High temperature mechanical testing of novel Cu-Nb nanocomposites

In previous investigations it was shown that nanoscaled body centered cubic (bcc) – face centered cubic (fcc) composites exhibit outstanding properties, for example higher hardness or better physical properties. 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, as can be seen in Fig. 1.

Fig. 1: Backscattered scanning electron microscopy image showing the microstructure of the composite. The grain size is in the range between 100 and 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 as shown in Fig. 2.

 

 

 

 

 

Fig. 2: Hardness of the investigated Cu-Nb composite (purple), different Cu-Nb composites (green), pure Cu (orange), and pure Nb (silver) 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 sub-grains and grain boundaries. With increasing testing temperature, the strain rate sensitivity raises to a maximum value of 0.106 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, as can be seen in Fig. 3. These findings were confirmed by the temperature dependent changes in hardness, activation volume, and activation energy.

 

 

 


Fig. 3: Strain rate sensitivity over temperature; show-ing a increase to 400°C (thermally activated disloca-tion processes at grain boundaries), followed by a decrease at 500°C (coarsened microstructure) The manufacturing, characterisation by electron microscopy and the microhardness testing was carried out at the Department of Materials Physics at the Montanuniversität Leoben. High temperature nanoindentation testing was performed at the Department for Nuclear Engineering at the University of California, Berkeley, USA.

 

 

 

 

Recent results on this research field can be found in the publications below or, for academic use only, in the manuscripts at the bottom of the page:

http://www.sciencedirect.com/science/article/pii/S092150931401507X

doi:10.1016/j.msea.2014.12.020

 

http://www.sciencedirect.com/science/article/pii/S0925838814030084

doi:10.1016/j.jallcom.2014.11.193

 

Acknowledgement: The master thesis was partly founded by the Marshall Plan Foundation, the Montanuniversität Leoben and the country of Lower Austria.