Acta Student Award for research on oxygen effects in complex alloys
Yujun Zhao honoured for her paper in the journal Scripta Materialia
At a glance:
- Award: Acta Student Award 2025 by the journal Scripta Materialia.
- Awardee: Dr Yujun Zhao, postdoctoral researcher at the Max Planck Institute for Sustainable Materials.
- Research question: How does oxygen influence the microstructure and properties of HfNbTaTiZr high-entropy alloy when exposed to high temperatures over extended periods?
- Outcome: Oxygen triggers the sequential formation of new phases, ultimately leading to hardening effects.
Dr Yujun Zhao, postdoctoral researcher at the Max Planck Institute for Sustainable Materials, has received an Acta Student Award 2025 for research that advances our understanding of high-entropy alloys - a class of metallic materials with exceptional strength and thermal stability that could play an important role in future aerospace applications.
The award-winning publication investigates how oxygen influences the microstructure and mechanical properties of a high-entropy alloy consisting of hafnium, niobium, tantalum, titanium and zirconium (HfNbTaTiZr) when exposed to high temperatures over extended periods. The findings reveal that oxygen is not merely an unwanted impurity, but actively drives structural changes within the material, affecting its long-term performance.
Understanding materials for extreme environments
High-entropy alloys, commonly containing several principal elements in nearly equal proportions, can combine high strength, good ductility and excellent thermal stability, making them attractive candidates for demanding applications such as aerospace engines and other high-temperature technologies.
The single-phase HfNbTaTiZr alloy has been considered as a competitive alternative for conventional superalloys. At elevated temperatures (> 1100 °C), it forms a stable body-centred cubic crystal structure. However, the constituting elements have a high affinity for oxygen, which could diffuse into the alloy and potentially affect the alloy's performance during long-term exposure to high temperatures.
Nonetheless, the oxygen ingress in this high-entropy alloy has been mainly focused in the context of surface oxidation.
Revealing the influence of oxygen on phase stability
Zhao and her colleagues introduced oxygen dynamically into the bulk at 1200°C, and the resulting precipitation sequence was unravelled by combining transmission electron microscopy and atom probe tomography.
“We revealed that oxygen is absorbed from the surrounding atmosphere and induces the formation of a zirconium-hafnium rich hexagonal-close packed phase”. explains Zhao. As oxygen uptake continues over time, the hexagonal-close packed (hcp) phase becomes increasingly enriched in both titanium and oxygen. This compositional evolution is likely driven by the formation and stabilization of ordered oxide precipitates with a structure similar to Ti3O within the hcp phase. This nanoscale ordered phase increases in number and size during prolonged ageing.
Stronger through oxygen
The study shows that the newly formed ordered phase and dissolved oxygen atoms reinforce the hcp phase, making it harder.
These observations highlight the crucial role of oxygen in governing phase stability and strengthening at high temperatures. Furthermore, this work shows the potential of exploiting nanometric interstitial oxides in high-entropy alloys and opens up new pathways to produce high-entropy composites for structural applications in future.
Recognition for outstanding early career research
The study was recognised with an Acta Student Award, which honours exceptional contributions by student researchers who published in one of the Acta journals. Each year, up to 16 awards are presented to students, highlighting excellence in materials science and engineering at an early stage of a research career. The awardees are encouraged to attend The Minerals, Metals and Materials Society (TMS) Conference of the following year and are supported by a reimbursement of their travel expenses.
Yujun Zhao conducted this research mostly at the Ruhr University Bochum during her PhD, where she was supervised by Prof. Tong Li and Prof. Guillaume Laplanche. She joined the Max Planck Institute during the final stage of her PhD and now works as a postdoctoral researcher.
