Lessons learned from nano scale specimens tested by MEMS based apparatus

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Lessons learned from nano scale specimens tested by MEMS based apparatus

  • Datum: 24.09.2019
  • Uhrzeit: 13:30 - 14:30
  • Vortragender: Prof. Taher Saif
  • Mechanical Science and Engineering University of Illinois at Urbana-Champaign, USA
  • Ort: Max-Planck-Institut für Eisenforschung GmbH
  • Raum: Large Conference Room No. 203
  • Gastgeber: Prof. Gerhard Dehm
Materials at small scale behave differently from their bulk counterparts. This deviation originates from the abundance of interfaces at small scale. Quantifying the properties and revealing the underlying mechanisms requires experiments with small samples in situ in analytical chambers. However, small size poses the challenge of sample handling, but offers the opportunity of in situ inspection of mechanism during testing in analytical chambers. In order to overcome the challenge and take advantage of the opportunity, we developed a MEMS based micro scale testing stage where the sample and the stage are co-fabricated. The stage suppresses any misalignment error in loading by five orders of magnitude. The stage allows in situ inspection of samples during testing in SEM and TEM. We employed the stage in two scenarios. (1) Exploring the effect of microstructural heterogeneity, such as grain size and orientation, on the deformation mechanisms in nano grained polycrystalline metals. Here the test specimens are free standing thin films subjected to uniaxial tension. We found that heterogeneity introduces two apparently dissimilar, but fundamentally linked, anomalous behaviors. The samples undergo plastic deformation during unloading, i.e., exhibit Bauschinger type phenomenon. Upon unloading, they recover a significant part of plastic deformation with time. The underlying mechanism, verified by in situ TEM inspection, is as follows: during loading, the relatively larger grains undergo plastic deformation and relax by employing dislocations, while the smaller grains remain elastically deformed. During unloading, the smaller grains apply reverse stress on the larger grains causing reverse plasticity resulting in a deviation from linear stress-strain response. Upon complete unloading, the residual stress of the elastically strained small grains continue to apply reverse stress on the larger grains resulting in biased jumps of dislocation in the larger grains and strain recovery. (2) Exploring the effect of size on brittle to ductile transition (BDT) temperature (540C) in single crystal silicon. Here the sample is a micro scale single crystal silicon beam subjected to bending which limits the high stress region to a small volume in the sample, and minimizes the probability of premature failure from random flaws. We found that silicon indeed deforms plastically at small scale at temperatures much lower than 540C. Ductility is achieved through a competition between fracture stress and the stress needed to nucleate dislocations from the surface. Our combined SEM, TEM and AFM analysis reveals that as a threshold stress is approached, multiple dislocation nucleation sites appear simultaneously from the high stressed surface of the beam with a uniform spacing of about 200 nm between them. Dislocations then emanate from these sites with time lowering the stress while bending the beam plastically. This process continues until the effective shear stress drops and dislocation activities stop. A simple mechanistic model is presented to relate dislocation nucleation with plasticity in silicon.
 
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