BESSY II: Local variations in the structure of High-Entropy Alloys
The analysis of the EXAFS data showed different local environments around the elements of the Cantor alloy depending on the annealing temperature, which indicate different ordering and diffusion processes. Manganese diffuses fastest in the high-temperature state, nickel in the low-temperature state. © HZB
High-entropy alloys can withstand extreme heat and stress, making them suitable for a variety of specific applications. A new study at the X-ray synchrotron radiation source BESSY II has now provided deeper insights into the ordering processes and diffusion phenomena in these materials. The study involved teams from HZB, the Federal Institute for Materials Research and Testing, the University of Latvia and the University of Münster.
The team analysed samples of a so-called Cantor alloy, which consists of five 3d elements: chromium, manganese, iron, cobalt and nickel. The samples of crystalline structures (face-centred cubic, fcc) were annealed at two different temperatures and then shock frozen.
The study focussed on unravelling local atomic structures in single crystalline samples cooled from either a high-temperature (HT) state annealed at 1373 Kelvin or a low-temperature (LT) state annealed at 993 Kelvin. To analyse the local environments of the individual elements in the samples, the team used a well established method: element-specific multi-edge X-ray absorption spectroscopy (EXAFS). To interpret the measurement data in the most precise and unbiased manner, the team carried out a Reverse Monte Carlo (RMC) based analysis.
"In this way, we have been able to reveal, both qualitatively and quantitatively, the peculiarities of the characteristic local environments of each principal components of the alloy at the atomic scale," explains Dr Alevtina Smekhova from HZB. In particular, the spectroscopic results also provide insights into the diffusion processes in HEAs. For example, it was directly demonstrated that the element manganese diffuses fastest in the HT samples, while the element nickel diffuses faster in the LT samples as it was found earlier from diffusion experiments.
"These results help us to better understand the relationship between the local atomic environment and the macroscopic properties in these alloys," explains Smekhova.
- Ultrafast dissociation of molecules studied at BESSY IIFor the first time, an international team has tracked at BESSY II how heavy molecules – in this case bromochloromethane – disintegrate into smaller fragments when they absorb X-ray light. Using a newly developed analytical method, they were able to visualise the ultrafast dynamics of this process. In this process, the X-ray photons trigger a "molecular catapult effect": light atomic groups are ejected first, similar to projectiles fired from a catapult, while the heavier atoms - bromine and chlorine - separate more slowly.
- Protons against cancer: New research beamline for innovative radiotherapiesTogether with the University of the Bundeswehr Munich, the HZB has set up a new beamline for preclinical research. It will enable experiments on biological samples on innovative radiation therapies with protons.
- Battery research with the HZB X-ray microscopeNew cathode materials are being developed to further increase the capacity of lithium batteries. Multilayer lithium-rich transition metal oxides (LRTMOs) offer particularly high energy density. However, their capacity decreases with each charging cycle due to structural and chemical changes. Using X-ray methods at BESSY II, teams from several Chinese research institutions have now investigated these changes for the first time with highest precision: at the unique X-ray microscope, they were able to observe morphological and structural developments on the nanometre scale and also clarify chemical changes.