Factors Affecting the Coarsening and Agglomeration of Alumina Particles in Cu–Al2O3 Composites Prepared with Internal Oxidation

Heat sink materials with outstanding thermal and mechanical properties to withstand very high energetic neutron irradiation and heat fluxes are desirable for future fusion reactors. Specifically, copper (Cu) alloys are potential candidates owing to the high thermal conductivity of the pure base metal. Although different strategies for strengthening Cu alloys are characterized by reduced thermal conductivity, secondary phase strengthening has proved more effective. Among the strengthened Cu alloys, CuCrZr has attracted significant research attention for heat sink applications and has been utilized in the international thermal-nuclear experimental reactor. However, its application in fusion reactors is restricted by their inadequate stability of its strengthening phases at high irradiation and elevated temperatures.

Alumina dispersion strengthened Cu (DS-Cu) alloys have been extensively investigated for heat sink applications. Although it has excellent irradiation resistance and thermal stability than CuCrZr, its mechanical properties are still characterized by some drawbacks at elevated temperatures. As the main sources of beneficial properties of DS-Cu, microscopic characteristics of nano-sized oxides such as shape, size and interfacial structures need further modification. While this can be achieved using several routes like mechanical alloying, the traditional internal oxidation route is more beneficial in reducing contamination and high scalability and mass production capability.

From the microstructure perspective, both the intra- and intergranular of alumina particles in traditional Cu-Al₂O₃ composites fabricated via internal oxidation are highly susceptible to agglomeration and coarsening, which may degrade irradiation and ductility properties. Therefore, it is important to tailor the nano-sized oxides to improve the properties of such DS-Cu fabricated via internal oxidation. This requires a deeper understanding of the relationship between internal oxidation experimental details and the microscopic characteristics.

On this account, Dr. Sixiang Zhao, Ms. Tingxiao Zhang and Mr. Minjing Wang from Lanzhou University of Technology investigated the various factors affecting the agglomeration and coarsening of alumina particles in Cu-Al₂O₃ composites fabricated through internal oxidation. The origins of agglomeration and coarsening were uncovered using a combination of Rhines-pack and powder metallurgy methods. The work is currently published in the journal, Advanced Engineering Materials.

The authors identified the factors contributing to the agglomeration and coarsening of oxide particles in Cu-Al₂O₃ composites manufactured via internal oxidation. These factors included grain boundaries, Al concentration, presence of water vapor and diffusion distance. An increase in the concentration of Al or diffusion distance led to coarsening of the particles. The presence of water vapor exacerbated the agglomeration and coarsening effects. While the grain boundaries were the most preferred sites for particle agglomeration, coarse and agglomerated surfaces were also formed on the powder surfaces during water atomization.

The effects of titanium doping which is less prone to activation when subjected to neutron irradiation were demonstrated. It proved to be an effective strategy for modifying the size, shape and distribution of the intra- and intergranular oxide particles in both dry and wet internal oxidation environments. This further improved the irradiation resistance and mechanical properties of the DS-Cu fabricated through a combination of internal oxidation route and powder metallurgy.

In a nutshell, an experimental study of the origin and factors contributing to agglomeration and coarsening of alumina particles were reported. By paying attention to the potential effects of the contributing factors, the findings revealed the importance of further optimization of the microscopic features of the nano-sized oxides via Ti doping. In a statement to Advances in Engineering, Dr. Sixiang Zhao explained that a combination of internal oxidation and powder metallurgy is a promising path for developing advanced and high-performance DS-Cu for future fusion reactors.