Researchers found that in both materials, tiny regions with different atomic structures form during heat treatment, but that are symmetrically related. The mismatch that evolves between these regions generates internal strain strong enough to create new, smaller grains within the metal – without first rolling, forging or mechanically deforming it.
Project leader Sandy Knowles, Professor in Nuclear Materials from the University of Birmingham, said: "Our discovery challenges conventional understanding that grain refinement typically requires extensive thermomechanical processing. We show that the PIX mechanism can be used to refine grain structure in varied materials systems.
"We can design alloys where strain is generated internally during heat treatment, opening exciting possibilities for materials that are difficult to process using conventional methods, including refractory metals such as tungsten and advanced alloys for aerospace applications, particularly for net-shape manufacturing."
Higher performance materials
Grain size plays a critical role in material performance. Large grains can provide easier paths for cracks to spread, while smaller grains create more barriers that can improve mechanical reliability and resistance to damage.
Tungsten is regarded as a leading candidate material for future fusion reactors because of its exceptional heat resistance and very high melting point of 3,422°C. However, it can be brittle, and radiation exposure can worsen this brittleness. PIX offers an alternative means of improving the grain size of tungsten-based alloys in extreme environments.
Ageing the tungsten-chromium alloy at 1,250°C generated enough internal stress to drive recrystallisation, reducing average grain size by around 60%.
Researchers also demonstrated the same underlying principle in a titanium-iron-molybdenum 'bcc-superalloy' – a class of material being explored for high-performance aerospace applications such as jet-engine compressor blades. Ageing the alloy at 750°C caused its average grain size to fall by around 90%, while increasing hardness by 60 HV (Vickers Hardness test).
PIX may represent a broader materials-design principle that could be applied across different alloy systems, giving researchers a new way to control grain structure and material properties through heat treatment alone. It could be particularly useful for materials that are brittle, difficult to shape, or produced using additive manufacturing techniques where traditional rolling and deformation processes may not be practical.
