When Tungsten Meets Steel: Designing Interfaces for Fusion

From materials challenge to fusion application

In future fusion reactors, plasma-facing components must withstand extreme heat loads while maintaining structural integrity. Tungsten is the preferred material for the surface exposed to the plasma, while ferritic–martensitic steels serve as the structural backbone. Combining these two materials into a reliable component remains a major engineering challenge.

Additive manufacturing offers a promising route to produce such multi-material systems, enabling complex geometries and tailored interfaces. However, the process involves rapid and repeated heating and cooling cycles, leading to highly non-equilibrium microstructures—particularly at the interface between tungsten and steel.

Phase formation at the tungsten–steel interface

At the boundary between tungsten and steel, the interaction of the two materials under extreme thermal gradients leads to the formation of several distinct phases. The study identifies intermetallic compounds, such as Fe₇W₆, as dominant phases in the interface region, alongside a complex mixture of microstructural zones.

These intermetallic phases are critical: while they form naturally during processing, they are often brittle and can compromise the mechanical integrity of the interface. Their formation is strongly influenced by melt pool dynamics, material mixing, and local thermal conditions during deposition.

The researchers show that processing strategies—such as energy grading during tungsten deposition—can significantly influence the resulting microstructure, offering a pathway to mitigate the formation of undesirable phases and improve interface stability.

Microstructural changes in the steel

The influence of tungsten deposition extends beyond the interface into the underlying steel. In regions close to the interface, the microstructure of the steel is strongly modified: grain coarsening occurs, and minor phases such as retained austenite can form locally, even within a predominantly martensitic matrix.

At the same time, the presence of tungsten in solid solution affects phase transformations during cooling, promoting the formation of large martensitic grains and altering the balance between phases. These subtle but important changes can impact mechanical properties and long-term performance under fusion-relevant conditions.

Capturing phase evolution in real time

To understand how these phases form, the team performed operando X-ray diffraction experiments during the additive manufacturing process. This approach allows the material to be probed while it is being built, providing direct access to transient phase transformations.

The measurements reveal how rapid heating and cooling cycles govern the formation and stability of different phases, linking processing conditions directly to the resulting microstructure. Such real-time insight is essential for understanding the mechanisms behind intermetallic formation and phase evolution at the interface.

Toward controlled interfaces for fusion materials

By combining advanced synchrotron imaging, electron microscopy, and neutron-based techniques, the researchers provide a comprehensive picture of how tungsten–steel interfaces develop during additive manufacturing. This work was carried out through close collaboration across multiple PSI research centers, bringing together expertise in materials science and large-scale characterization methods.

The results highlight that controlling phase formation—both at the interface and within the steel—is key to designing reliable multi-material components for fusion applications. With improved understanding of these processes, additive manufacturing moves closer to enabling the next generation of fusion energy systems.