Australian and Chinese researchers have made progress in understanding the mechanical properties of a new class of materials for use in molten salt reactors (MSRs). The Australian Nuclear Science and Technology Organisation (Ansto) said yesterday that NiMo-SiC alloys - prepared from nickel molybdenum metal powders with added silicon carbide particles - have superior corrosion resistance and radiation damage resistance.

Although there are no commercial MSRs in operation, there is an MSR and thorium energy research and development program at the Shanghai Institute of Applied Physics (Sinap), with which Antso has a partnership agreement. A number of Ni-MoSiC alloy specimens containing varying amounts of silicon carbide were prepared in Sinap laboratories before being characterised at Antso.

"Structural materials for MSRs must demonstrate strength at high temperatures, be radiation resistant and also withstand corrosion," Antso said.

In a paper published in Materials and Design, researchers from the two organisations reported that NiMo-SiC alloys "possess superior mechanical properties owing to the precipitation, dispersion and solid-solution strengthening of the NiMo matrix".

The preparation includes mechanical alloying, spark plasma sintering, rapid cooling, high temperature annealing and water quenching.

"Although the dispersion strengthening benefits of adding silicon carbide particles to nickel had been known, it was not considered satisfactory for use in MSRs due to the low high-temperature strength," Antso said. This issue has been addressed in the newly developed NiMo-SiC alloys by the presence of nickel silicide nano-particles, which "fill the space between silicon carbide particles, and thus hinder the dislocation motion", it added.

These alloys are developed via a powder metallurgy process and the silicon carbide and nickel silicide are uniformly distributed within the NiMo matrix, it said. Such uniform distribution of silicon carbide particles would not be possible to achieve using standard metallurgy processes, it added.

"The strength of these alloys stems therefore from the combination of dispersion strengthening by silicon carbide particles, precipitation strengthening by nickel silicide and solid-solution strengthening by molybdenum," it said.

In addition, the NiMo matrix is strengthened by the presence of molybdenum atoms, which form the solid-solution of NiMo.

"As well as superior high-temperature strength, these newly developed alloys have superior corrosion resistance and radiation damage resistance," Antso said. "The nano-particles present in the microstructure not only provide the obstacles for dislocation motion, but also provide sites/traps for radiation damage effects," it added.