Strained Ion Tracks in Amorphous Solids: Origin of Plastic Deformation
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ABSTRACT Track formation in amorphous solids is treated in terms of viscoelastic shear stress relaxation in thermal spike regions which is followed by the freezing-in of the associated strain increment. The resulting strained tracks are considered to be the mesoscopic defects responsible for anisotropic creep and growth. A recently presented approximate quantitative approach to the problem is reviewed. In addition, a new set of constitutive equations describing the viscous flow in thermal spike regions is suggested and general solutions are discussed. INTRODUCTION Bombardment with heavy ions is an efficient way to drive solid systems far from equilibrium. Extreme conditions may be thus realized: high overall defect production rates as well as strongly localized short-term (nm/ps) thermal spikes associated with quenching rates of up to the theoretical limit of about 1015 K/s. Under such conditions, crystalline materials may become amorphous, and amorphous materials may be driven into less stable states associated with material modifications and dimensional changes. An energetic ion penetrating a solid material slows down by two different interactions: quasielastic collisions characterized by a "nuclear stopping power", Sn, and electronic excitations and ionization characterized by an "electronic stopping power", S. [1]. With increasing ion energy, the stopping power changes (in the MeV range) from a low energy region where the former
mechanism dominates to a high energy region where the latter prevails. The two energy loss mechanisms differ qualitatively in the morphology of the primary damage associated with them: Collision cascades and electronic excitations represent collective excitations of approximately spherical and cylindrical shape, respectively. Tracks of energetic ions in solid materials, particularly in crystalline ceramics, have in the past been studied intensively and various ion track techniques have been developed, ranging from fission track dating to the production of microfilters [2,3]. Ion tracks in crystalline materials can be revealed by transmission electron microscopy as well as chemical etching. High resolution electron microscopy shows ion tracks in crystalline (ceramic) materials to represent well bounded long amorphous inclusions embedded into their crystalline environment [4,5]. Ion tracks in amorphous materials can not be imaged by electron microscopy but may be revealed by chemical etching similar to tracks in crystalline materials [2,6]. Their preferential etching shows the track regions to be in an amorphous state which is modified and less stable than the virgin state. For the preferential etching of tracks in vitreous silica, for instance, this modification may be correlated with the observed compaction. For the preferential etching of tracks in metallic glasses [6] where no obvious structure and density changes have been observed the character of the modification has not been clarified so far. An old and conceptionally simple idea to explain damage creation along ion tracks is the
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