Zyklische Plastizität an Mikrobiegeproben

Translated title of the contribution: Cyclic Plasticity at the Micron-Scale: Bending Fatigue Experiments

Wolfgang Liegl

Research output: ThesisDiploma Thesis

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Abstract

The response of materials to plastic deformation in the micrometer and sub-micrometer regime experienced an increasing interest through the ongoing miniaturisation of devices in different disciplines. Many different methods were developed to explore the mechanical properties on the micrometer scale and, as a matter of fact, a better understanding of the governing mechanisms at monotonic applied loads was achieved. Not only the behaviour under monotonic loading, but also the response of materials to fatigue loadings is important. At the macroscopic sale a vast number of investigations were carried out on the fatigue behaviour of different materials, but little is known on the micro scale, despite of the thermo-mechanical fatigue of thin films. To get an better insight in the fatigue mechanisms and size dependence of fatigue life of micro scaled samples in-situ fatigue experiments were performed. Bending beams were prepared with a Focused Ion Beam (FIB) technique out of electrochemically etched copper single crystal rods in single slip orientation, (234) bending axis. The coarse shape of the beams had dimensions of 10x10x50 µm³. Subsequently, thinner gage sections with sizes of 2 and 8 µm in thickness were milled. The aspect ratio of the gage section was kept constant in both cases, 1x2x1.25. In-situ cyclic bending experiments were carried out in a Scanning Electron Microscope (SEM) with a micro-indenter installed working under displacement control. The displacement was applied sinusoidally and completely reversible, therefore a load ratio of R=-1 was achieved. Low cycle fatigue experiments with up to 14.000 load cycles and varying plastic strain amplitudes were conducted and the evolution of the surface damage continuously monitored in the SEM. Additionally, Transmission Electron Microscope (TEM) lamellas were prepared out of the fatigued beams by FIB lift out, to attain an insight into the evolved dislocation structure. The larger samples show a cell like structure at the surface. However, this damage characteristic only appears after a certain threshold in the plastic strain amplitude. If the amplitude is lower then this the sample shows only uniformly distributed glide steps, but no pronounced surface morphology even at a higher number of load cycles. After the same plastic strain amplitude the 2 µm sample surface does not exhibit such a cell like structure as observed in the 8 µm specimen, indicating that this cell formation threshold is size dependent. These surface observations were complemented by the TEM analysis of the subsurface dislocation arrangement showing cell formation at higher plastic strain amplitudes and absence of such substructure formation for lower strain amplitudes for the 8 µm sample. No structure formed for the 2 µm specimen, confirming the SEM observations. Our results indicate a size dependent formation of substructure during low cycle bending tests. This demands further investigation to understand the small scale fatigue mechanisms.
Translated title of the contributionCyclic Plasticity at the Micron-Scale: Bending Fatigue Experiments
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Pippan, Reinhard, Supervisor (internal)
Award date1 Jul 2011
Publication statusPublished - 2011

Bibliographical note

embargoed until null

Keywords

  • LCF
  • Cu
  • micromechanic

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