Research Highlight

Order in the Chaos: Ultraelastic Chemically Complex Alloy from Atomistic Scale Ordering
Division of Mathematics and Physical Sciences: Novel Chemically Complex Material

Alloy materials have played a crucial role in the evolution of civilization since the Bronze Age. However, the sizes of atoms in alloys cannot differ too much, otherwise stable crystal materials cannot be formed. Dr. Chun-Wei Pao from the Research Center for Applied Science and international collaborators have discovered a new approach to developing novel complex alloys by ingeniously arranging atoms at atomic scale. Through this method, they were able to combine five different types of atoms with a size mismatch up to 11%, while forming a stable and highly elastic complex alloy. This discovery provides a fresh perspective on the development of future complex materials.

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Alloy materials have been continuously developed and improved since the Bronze Age, and high entropy alloys (HEAs) - which are composed of multiple elements in similar concentrations - have emerged as an important breakthrough in materials science in recent years. The concept of HEAs was first proposed by Professor Yeh, Jien-Wei from the National Tsing Hua University in 2004. Since then, researchers have been working to further deepen the understanding and applications of HEAs. In addition to HEAs, the materials science community is actively developing other chemically complex materials, such as high entropy ceramics and 2D materials, which are expected to have many promising applications in fields such as batteries, hydrogen production, and optoelectronics. The Research Center for Applied Sciences (RCAS) has also been actively involved in this emerging field. RCAS researcher Dr. Pao, Chun-Wei and his international collaborators have successfully developed a new ultraelastic chemically complex alloy by collaborative efforts of theoretical simulations and experiments. This new alloy can withstand larger external stresses and strains, has an extremely high elastic limit, and has very low elastic energy losts (Figs. 1B, C). The research findings have been published in the renowned international academic journal Nature in 2022.

The new alloy is composed of five elements - cobalt, nickel, hafnium, titanium, and zirconium (Fig. 1A). Both computer simulations and experiments indicate that, despite of a gigantic atomic size mismatch of up to 11%, the crystal structure is stable through cleverly arranged short-range chemical order (Fig. 2 and Fig. 3A). The atomic scale simulations also indicate that the huge atomic size differences make the constituent atoms of the alloy endure lattice distortions of up to 9% (Fig. 3B), far higher than the typical 2% observed in most HEAs. This makes it difficult for permanent deformation to occur due to the increased difficulty of atomic dislocation gliding. This new alloy is different from traditional metals that soften after being heated, exhibiting the Elinvar effect. Even when heated to around 726 degrees Celsius, its rigidity remains comparable to that at room temperature. The elastic limit of this new alloy is one of the highest known standardized strengths, and as such, it could be used in the future for high-precision components in industries such as consumer goods, biomedicine, energy, and aerospace.

Figure of Paper
A. Co, Ni, Hf, Ti, and Zr can form homogeneously-mixed complex alloy.
B. this complex alloy yields an ultra-high yield strain of c.a. 2%;
C. the extraordinary elastic limit of this complex alloy outperforms conventional alloy materials and is compatible with bulk metallic glasses.