A new generation of electron microscopes is now sharp enough to see individual atomic defects inside tungsten crystals—defects that determine whether the metal bends or shatters under stress. This matters because tungsten is the leading candidate for the divertor, the component inside a fusion reactor that must withstand temperatures above 1000°C, power densities of roughly 20 MW/m², and intense neutron bombardment. At room temperature, tungsten is brittle; it only becomes ductile above its brittle-ductile transition temperature (BDTT). Alloying can lower that temperature, but no one understands why. Without that understanding, engineers cannot rationally design tougher tungsten alloys. The research team—at Oxford, York, and the University of Tokyo—will combine aberration-corrected electron microscopy with 4D-scanning transmission electron microscopy (4D-STEM) to directly image the screw dislocations that control plasticity. By focusing the microscope at nanometre-scale depths, they can see how these dislocations behave and how alloying elements alter them. If successful, this work will provide the first rational, experimentally grounded explanation of tungsten ductility. That could guide the development of alloys for fusion reactor components—and, more broadly, for any high-temperature structural application where refractory metals are essential.
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Fusion reactors have the promise to provide high quantities of power with zero carbon and greenhouse gas emission during generation. In addition to major, international initiatives such as ITER, recent years have seen a proliferation of small and mid-size enterprises presenting innovative approaches to fusion generation. Unlocking the capabilities of fusion power can form a key component of addressing the UN’s Sustainable Development Goal Number 7 “Ensure access to affordable, reliable, sustainable and modern energy for all” and Goal Number 9 “Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation”. The research proposed here seeks to address a question of key importance in developing fusion reactors: What controls plasticity in refractory metals? This understanding will unlock the development of rationally designed alloys that can form key components in a fusion reactor? A key component of a tokamak is the divertor, which is located near the base of the tokamak and extracts heat and ash produced by the fusion reaction, minimizes plasma contamination, and protects the surrounding walls from thermal and neutronic loads. As such the diverter must withstand extremely high temperatures of over 1000 C at steady state, power densities of ~20MW/m2 and substantial neutron irradiation while maintaining its shape and structural integrity. The materials that can withstand such conditions are based on refractory metals, with tungsten being the preferred candidate due to its low activation and excellent sputtering resistance. Most refractory metals, including tungsten, are brittle at low temperatures but can become more ductile at higher temperatures. The temperature associated with this activation is known as the brittle-ductile transition temperature (BDTT) which can be above room temperature and make such materials unusable as a structural component. Alloying can improve the ductility of refractory metals at low temperatures, but there is little rational understanding of why this occurs. The proposed work brings together capabilities at two UK universities (Oxford and York) and one Japanese (University of Tokyo) to finally unlock a rational explanation of refractory metal ductility by directly imaging dislocation core structures. The key new technology that has unlocked the ability to image the screw dislocation is a combination of aberration correctors in electron microscopy allowing a very high semi-angle of convergence in the probe forming optics combined with the emerging technique of 4D-scanning transmission electron microscopy (4D-STEM). The depth of field of an optical system varies inversely proportionately with the square of the numerical aperture, and so the high convergence angles allowed by the latest generation of aberration correctors leads to a very low depth of focus at nanometre scale. This reduced depth of field can be used to focus on features at specific depths in a sample – an approach known in light optics as optical sectioning. Samples produced and with initial characterisation at Oxford will then be examined at high resolution using unique instrumentation at Tokyo guided by simulations and data processing algorithms developed in work led by York. The aim is to develop a rational understanding of tungsten plasticity to guide the development of alloys to enable the formation of a key component in fusion energy.
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