Metal alloys to support nuclear fusion energy

27 January 2023

Scientists at the US Pacific Northwest National Laboratory (PNNL), the Virginia Polytechnic Institute and State University (Virginia Tech) are investigating tungsten heavy alloys as possible materials for use in advanced nuclear fusion reactors. Before fusion energy can be used as a power source, it will be necessary to develop advanced nuclear fusion reactors that can withstand the high temperatures and irradiation conditions that result from fusion reactions.

Tungsten has one of the highest melting points of any element, but it can also be very brittle. Mixing tungsten with small amounts of other metals, such as nickel and iron, creates an alloy that is tougher than tungsten alone while retaining its high melting temperature. Moreover, thermomechanical treatment of these tungsten heavy alloys can alter properties such as tensile strength and fracture toughness.

The researchers found that a particular hot-rolling technique produced microstructures in tungsten heavy alloys that mimic the structure of nacre – mother-of-pearl found in seashells – which is exceptionally strong. The PNNL and Virginia Tech teams investigated these nacre-mimicking tungsten heavy alloys for potential nuclear fusion applications.

“This is the first study to observe these material interfaces at such small length scales,” said Jacob Haag, first author of the research paper recently published in Scientific Reports. “In doing so we revealed some of the fundamental mechanisms which govern material toughness and durability.” He added: “We wanted to understand why these materials exhibit nearly unprecedented mechanical properties in the field of metals and alloys.”.

To get a closer look at the microstructure of the alloys, Haag and his team used advanced materials characterisation techniques, such as scanning transmission electron microscopy to observe atomic structure. They also mapped the nanoscale composition of the material interface using a combination of energy dispersive x-ray spectroscopy and atom probe tomography.

They found that, within the nacre-like structure, the tungsten heavy alloy consists of two distinct phases: a ‘hard’ phase of almost pure tungsten, and a ‘ductile’ phase containing a mixture of nickel, iron, and tungsten. The research suggests that the high strength of tungsten heavy alloys comes from an excellent bond between the dissimilar phases, including intimately bonded ‘hard’ and ‘ductile’ phases.

“While the two distinct phases create a tough composite, they pose significant challenges in preparing high-quality specimens for characterisation,” said Wahyu Setyawan, PNNL computational scientist and co-author of the paper. “Our team members did an excellent job in doing so, which enable us to reveal the detail structure of interphase boundaries as well as the chemistry gradation across these boundaries.”

“If these bi-phase alloys are to be used in the interior of a nuclear reactor, it is necessary to optimise them for safety and longevity,” said Haag. The findings are already being further expanded in PNNL and in the scientific research community. Multiscale material modelling research is underway at PNNL to optimise structure, chemistry, and test the strength of dissimilar material interfaces, as well as experimental investigations to observe how these materials behave under the extreme temperatures and irradiation conditions of a fusion reactor.

“It is an exciting time for fusion energy with renewed interests from the White House and the private sectors. The research that we do in finding material solutions for pro-longed operations is critically needed to accelerate the realisation of fusion reactors.” said Setyawan.

Image: Researchers have found that a particular hot-rolling technique produced microstructures in tungsten heavy alloys that mimic the structure of nacre - mother-of-pearl found in seashells (courtesy of Scientific Reports)

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