Ermüdungseigenschaften und Mikrostrukturevolution von mikro- und nanokristallinen Mikrobiegeproben

Translated title of the contribution: Fatigue properties and microstructure evolution of micro- and nanocrystalline micro beams

Research output: ThesisDiploma Thesis

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Abstract

Ultra-fine-grained materials exhibit a far higher strength during monotonic loading than their coarse-grained counterparts. This sparked an interest in using this material class for technical applications. For example, in medical applications and for superplastic deformation UFG-materials have favorable properties. For use in actual devices not only the monotonic material properties are important, but especially the properties under cyclic loading, because failure in technical devices is primarily caused by cyclic fatigue. The most promising fabrication methods for producing UFG-materials in „bulk”- form are methods of severe plastic deformation. During SPD-processing the defragmentation of the microstructure occurs and submicrocrystalline grains are obtained. Additionally, the defect density is increased in the material, and the new microstructure is away from the thermodynamical equilibrium. Due to this fact the material is susceptible to microstructural change during use. For example, deformation induced grain growth at room temperature was reported by various work groups. A change of microstructure is always accompanied by a change in the performance characteristics or reliability of the system. Therefore a concise knowledge about the fatigue properties and microstructural evolution during cyclic loading is important. In this master thesis the fatigue properties and damage evolution of OFHC-Copper is addressed. By electrolytic etching and FIB-machining, micro bending beams with a thickness of 5 microns were fabricated from an UFG-copper disc produced by high pressure torsion. The loading of the samples was done in-situ with a micro-indenter of the company ASMEC. Experiments were conducted solely under displacement-controlled conditions. A sinusoidal strain signal was imposed on the sample. Several experiments with different plastic strain amplitudes in the low cycle fatigue regime were conducted. Dependent on the chosen plastic strain amplitude 80 to 10,000 cycles were carried out. During the experiments the fatigue properties of the material were examined and the damage evolution was traced regularly through scanning electron microscopy. Additional attention was put on the stability of the microstructure. In this area several experiments on macroscopic samples were conducted according to the literature. During the experiments at low plastic strain amplitudes there was still grain growth in specific grains and near geometrical constraints. In the enlarged grains fine slip bands were found. During the experiments the roughness of the surface was increased, because grains were pushed out of the surface. At the grain boundaries of these grains, grooves were formed. This grooves developed into surface cracks as the damage of the sample became more and more distinct. Information about the orientation and the texture of the enlarged grains were gathered with EBSD (electron backscattered diffraction). Due to the small number of enlarged grains only a qualitative evaluation of this data was possible. Further experiments need to be conducted to increase the understanding of the behavior of UFG-materials during fatigue loading. If the UFG-materials exhibit a pronounced sample size effect, when the sample size is decreased further is currently unknown. By means of heat treatment several work groups try to adjust the properties to the specific application and to increase the ductility. How this is affecting the stability of the microstructure isn’t investigated yet.
Translated title of the contributionFatigue properties and microstructure evolution of micro- and nanocrystalline micro beams
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Pippan, Reinhard, Supervisor (internal)
Award date14 Dec 2012
Publication statusPublished - 2012

Bibliographical note

embargoed until null

Keywords

  • ultra-fine-grained
  • severe plastic deformation
  • high pressure torsion
  • OFHC-copper
  • micromechanics
  • fatigue
  • microstructural evolution
  • grain growth
  • in-situ
  • microbending

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