Dynamics of viscoelastic flow in ion tracks: origin of plastic deformation of amorphous materials

In the present paper, track formation and plastic deformation (creep and anisotropic growth) of amorphous solids under heavy ion bombardment are discussed in terms of viscoelastic shear stress relaxation in thermal spike regions and the subsequent freezing-in of the associated strain increments. The resulting strained tracks are considered to represent the (symmetry-breaking) mesoscopic defects responsible for anisotropic macroscopic deformation. Recent theoretical approaches to the problem are critically reviewed. A system of viscoelastic continuum equations is proposed which includes the recently used "effective flow temperature approach" as a limiting case. General solutions of this system are presented and discussed. These solutions, completed by a model for the ion beam induced reduction of the viscosity, appears to be suited to fully describe the deformation characteristics of amorphous materials including the threshold behaviour with respect to electronic stopping power and ion fluence ("incubation dose") as well as the target temperature dependence. An evaluation of appropriate experimental studies on the basis of the presented theory is expected to provide valuable information on the lion beam reduced) viscosity of amorphous materials around ambient and thermal spike temperatures. (C) 1998 Elsevier Science B.V. All rights reserved.