Oxidstabilisierte nanokristalline Metalle

Translated title of the contribution: Oxide-stabilized Nanocrystalline Metals

Reinhard Fritz

Research output: ThesisDiploma Thesis

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Abstract

Oxide-stabilized nanocrystalline metals are made by consolidation of powders and high pressure torsion (HPT). The naturally formed oxide layer on the surface is used as second phase to stabilize the nanocrystalline matrix. During HPT deformation the oxides get fragmented to nanometer sized particles and remain along the grain boundaries. Thus, grain boundaries are pinned against thermally induced grain growth. Such materials show excellent strength but only moderate ductility. Annealing at relatively large homologous temperatures leads to growth of the grains, wherby the oxide particles get into the grain interior. These dispersion strengthened materials would combine high strength levels with good ductility. However cracks and porosities were formed during the annealing process. Due to entrapped moisture during storage in air or imperfect compaction can be responsible for outgassing. The aim of this work was to find and to evaluate processes which can prevent the forming of porosity and cracks. Therefore an experimental plan was established and with statistical design of experiments factorial tests were performed. As-prepared Nickel powder was annealed in vacuum, consolidated in vacuum and also annealed at different temperatures. After consolidation, the powder was HPT-deformed at elevated temperatures and subsequently annealed under hydrostatic pressure. Of the above mentioned processing possibilities, the porosity could be minimized. The achievement of the investigation is that an annealing process of the as-prepared powder in a vacuum furnace with subsequent HPT-forming at elevated temperatures and subsequent annealing under hydrostatic pressure leads to samples with minimized porosity. Furthermore the formation of pores can be postponed to higher annealing temperatures.
Translated title of the contributionOxide-stabilized Nanocrystalline Metals
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Renk, Oliver, Co-Supervisor (internal)
  • Pippan, Reinhard, Supervisor (internal)
Award date4 Apr 2014
Publication statusPublished - 2014

Bibliographical note

embargoed until null

Keywords

  • oxide-stabilization
  • nanocrystalline
  • powder consolidation
  • SPD
  • severe plastic deformation
  • HPT
  • high pressure torsion
  • pores
  • cracks
  • porosity
  • oxide layer
  • factorial test
  • statistical design of experiments

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