„Anisotropy – Influence of Topological Anisotropy on the Mechanical Properties of Silicate Glasses“
German Science Foundation
Glasses are generally thought-of as isotropic materials. However, it is also well known that anisotropy can easily be introduced during the processing of glasses. The object of the present project is the question as to how anisotropy develops in silica, phosphate and silicate model glasses and how it affects the stress-strain response of these glasses. Anisotropy can manifest in many forms, and our investigations clearly show the need to distinguish between (i) transient anisotropy, which is only exhibited by a material under load, as is the case, e.g., in stress-induced reversible birefringence; (ii) persistent, structural, anisotropy which is reversible only upon thermal annealing and can, e.g., be caused by plastic deformation, or by freezing-in the structure of a glass flowing under load, like in the case of wire drawing; and, finally, (iii) spatio-temporal transient anisotropy which describes localized directional deformation processes which take place during deformation, and which are required to occur in order to develop persistent anisotropy, e.g., the self-organization of shear transformation zones (STZ) in shear bands (SB) in bulk metallic glasses (BMG). The relation between these different aspects, which are usually subsumed in the empirical concept of macroscopic anisotropy as determined by experiment, and the underlying glass structure and topology are currently not fully understood. For example, tin phosphate glasses show negligible stress-induced transient anisotropy as measured by birefringence. At the same time, this does not immediately correlate to its potential for generating persistent anisotropy through thermal straining. On the structural level, it is currently unclear to which degree the macroscopically measured persistent anisotropy depends on the degree of heterogeneity of plastic deformation. This is of particular importance as most processes to induce persistent anisotropy involve deformation gradients, while computer simulations typically only study homogeneous deformation of comparatively small volumes. Furthermore, the role of short range order (SRO) and medium range order (MRO) on the time-dependence and the anisotropy of transient properties like the Poisson ratio during deformation have not yet been investigated. This is a collaboration with Profs. Erik Bitzek and Erdmann Spiecker of the University of Erlangen-Nuremberg.