News Source: Cao Tangqing, Xue Yunfei, School of Materials Science and Engineering

Photography: School of Materials Science and Engineering

Editor: Mou Xuejiao

Reviewer: Cheng Xingwang

Recently, Prof. Yunfei Xue from School of Materials Science and Engineering, Beijing Institute of Technology, together with Prof. Xiaoyan Li from School of Aeronautics and Astronautics, Tsinghua University, published a research paper in Acta Materialia, the top journal in the field of metallic materials, entitled "Dynamic deformation behaviors and mechanisms of CoCrFeNi high-entropy alloys". Dr. Tangqing Cao of BIT and Dr. Qian Zhang of Tsinghua University are the co-authors, and Prof. Yunfei Xue and Prof. Xiaoyan Li are the co-corresponding authors of this paper.

The research team investigated the deformation behavior of CoCrFeNi high-entropy alloys (HEAs) with face-centered cubic (FCC) structure across a range of eight orders of magnitude of strain rate (5.0×10-5 /s-6.5×103 /s) by quasi-static and dynamic mechanical tests. It is found that (i) the CoCrFeNi HEA exhibited a more remarkable strain-hardening capability than the conventional FCC metal, and (ii) the strain-rate sensitivity coefficient shift corresponds to a significantly lower critical strain rate than that of the conventional FCC metal. Based on these findings, the research team combined experimental characterization and large-scale molecular dynamics simulations to investigate the microscopic mechanism of the dynamic mechanical response of typical FCC-phase HEAs.

It is shown that the significant strain rate effect of FCC HEAs is mainly related to the continuous lattice distortion effect of HEAs. On the one hand, the resistance to dislocation motion in HEAs is mainly the short-range stress field brought about by this continuous lattice distortion, which acts within only a few tens of atoms, so this resistance is strongly affected by thermal activation, i.e., it exhibited an obvious rate correlation; on the other hand, the high phonon drag effect brought about by the continuous lattice distortion leads the high-entropy alloy to approach the upper limit of the dislocation velocity earlier during the deformation at high strain rate, forcing the dislocations to proliferate rapidly leading to the strain-rate sensitive coefficient of the sudden increase in the phenomenon of the shift to a low strain rate; the significant strain-rate reinforcement of the FCC HEAs and the characteristics of the sudden increase in a much lower strain rate can take place, so that it is more likely to reach the threshold of the twisted-crystal-activated stress during the loading at a high strain rate, inducing a large number of deformed nanotwins and generating a dynamic Hall-Peach effect, resulting in the significant enhancement of work hardening capacity.

The study sheds light on the plastic deformation mechanisms of HEAs under dynamic loading, providing guidance for the design and fabrication of HEAs with excellent dynamic mechanical properties.

https://www.sciencedirect.com/science/article/abs/pii/S1359645423006730

（DOI: 10.1016/j.actamat.2023.119343）

Fig. 1. (a-b) Variations in yield stress and flow stress at a strain of 5% with strain rate; variations in flow stresses at different strains with strain rate; (c-d) Dislocation evolution during deformation of the simulated sample at different strain rates.

Fig. 2. Microstructures of CoCrFeNi HEAs after deformation at different strain rates. Under dynamic loading, deformation twins of CoCrFeNi HEA occur earlier, more and even secondary twins are formed: (a) deformed structure at 4500s-1 strain rate; (b) deformed structure at 10-3s-1 strain rate; (c) the difference distribution statistics show that the twin content is higher under dynamic conditions.