CityU materials scientists find a new way to make thermally stable alloys with high entropy

CityU materials scientists find a new way to make thermally stable alloys with high entropy

Image: Increased thermal stability in the investigated alloys due to the addition of cobalt (0, 15 and 30 atomic percent (at.%)). a–c Typical SEM images of nanoparticles in the three chemically complex alloys aged at 1000 °C for 240 h and their average diameter. d–f The evolution of the mean size of nanoparticles in the three chemically complex alloys aged at 800, 900 and 1000 °C for different durations (24 h, 72 h, 168 h and 240 h).
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Source: Xiao, B. et al., (Source:

Nanoparticles have been used to develop high-strength materials for structural applications. But these nanoparticles are often thermally unstable, leading to rapid coarsening in a high-temperature environment. The latest research, led by materials scientists at the City University of Hong Kong (CityU), found that adjusting the concentration of cobalt in high-entropy alloys (also known as chemically complex alloys) can protect nanoparticles from rapid coarsening at high temperatures. This novel stabilization strategy opens a new avenue to develop novel thermally stable chemically complex alloys for various technical fields in the future.

Nanoparticle reinforcement technology – strengthening alloys by adding nanoparticles during the alloying process – is recognized as a powerful strategy for creating materials with unique structural and functional properties. This has been widely applied to renew high-strength materials such as advanced aluminum alloys, steels and super alloys. But these fine, nanoscale particles have poor thermal stability and tend to coarsen rapidly at high temperatures, dramatically reducing the load-bearing capacity of the host materials and consequently leading to their rupture or other catastrophic failures.

To overcome this obstacle, a research team co-led by CityU materials scientists recently found that adjusting cobalt concentration can controllably control the “sluggish lattice diffusion” effect of high-entropy alloys in a quantitative manner, essentially preventing nanoparticles from rapid coarsening At high temperatures, temperatures up to 1,000 °C are protected.

“Our results pave a highly effective path for the targeted design of high-performance alloys with excellent thermal and mechanical properties for high-temperature structural applications,” he said dr Yang tao at CityU’s Department of Materials Science and Engineering (MSE), who led the study. The research results were published in the journal nature communication under the title “Achievement of thermally stable nanoparticles in chemically complex alloys by controllable sluggish lattice diffusion”.

The sluggish lattice diffusion effect means that single element diffusion is slower in alloys with higher configurational entropy than in those with lower configurational entropy. This can potentially give several high entropy alloys remarkable thermal stability. But the underlying mechanism of the sluggish lattice diffusion effect is still unknown.

In this study, through a combination of various complementary experimental techniques and theoretical simulations, the research team found that cobalt can effectively induce a unique sluggish lattice diffusion effect in the nickel-cobalt-iron-chromium-aluminum-titanium (NiCoFeCrAlTi) alloy system by reducing the interdiffusion coefficient (a parameter for Description of atomic mobility in a material) of other elements. They found that increased concentrations of cobalt can significantly reduce the average particle size and further improve the thermal stability of these nanoparticles.

Furthermore, adjusting the cobalt concentration led to a significant reduction in the interdiffusion coefficients of all main components of high entropy alloys, especially aluminum, at 800 °C.

The controllable sluggish lattice diffusion strategy developed by the research team can achieve ultra-stable nanostructures in high-entropy alloy systems at 800 to 1,000 °C.

“We have discovered a novel nanoparticle stabilization mechanism that differs significantly from the conventional wisdom that nanoparticle stabilization is achieved through the addition of refractory elements such as rhenium,” explained Dr. Yang.

“This new strategy can further advance the development of novel chemically complex alloys with superior microstructural stability and potentially be applied to other metallic alloys.” This paves the way for the development of strong, high-entropy, next-generation alloys that can be used in an extreme high-temperature environment in various engineering fields such as aerospace, automotive design and nuclear,” he said.

The first author of the study is dr XiaoBo. The corresponding author is dr Yangfrom MSE and Professor Kaijijungby MNE. Other cooperation partners are e.g Professor Liu Chain-tsuan, dr Luan Junhua and dr Zhao Shijunby CityU, and researchers from Central South University, Harbin Institute of Technology (Shenzhen), and Tianjin University.

Funding sources for the research were CityU, Hong Kong Research Grant Council, National Natural Science Foundation of China, and Guangdong Basic and Applied Basic Research Foundation.

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