TY - THES

T1 - High temperature mechanical testing of novel Cu-Nb nanocomposites

AU - Primorac, Mladen-Mateo

N1 - embargoed until null

PY - 2013

Y1 - 2013

N2 - In general, composites are promising engineering materials for prospective purposes in various fields, ranging from applications in medicine to the energy technology. Furthermore, it is well known that a decrease of the structure size of a material to the nanocrystalline (nc) or ultrafine grained (ufg) range results for example in a higher hardness, or in better physical properties. Thus, it is evident that a composite material with a structure size in the nc or ufg regime should possess outstanding properties. In previous investigations it was shown that nanoscaled body centered cubic (bcc) – face centered cubic (fcc) composites exhibit such outstanding properties in various fields. One of these materials is the composite assembled by the fcc element copper (Cu) and the bcc element niobium (Nb). Layered Cu-Nb composites have shown, for example, a high thermal stability as well as a high radiation damage tolerance, making them interesting for prospective use in nuclear reactors. The aim of this work is to create a nanocristalline Cu-Nb composite for high temperature applications in harsh radiation environments. The composite is manufactured via a severe plastic deformation process. In order to create a bulk Cu-Nb composite in the ufg regime, the two step high pressure torsion technique was used. The microstructure was examined in the scanning electron microscope, exhibiting a grain size of approximately 100 to 200 nm. After first microhardness measurements at room temperature, high temperature nanoindentation to a maximum testing temperature of 500°C was used to investigate mechanical properties as a function of temperature. The focus was set on basic elastic and plastic properties – Young’s modulus and hardness – as a function of time to estimate the maximum operating temperature of the composite. With 5.33 GPa the hardness at room temperature is higher compared to comparable composites, pure Cu, or Nb. Furthermore, rate-depending material parameters – strain rate sensitivity and activation volume – were determined to examine the governing mechanism for plastic deformation, giving more detailed insights into the materials behaviour. Finally, the activation energy for plastic deformation was evaluated. The plastic deformation is governed by an interaction of dislocations with subgrains and grain boundaries. With increasing testing temperature, the strain rate sensitivity raises to a maximum of 0.106 value at 400°C, indicating a deformation governed by thermally activated dislocation interaction in a bimodal microstructure. This increase is followed by a drop to 0.069 at the maximum testing temperature of 500°C, indicating a coarsening of the microstructure, and the limit of thermal stability of the composite. These findings were confirmed by the temperature dependent changes in hardness, activation volume, and activation energy.

AB - In general, composites are promising engineering materials for prospective purposes in various fields, ranging from applications in medicine to the energy technology. Furthermore, it is well known that a decrease of the structure size of a material to the nanocrystalline (nc) or ultrafine grained (ufg) range results for example in a higher hardness, or in better physical properties. Thus, it is evident that a composite material with a structure size in the nc or ufg regime should possess outstanding properties. In previous investigations it was shown that nanoscaled body centered cubic (bcc) – face centered cubic (fcc) composites exhibit such outstanding properties in various fields. One of these materials is the composite assembled by the fcc element copper (Cu) and the bcc element niobium (Nb). Layered Cu-Nb composites have shown, for example, a high thermal stability as well as a high radiation damage tolerance, making them interesting for prospective use in nuclear reactors. The aim of this work is to create a nanocristalline Cu-Nb composite for high temperature applications in harsh radiation environments. The composite is manufactured via a severe plastic deformation process. In order to create a bulk Cu-Nb composite in the ufg regime, the two step high pressure torsion technique was used. The microstructure was examined in the scanning electron microscope, exhibiting a grain size of approximately 100 to 200 nm. After first microhardness measurements at room temperature, high temperature nanoindentation to a maximum testing temperature of 500°C was used to investigate mechanical properties as a function of temperature. The focus was set on basic elastic and plastic properties – Young’s modulus and hardness – as a function of time to estimate the maximum operating temperature of the composite. With 5.33 GPa the hardness at room temperature is higher compared to comparable composites, pure Cu, or Nb. Furthermore, rate-depending material parameters – strain rate sensitivity and activation volume – were determined to examine the governing mechanism for plastic deformation, giving more detailed insights into the materials behaviour. Finally, the activation energy for plastic deformation was evaluated. The plastic deformation is governed by an interaction of dislocations with subgrains and grain boundaries. With increasing testing temperature, the strain rate sensitivity raises to a maximum of 0.106 value at 400°C, indicating a deformation governed by thermally activated dislocation interaction in a bimodal microstructure. This increase is followed by a drop to 0.069 at the maximum testing temperature of 500°C, indicating a coarsening of the microstructure, and the limit of thermal stability of the composite. These findings were confirmed by the temperature dependent changes in hardness, activation volume, and activation energy.

KW - Nanoverbundwerkstoffe

KW - Nanoindentierung

KW - Dehnratenempfindlichkeit

KW - Aktivierungsvolumen

KW - nanocomposite

KW - nanoindentation

KW - strain rate sensitivity

KW - activation volume

M3 - Diploma Thesis

ER -