Nanoindentation of ultrafine grained and single crystalline Chromium and Tantalum at elevated temperatures

Josef Pörnbacher

Research output: ThesisDiploma Thesispeer-review

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Abstract

Society’s ever growing demands regarding the abilities and performances of products led materials scientists to explore new materials with outstanding properties. Ultrafine grained materials were subject to intense investigations over the last decades. This rose from the extraordinary characteristics, such as high strength, hardness and further functional features that type of materials offer. The objective of this works was to determine the mechanical behavior of less investigated body centered cubic materials, comparing ultrafine grained chromium and tantalum samples with their single crystalline counterparts, regarding their temperature dependent strength and deformation mechanism. In order to test the mechanical properties of the different materials below and above their specific critical temperature, nanoindentation tests were conducted at room temperature and at up to 300 °C. The resulting indents were investigated by means of atomic force microscopy mode analysis and light microscopy for pile-ups and sink-ins. The actual measurement procedure was load controlled and based on a load and partial unload principle, which enables the determination of hardness and Young’s modulus at several different depths for one indent. These data were fitted with the Nix-Gao model for the indentation size effect, yielding macroscopic hardness and internal length scale values. The macroscopic hardness was further compared with microhardness tests and results from a different nanoindentation system. The ultrafine grained material, both owning an average grain size of ≈ 100 nm, exhibited hardness increases by a factor of 3.5 - 4 at room temperature compared to the single crystals. Tantalum samples of both microstructures exhibited a significant hardening phenomenon at 250 °C and above. Cross sections of the according indents were made using a focused ion beam workstation and examined by scanning electron microscopy to obtain possible oxide growth. Since no significant oxygen layer was found, a theoretical model was developed proposing oxygen diffusion and solid solution hardening as cause for the phenomenon, which describes the experimental data well. Except for the described tantalum hardening, hardness decreased with increasing temperature. While the hardness of ultrafine grained material further decreased between the assumed critical temperature of approximately 200 °C and 300 °C, the single crystalline sample exhibited no further hardness decrease. Load displacement data were also used to analyze incipient plasticity in the case of the single crystals. The shear stress at the onset of plasticity was calculated to be a significant fraction of the theoretical shear stress. Due to the pronounced distribution, however, it was concluded that several mechanisms such as homologous and heterogeneous dislocation nucleation or activation of preexisting dislocations might be accountable. Creep tests were conducted to obtain strain rate sensitivity and activation volume, both feasible parameters to investigate the governing deformation mechanism. Whereas the strain rate sensitivity at room temperature was found to be significantly decreased for the ultrafine grained materials ( ≈ 0.01) compared to the single crystals ( ≈ 0.05), the activation volume was similar ( ≈ 10 b3). This activation volume range is linked to the predominance of the kink-pair nucleation, typical for bcc metals. At 100 °C the strain rate sensitivity decreases for the single crystals, whereas it is raised for the ultrafine grained samples. This was assumed to be the result of diminishing contribution from the Peierls stress in terms of the single crystal, and additionally, the increasing importance of grain boundary contributions in terms of the ultrafine grained sample. However, at 200 °C and above results are assumed to be influenced by thermal drift, exacerbating conclusions from these creep tests.
Translated title of the contributionHochtemperatur-Nanoindentierung von ultrafeinkörnigem und einkristallinem Chrom und Tantal
Original languageEnglish
QualificationDipl.-Ing.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Kiener, Daniel, Supervisor (internal)
Award date30 Oct 2015
Publication statusPublished - 2015

Bibliographical note

embargoed until null

Keywords

  • nanoindentation
  • base-centered-cubic
  • hardness
  • elevated temperatures
  • kink-pair nucleation

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