Structural Investigation of Size Effects in Plasticity using Indentation Techniques

Martin Leopold Rester

Research output: ThesisDoctoral Thesis

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It was found that contrary to the predictions of the classic continuum plasticity theory, the plastically deformed zone below nano-, micro- and macroindentations is not self-similar. Rather, different stages of deformation associated with varying sizes of the deformed regions were detected. Examining cross-sections through nanoindentations in copper by means of electron backscatter diffraction technique shows that different characteristic deformation patterns occur. For large nanoindentations a plastically deformed zone consisting of three characteristic regions is found, while for shallow ones only two sections appear. Due to these findings it can be assumed that a change in the deformation mechanism between large and shallow nanoindentations takes place. Analysis of the corresponding hardness data in terms of geometrically necessary dislocations using the Nix-Gao model supports the assumption of a mechanism change. To explain the observed behavior, two models based on possible dislocation arrangements are suggested and compared to the experimental findings. The model presented for large nanoindents is similar to the dislocation pile-up model explaining the Hall-Petch effect, while the model for shallow nanoindentations uses far-reaching dislocation loops to accommodate the indentation. The plastic deformation zone below microindentations can as well be divided into three characteristic regions. Noticeable is, that the dimension of the zone where significant changes of the orientation occur, is proportional to the size of the imprint. For macroindentations the plastically deformed zone consists of only two characteristic regions, showing a structure typical for low and medium deformed face-centered cubic single crystals of pure metals. With increasing load dislocation substructures, exhibiting orientation fluctuations in the micron regime, occur. Summarizing the microstructural results of all examined indentations it becomes apparent that the size of the indentations covers a wide range of the different scales of structural evolution, appearing during the deformation of a single crystal. It seems that the hardness of a material varies with the size of indentation, as the flow stress of a single crystal with the evolving substructure.
Translated title of the contributionStrukturelle Untersuchung des Größeneffekts bei der plastischen Verformung mittels Härteeindruckverfahren
Original languageEnglish
  • Pippan, Reinhard, Assessor A (internal)
  • Clemens, Helmut, Assessor B (internal)
Publication statusPublished - 2008

Bibliographical note

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  • size effect
  • plasticity
  • nanoindentation
  • microindentation
  • macroindentation
  • electron backscatter diffraction technique
  • Nix-Gao model
  • dislocations

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