Mechanical behaviour of ultrafine-grained nickel: The role of purity and strain path

Georg Benedikt Rathmayr

Research output: ThesisDoctoral Thesis


This work is devoted to the mechanical characterisation of severe plastically deformed (SPD) materials as these materials exhibit interesting and outstanding mechanical properties. In the field of high purity materials a lot of research has been conducted to understand the process of grain refinement during SPD. Nearly no work exists currently about how small impurity levels in the material combined with different deformation conditions (deformation temperature, strain rate, strain speed and path, etc) affect the mechanical properties. Therefore systematic studies have been conducted to investigate these influences. As model material high purity nickel (>99.5wt%) was used which is one of the most used materials in the SPD community. Besides the impurity question another research focus was related to the influence of the aligned SPD microstructure on the mechanical properties. The mechanical properties itself have been determined by the use of micro hardness tests and micro tensile tests. The main results of the mechanical characterisation part of this thesis, some of them quite surprising, are as follows. The ultimate tensile strength can be increased from 1000 MPa up to 1700 MPa by adding 0.06 wt% carbon without any detrimental influence on the ductility. Furthermore inclusions do not deteriorate the strength but strongly govern the ductility as shown for inclusion free and inclusion containing tensile samples. A grain aspect ratio orientation dependency is observed for the reduction in area but not for the strength itself. A current problem in the SPD community is the size limitation of the material synthesis which requires tensile tests which are not covered by the ISO or ASTM standards. As different sample geometries, sample fabrication methods and test setups are used a comparison of results obtained from different research groups is nearly impossible due to the experimental uncertainties. Therefore a new tensile sample fabrication method is presented based on a circular grinding process. This tool provides the possibility to get perfectly shaped samples with cross section diameters ranging from 3 mm down to 100 µm. Furthermore a contact free, sample related video based displacement measurement system is presented. This system requires no additional markers on the sample as the sample contour itself is used. Furthermore the evaluation of true stress strain curves is possible with this new developed procedure.
Translated title of the contributionMechanische Eigenschaften von hochverformtem, nanokristallinem Nickel: Der Einfluss von Reinheit und Verformungspfad
Original languageEnglish
  • Clemens, Helmut, Assessor A (internal)
  • Pippan, Reinhard, Supervisor (internal)
Publication statusPublished - 2012

Bibliographical note

no embargo


  • severe plastic deformation
  • high pressure torsion
  • nickel
  • impurity
  • micro tensile test

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