Numerische Analyse der Martensit Morphologie in nanokristallinen NiTi Legierungen

Translated title of the contribution: Numerical analysis of the martensite morphology in nanocrystalline NiTi alloys

Wolfgang Pranger

Research output: ThesisDiploma Thesis

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Abstract

NiTi are the most common shape memory alloys. It transforms upon cooling from the cubic orderd lattice structure of austenite to the monoclinic martensite lattice. The resulting stresses are relaxed by twinning mechanisms. In nanocristalline NiTi with a grain-size of about 100nm so called (001) compound twins occur, which are not observed in coarse-grained NiTi. Different morphologies may appear depending on the grain-size. While in smaller grains (less than 100nm) a single laminate of alternating variants of twinned martensite can be found, a sequence of such laminats is formed in larger grains (greater than 100nm). The latter is called herringbone structure due to their angled position with relative to each other resembling a fishbone pattern. The different morphologies are analyzed in terms of the energies involved. An important part is the elastic strain energy occuring upon the transformation from austenite to martensite. This transformation is mathematically described by the deformation gradient leading to the transformation strains. The elastic strain energies of the different morphologies are calculated with the finite element method. To carry out a parameter study it is necessary to automatize the creation und analysis of the models. To this end the recovery and replay files of ABAQUS CAE are modified by a Visual Basic for Applications program. The other energy terms like the surface energies und dissipated energy are added to the elastic strain energy. This allows to calculate the grain-size for the morphology change and the optimum parameters of the herringbone structure. The results of the simulation are in good agreement with the experimental data.
Translated title of the contributionNumerical analysis of the martensite morphology in nanocrystalline NiTi alloys
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Dehm, Gerhard, Supervisor (internal)
  • Antretter, Thomas, Co-Supervisor (internal)
Award date16 Dec 2011
Publication statusPublished - 2011

Bibliographical note

embargoed until null

Keywords

  • NiTi
  • twinning
  • phase transformation
  • elastic strain energy
  • finite element method
  • micromechanic

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