The physical and mechanical properties of glasses strongly depend on their bonding configuration and topology, which includes near, intermediate and long range order . It is well-known that controlled application of mechanical load during cooling of glass melts can lead to topologically modified network structures [2,3]. Also uniaxial compression experiments can be used to introduce structural anisotropy into various glasses [4,5]. Moreover, moderate electron beam (e-beam) irradiation in the transmission electron microscope (TEM) [6,7] and scanning electron microscope (SEM)  can be exploited to induce enormous ductility in nanoscale silica spheres under mechanical load. However, still the question remains whether e-beam irradiation in combination with compression can lead to anisotropic glasses and how this affects their mechanical properties.
Here we present a novel approach to perform athermal mechanical quenching experiments in the TEM and evidence its impact on mechanical properties of nanoscale silica spheres . Nanoscale silica spheres are compressed in the TEM under different e-beam conditions and loading scenarios by using the Hysitron PI95 TEM PicoindenterTM (Fig. 1). Prior to compression the silica spheres are irradiated with an e-beam current density of 0.09 A/cm2, leading to a shrinkage of 15-18% . In experiment 1 the silica sphere is compressed at beam-off conditions and exhibits an elastic-plastic deformation behavior without fracture . In experiment 2 the silica sphere is quenched under load. To achieve this the compression is started under e-beam irradiation (which we use to mimic temperature) and the e-beam is switched off during compression. The sudden absence of the e-beam quenches-in the modified silica network structure. Surprisingly, starting from the quenching point the slope of the force-displacement curve increases drastically, while a completely elastic loading-unloading behavior is obtained. In case of experiment 3 directly after the beam-on compression a holding segment is used, allowing for relaxation of stresses. During the following deformation at beam-off conditions the silica sphere shows a completely elastic loading-unloading behavior. Interestingly, complementary finite element method simulations reveal that the Young’s moduli (E) of silica spheres are altered: E values of 45 GPa, 38 GPa and 29 GPa are obtained for silica spheres from experiments 1, 2 and 3, respectively. As a direct reason for this observation structural anisotropy is proposed (Fig. 2) . Quenching of silica spheres under load leads to a partially anisotropic silica network, while quenching after relaxation generates an even more anisotropic structure. During the relaxation period the silica sphere is in a compressed and confined state, during which structural re-organization is restricted along the compression direction . This mechanism is further favored by residual tensile stresses acting perpendicular to the loading direction [10,11], which maintains the development of structural anisotropy reported here .
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Financial support by the DFG through the SPP1594 “Topological Engineering of Ultra-Strong Glasses”, Cluster of Excellence EXC 315 “Engineering of Advanced Materials” and GRK1896 “In situ microscopy with electrons, X-rays and scanning probes” is gratefully acknowledged. We thank R. Klupp-Taylor and M. Hanisch for providing the silica spheres and S. Romeis for valuable discussions.
To cite this abstract:Mirza Mačković, Florian Niekiel, Lothar Wondraczek, Erik Bitzek, Erdmann Spiecker; In situ mechanical quenching of nanoscale silica spheres in the TEM. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/in-situ-mechanical-quenching-of-nanoscale-silica-spheres-in-the-tem/. Accessed: December 12, 2018
EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-mechanical-quenching-of-nanoscale-silica-spheres-in-the-tem/