low irradiation tempera ture, simple asymptotic expressions are derived for the anisotropic fl uidity (or viscosity), the normalized anisotropic growth rate and the steady-state zero-creep stress. A comparison of these expressions with experimental results yields good agreement which is considered justif ication of the basic assumptions of the model. Model predictions suite d for testing details of the model assumptions are discussed.