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In situ probing of size dependent properties of nano pillars

PhD student: Alexey Kuzmin

The project is aimed at scrutinizing our hypotheses about the interplay between relevant length scales and size effects affecting the thermo-mechanical stability of devices. The hypotheses read:

  • Upon decreasing size, when going from crystalline to metallic glassy materials, we move from a 'smaller is stronger' into a 'smaller is tougher' regime.
  • Devices thinner than a critical thickness will not exhibit catastrophic failure depending on the specific materials design.

This proposal sets apart from the current work in this field by focusing on what is the interplay of the internal characteristic length scale, e.g. the size of the shear transformation zone, and the external dimensions of the amorphous material system, and how this could be tuned to yield an optimal performance. Understanding and controlling shear localization is one of the principal tasks in research on metallic glasses. However, a major drawback of experimental and theoretical research so far is that - not surprisingly- almost all of the microscopy work has been concentrated on static structures and as a consequence the mechanical response of amorphous materials is a subject area which is still shrouded with considerable confusion. Our approach is to concentrate on in situ TEM/SEM studies of amorphous systems (pillars) and amorphous-controlled nanocrystalline multilayers under compression/tension and load/displacement controlled conditions.

pillars_bending

Figure: Metallic glass nano-pillar being bent at a rate of 20 nm/s, with images (b)-(d) acquired at different deformation intervals. The pillar shows at the middle height a bump under compression and concave/necking upon tension (indicated by solid and open white arrows respectively), and fully homogeneous flow near the base, typical of ductile deformation.