Key Takeaways & Executive Findings
- •• Deep cryogenic treatment (DCT) for 36 hours significantly enhances both tensile strength (by 5.74%) and ductility (by 11.79%) of AlCoCrFeNi2.1 eutectic high-entropy alloy, achieving a balanced improvement in mechanical properties. • DCT increases dislocation density in the FCC phase and induces the formation of fine spherical BCC precipitates within the B2 phase, which are key microstructural mechanisms for strengthening. • The study demonstrates that DCT is an environmentally friendly alternative to traditional heat treatment processes, reducing pollution while improving material performance. • The findings provide practical guidance for applying DCT to strengthen 'lamellar + block' type eutectic high-entropy alloys, expanding their potential for advanced structural applications.
Abstract
As a typical eutectic high-entropy alloy (EHEA), AlCoCrFeNi2.1 exhibits excellent casting properties. However, the imbalance between strength and plasticity hinders its application as an advanced structural material. In order to address this challenge, deep cryogenic treatment (DCT) as a new process applied in the field of EHEAs was proposed in this study. The effects of different DCT times on the microstructure and mechanical properties of AlCoCrFeNi2.1 EHEAs were studied, mainly focusing on the flake structure of FCC+B2 layer. The experimental results suggest that with the extension of the DCT time, the dislocation density in the FCC phase increases significantly. The spherical BCC precipitate phase is generated within the B2 phase, and the average size of this newly generated precipitate phase gradually decreases. Increasing the number of dislocations and precipitate phases is of great significance to improve the mechanical properties. The AlCoCrFeNi2.1 EHEA exhibits excellent comprehensive mechanical properties after DCT for 36 h. Compared with the as-cast state, the tensile strength at room temperature reaches 1,034.51 MPa, increased by 5.74%. The plasticity reaches 21.72%, which is increased by 11.79%. The results show that the tensile strength and ductility of AlCoCrFeNi2.1 EHEAs are balanced and improved after DCT, which are more suitable as advanced structural materials. In addition, the introduction of the DCT process to EHEAs solves the problem of environmental pollution caused by traditional heat treatment process. This study provides useful guidance for using the DCT process to strengthen the mechanical properties of “lamellar + block” type EHEAs.
1. Introduction
AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) has the characteristics of both eutectic alloy and high-entropy alloy. Due to its good casting fluidity, high breaking strength, and high wear resistance, it has excellent application prospects in the preparation of special environmental materials such as ship propellers, icebreakers, and deep-sea drill bits [1-4]. In engineering applications, the balance between strength and plasticity is critical to improving the safety and longevity of metal structural materials [5]. However, the strength and plasticity of cast alloy materials are usually mutually restrictive. Therefore, how to develop a high-entropy alloy with good casting fluidity and high strength plasticity is an important problem in the field of new materials.
At present, there are three mainstream methods to improve the strength and plasticity of high-entropy alloys. (І) Optimizing the alloy composition by adding elements. (II) Utilizing new preparation technologies such as high-energy ball milling, plasma spraying, and rapid solidification. (III) Using heat treatment process to optimize the grain size, grain boundary characteristics, and precipitate phase number of high-entropy alloys to improve the mechanical properties of the alloys. Huo et al. [6] found that by adding of 0.4at.% Zr to the CoCrFeNi alloy, a eutectic structure composed of FCC phase and Laves phase was formed, which can effectively delay high-temperature creep and thus has great potential in high-temperature resistant materials. Tan et al. [7] found that the alternating arrangement of FCC and tetragonal Mn7Pd9 structures in the CoCrFeNiMn alloy with Pd would lead to the weakening of the interfacial energy anisotropy, resulting in a “seaweed-like” eutectic structure. He et al. [8] found that when 0.25at.% Nb was added to the CoCrFeNi alloy, a lamellar structure composed of an FCC phase and a hard HCP phase was formed in the CoCrFeNiNbx high-entropy alloy with good strong plasticity. The compressive yield strength and compressive strain of CoCrFeNiNb0.25 can reach 2,024.6 MPa and 38.8%, respectively. Tao et al. [18] prepared CoNiCrMoNb high-entropy alloys by plasma spraying, and the results showed that the hardness and wear resistance of high-entropy alloys were improved with the increase of amorphous alloy content in the matrix as the spraying power increased. Nassar et al. [9] prepared AlCoCrFeNi2.1 EHEA by rapid solidification technology. It was found that with the increase of cooling rate, the microstructure of AlCoCrFeNi2.1 EHEA gradually changed from a regular eutectic to a “
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Si-ruo Zhang, Cheng-hao Liu, Hao Qi, Hao-kai Wu, Guang-yu Yang, Ting-shuai Tan, Ying-dong Qu, Guang-long Li (2025). Effect of deep cryogenic treatment on microstructure and mechanical properties of AlCoCrFeNi2.1 eutectic high-entropy alloy. China Foundry. https://doi.org/10.1007/s41230-025-4047-8
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Frequently Asked Questions
What is deep cryogenic treatment (DCT) and how does it affect AlCoCrFeNi2.1 eutectic high-entropy alloy?
Deep cryogenic treatment (DCT) is a process that involves cooling the alloy to very low temperatures (typically below -150°C) to modify its microstructure. In AlCoCrFeNi2.1 EHEA, DCT increases dislocation density in the FCC phase and induces the formation of fine spherical BCC precipitates within the B2 phase, leading to improved strength and ductility.
What are the key mechanical property improvements after DCT for 36 hours?
After 36 hours of DCT, the tensile strength at room temperature reaches 1,034.51 MPa, which is a 5.74% increase compared to the as-cast state. The plasticity reaches 21.72%, an 11.79% increase. This demonstrates a balanced improvement in both strength and ductility.
Why is DCT considered an environmentally friendly alternative to traditional heat treatment?
Traditional heat treatment processes often involve high temperatures and can produce harmful emissions or require significant energy. DCT, on the other hand, uses cryogenic temperatures and is a cleaner process, reducing environmental pollution while still effectively enhancing material properties.
What is the significance of the 'lamellar + block' type EHEA in this study?
The study focuses on AlCoCrFeNi2.1 EHEA, which has a 'lamellar + block' microstructure. The findings show that DCT can effectively strengthen this type of alloy, providing guidance for improving the mechanical properties of similar eutectic high-entropy alloys.
What are the potential applications of AlCoCrFeNi2.1 EHEA after DCT?
With improved strength and ductility, the DCT-treated AlCoCrFeNi2.1 EHEA is more suitable for advanced structural materials, including applications in ship propellers, icebreakers, and deep-sea drill bits, where high performance and reliability are required.
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