Kinetic and structural properties of shear bands in bulk metallic glasses

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The plastic flow localization in shear bands of Bulk metallic glasses (BMGs) attracted a lot of attention although its nature is still controversially discussed or even obscure. Additionally, shear banding as the intrinsic response to plastic straining of metallic glasses presents also a technological bottle neck for utilizing the unique mechanical properties of this class of materials, since it often leads to early failure during the development of so-called “catastrophic shear bands”. In order to develop glassy materials with intrinsic structures or inherent properties that lead to branching of shear bands or that utilize other transformation pathways to yield macroscopic pseudo-plastic behavior, the initiation and properties of shear bands need to be understood in more depth. In order to do so, we propose combining detailed studies of the diffusion kinetics in BMGs with induced shear bands and structural investigations and strain measurements based on electron microscopy with atomistic simulations of plastic flow localization that are to be carried out by the Researchgroup. The model glass former Pd-Ni-P is proposed to be investigate due to its excellent glass forming ability and the associated high stability against crystal formation, the opportunity to produce large-scale samples, the availability of structural and thermodynamic data and the existence of suitable radioisotopes.The present project is focused on two mayor aspects: the investigation of the diffusion kinetics in deformed BMGs which is extremely sensitive to the strain/stress fields and the free volume distribution and the experimental analysis of the free volume and strain distribution based on electron microscopy methods. The combination of these measurements of the shear band properties is a powerful tool to render deep insight into the atomistic nature of shear band initiation and propagation as well as concerning the properties of the shear bands.The kinetic properties of shear bands will be studied by the radiotracer technique and in-situ and postmortem experiments. The effect of thermomechanical treatments will be examined in order to figure out the response of the pertinent structure of shear bands on the free volume redistribution. The experiments will be focused on the excess free volume distribution inside the shear bands as a function of their thermomechanical history. Additionally, the role of the free volume and strain distribution shall also be addressed directly. Therefore it is planned to measure the density of shear bands by a new technique using the signal of the Rutherford-scattered electrons in the high-angle annular dark-field (HAADF) detector and the information of the low loss part of an electron energy loss spectrum simultaneously. The second aspect of interest is the strain development in a metallic glass during deformation. Our new approach is to measure the strain via a reference lattice, which is brought onto the sample by vapour deposition through nanometre-sized, selforganized Alumina masks (a technique which is available in-house). The periodicity of the reference lattice is then sensitive to any deviation introduced by deformation. These experimental results shall be directly compared with atomistic simulations on a model glass forming system performed in the Researchgroup.