Recently, researchers at the University of Michigan have developed significant advancements in producing a new iron-chromium steel alloy. This groundbreaking new material is known as castable nanostructured alloy #9 (CNA9). It was the Oak Ridge National Laboratory fusion materials team that had designed this Reduced Activation Ferritic/Martensitic (RAFM) steel. Their overarching aim is to improve the performance and lifespan of materials used in fusion energy environments. The particle accelerator was used for dual ion beam tests by this research team. This new approach better replicates the extreme conditions present in fusion energy environments.
The national project’s aim is to better understand the dose-dependent material behaviors of RAFM steel. Researchers carefully calibrated the levels of radiation damage from one to 100 displacements per atom (dpa). In addition, researchers increased helium concentrations and temperatures to better explore the material’s durability at higher levels of exposure. Kevin Field, a professor of nuclear engineering and radiological sciences at the University of Michigan and a senior author on both studies. Of that new cohort, T.M. Kelsy, PhD, also a doctoral graduate from the same department, stepped up to take the lead as the first author.
Innovative Testing Methodology
For their work, the research team used a synchrotron-based particle accelerator at the Michigan Ion Beam Laboratory. At the same time, they bombarded CNA9 steel samples with both an iron ion beam and a helium ion beam. This dual ion beam technique means that they can better replicate the radiation environment that future fusion reactors will see. The research team adjusted various parameters, including the level of radiation damage and temperature. Their objective was to determine a better understanding of how RAFM steel performs in these extreme conditions.
One critical component of the testing process was to increase levels of radiation damage from a low 1 dpa all the way to 100 dpa. This range enabled researchers to study how varying levels of radiation impact the structural integrity and functionality of CNA9 steel. You have the ability to vary helium concentrations from 10 to 25 atomic parts per million per displacements per atom (dpa). This unique capability allows for an unprecedented study of the interaction of helium with the alloy under irradiation conditions.
Findings on Titanium-Carbide Precipitates
These tests resulted in key findings related to the control of titanium-carbide precipitates in CNA9 steel. These precipitates displayed exceptional thermal stability at elevated temperatures, notably within the range of 500–600 °C. They were consistent at lower radiation doses below 15 dpa. At high damage levels (50-100 dpa), researchers noted the complete dissolution of the titanium-carbide precipitates. Notably, this was the case even in cooler temperatures.
In addition, any helium that wasn’t taken up by the titanium-carbide precipitates bubbled up within the bulk steel itself. This weird behavior led to an overall increase in the alloy’s size of around 2% at peak levels of irradiation. Interestingly, titanium-carbide particles were able to trap some helium as bubbles on their surfaces, which worked best at temperatures around 500 °C.
Implications for Future Research
These results are rich with information that can help improve CNA9 steel for future fusion energy applications. Increasing the concentration of titanium-carbide precipitates by as much as 1,000 times might significantly enhance the material’s resistance to swelling, researchers suggest. This advancement would enable exascale researchers to avoid disasters caused by helium bubble formation. This seemingly small change has the potential to produce a much tougher and more consistent alloy. It would be potentially well-suited to tolerating the extreme conditions encountered in fusion reactors.