Nanoindentation of ultra-fine grained and nanoporous Au at elevated temperatures

Alexander Leitner

Research output: ThesisDiploma Thesispeer-review

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Abstract

Nanostructured materials provide unique mechanical properties and hence are convenient for high-performance applications. However, high temperature data is seldom and the governing deformation mechanisms are insufficiently explored so far. In order to investigate the behavior of face centered cubic metals in harsh environments, such as high temperature conditions, Au lend itself as an appropriate candidate to study deformation dynamics due to its high oxidation resistance. The influence of different interphases was examined by opposing ultra-fine grained gold (ufg Au) with a grain size of about 250 nm to nanoporous gold (np Au). Gold powder was used as a base material and subsequently consolidated via high pressure torsion. Likewise a Au/Fe powder mixture was densified to create a nanostructured composite. For the latter a subsequent selective etching process removed the iron and left behind a novel foam with ligaments in the order of 100 nm and a porosity of 50 %. Additionally, a sample each with coarse grained (cg) and nanocrystalline (nc) structure was prepared through eligible annealing and by adding low amounts of Cu, respectively. The following microstructural evolution, using amongst other scanning electron microscopy, electron backscatter diffraction and energy dispersive X-ray spectroscopy, allowed a precise characterization of the fabricated nanomaterials. Depth sensing nanoindentation enables the determination of fundamental mechanical properties such as hardness and Young's modulus. Strain-rate sensitivity m and activation volume V* were inferred from relaxation data of a hold segment at maximum. The latter two are feasible parameters for the indication of the predominating deformation mechanism. Accurate nanoindentation experiments were performed at ambient temperature for all four types of samples and at high temperature up to 300 °C for ufg Au as well as np Au. Subsequently, cross-sections of indents were prepared with a focused ion beam. Those allowed to show the plastic deformation beneath the indenter tip. In case of np Au porosity maps could be created to illustrate zones of significant densification. The calculated yield strength of the ligaments of nanoporous Au at RT was determined to be 1.6 GPa and thus close to the theoretical strength of gold. A layer was absorbed to the surface at HT, most likely through insufficient thermal stability of ceramic components of the experimental setup. Nevertheless, nanoindentation tests provide feasible data and exhibit a decrease of hardness at elevated temperatures. Ufg Au exceeds the hardness of its coarse grained counterpart by far. High temperature data show, however, that the hardness decreases to less than 15 % with regard to the RT value of 1.6 GPa when 300 °C are reached. Within the entire tested temperature range np Au and ufg Au show a significant strain-rate dependence (m > 0.03) and low activation volumes (<75 b³). Hence, it is suggested that for both, np Au and ufg Au, that the plastic deformation is controlled by dislocation/interphase interactions. While the microstructure has a crucial impact on the governing deformation mechanism, the type of interphase does not have a decisive influence. On the other hand, cg Au exhibits a high activation volume at RT (> 100 b³), which is associated with the cutting of forest dislocations. Eventually, this study demonstrates the influence of interphase type and structure size on the mechanical properties and deformation dynamics of ultra-fine grained and nanoporous face centered cubic metals. Thus, this thesis contributes considerably to a better understanding of the mechanical behavior of novel metal nanostructures at room and elevated temperatures.
Translated title of the contributionHochtemperatur Nanoindentierung von ultrafeinkörnigem und nanoporösem Au
Original languageEnglish
QualificationDipl.-Ing.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Kiener, Daniel, Supervisor (internal)
Award date19 Dec 2014
Publication statusPublished - 2014

Bibliographical note

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Keywords

  • nanoindentation
  • nanoporous Au
  • ultra-fine grained Au
  • strain-rate sensitivity
  • activation volume
  • porosity maps

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