A precipitate-free AlCoFeNi eutectic high-entropy alloy with strong strain hardening

doi:10.1016/j.jmst.2021.03.005

Abstract

Abstract:

Over recent years, eutectic high-entropy alloys (EHEAs) have intrigued substantial research enthusiasms due to their good castability as well as balanced strength-ductility synergy. In this study, a bulk cast Al_19.25Co_18.86Fe_18.36Ni_43.53 EHEA is developed with fine in-situ lamellar eutectics. The eutectics comprise alternating ordered face-centered-cubic (L1₂) and ordered body-centered-cubic (B2) phases with semi-coherent interfaces. The resulting microstructure resembles that of most reported as-cast EHEAs, but the B2 lamellae are devoid of nano-precipitates because of the Cr-element removal in current tailored eutectic composition. Surprisingly, the B2 lamellae still feature much higher deformation resistance than the L1₂ lamellae, so that less lattice defects are detected in the B2 lamellae until the fracture. More interestingly, in the L1₂ lamellae we identify a dynamic microstructure refinement that correlates to extraordinary strain hardening in tension. The precipitate-free EHEA consequently shows excellent tensile ductility of ∼10 % and high ultimate strength up to ∼956 MPa.

Key words: High-entropy alloy, Lamellar eutectic, Deformation resistance, Microstructure refinement, Strain hardening

Peijian Shi, Yi Li, Yuebo Wen, Yiqi Li, Yan Wang, Weili Ren, Tianxiang Zheng, Yifeng Guo, Long Hou, Zhe Shen, Ying Jiang, Jianchao Peng, Pengfei Hu, Ningning Liang, Qingdong Liu, Peter K. Liaw, Yunbo Zhong. A precipitate-free AlCoFeNi eutectic high-entropy alloy with strong strain hardening[J]. J. Mater. Sci. Technol., 2021, 89: 88-96.

Figures/Tables 9

Fig. 1. Typical eutectic microstructure of the developed as-cast EHEA: (a) BSE-SEM image; (b) EBSD phase map and (c) inverse pole figure.

Fig. 2. (a) HAADF-STEM image showing the alternating dual-phase lamellae; (b) Two SADPs of B2 and L12 phases, and superlattice diffraction spots are marked by circles; (c) EDS maps for individual elements of Fe, Ni, Co and Al.

Fig. 3. Phase-interface characteristics at an atomic scale: (a) HR-TEM image of the L12 and B2 phases; (b) FFT and (c) IFFT images of the marked region in (a). ⊥, misfit dislocation.

Fig. 4. Tensile response of the developed as-cast EHEA. These red circles and blue trigons are the interrupted strain points for SEM and TEM analyses. The inset shows the corresponding strain-hardening curve.

Fig. 5. Typical BSE-SEM images showing the deformation microstructures under two interrupted strains: (a) and (b) ～3 %; (c) and (d) ～6 %.

Fig. 6. Typical BSE-SEM images showing the deformation microstructures of the post-fracture EHEA.

Fig. 7. Microscopic profile analysis (a) and (b), attached to the SEM images of Figs. 5(b) and 6 (d), respectively. The color bar indicates the relative change of surface height.

Fig. 8. Typical TEM images showing the deformation microstructures under different strain conditions: (a-c) ～3 %, (d) ～6 % and (e, f) ～10 %. The phase interfaces are marked by yellow dotted lines.

Fig. 9. Typical SE/BSE-SEM images showing a trench-like fracture morphology featuring the mixed ductile and brittle fracture modes.

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