Steel used in nuclear and high-temperature power applications must do more than carry load. It must resist heat, stress, ageing, and in nuclear service, radiation damage. One of the biggest challenges is that neutron irradiation often makes steel stronger but less ductile. In simple terms, the steel hardens, but it can also become more brittle.

A recent study on Grade 91 ferritic/martensitic steel offers an important new insight into how this tradeoff might be reduced.

Researchers compared conventionally cast Grade 91 steel with Grade 91 made using powder metallurgy with hot isostatic pressing, commonly known as PM-HIP. After neutron irradiation, the PM-HIP steel showed unusually strong ductility retention. At around 1 dpa, a standard measure of radiation damage, the PM-HIP Grade 91 retained more than 90% of its original uniform elongation, while still showing irradiation-induced strengthening.

The reason appears to lie in a nanoscale feature called a linear complexion.

A linear complexion is a tiny chemical and structural change that forms along a dislocation line inside the steel. Dislocations are crystal defects that allow metals to deform plastically. When their movement is blocked too severely, steel becomes stronger but less ductile.

In the PM-HIP Grade 91, silicon atoms moved to screw dislocations during irradiation and formed silicon-rich β-FeSi₂ linear complexions. Instead of simply blocking dislocation movement, these features appear to pin and release dislocations in a more controlled way during deformation. This helped the steel continue to deform rather than losing ductility rapidly.

This is important because irradiation damage is usually seen as purely harmful. Here, the material’s processing route helped create a microstructure that responded to irradiation in a more useful way.

For steel professionals, the message is clear: manufacturing route, chemistry, and microstructure must be considered together. PM-HIP was not just an alternative production method in this study. It helped create the defect structure needed for the beneficial irradiation response.

The finding does not mean that PM-HIP Grade 91 is automatically ready for every nuclear application. More work is needed to understand long-term stability, higher-dose behaviour, weldability, and component-scale performance. However, the research points to an exciting direction: steels that are designed not only to resist radiation damage, but to develop helpful nanoscale features during service.

If this mechanism can be reliably controlled, it could support the next generation of radiation-tolerant steels for nuclear power, advanced reactors, and other demanding high-temperature environments.