Research background

    High-entropy alloys (HEAs) possess numerous advantages including high strength, excellent hardness, superior wear resistance, strong thermal stability, and good corrosion resistance, demonstrating significant application potential and being recognized as one of the breakthroughs in the three major alloying theories. Currently, the characterization of material mechanical properties is predominantly based on uniaxial stress states. However, in practical applications, materials often experience complex stress states. When subjected to impact loads, mechanical property parameters derived from uniaxial stress states become unreliable references, and designing structures based on these parameters is unsafe. Scientific research employing the decomposition of complex stress states into simple stress states followed by geometric superposition fails to adequately reflect the mechanical behavior of composite components under actual working conditions. Therefore, investigating the mechanical properties of materials under dynamic biaxial loading has become increasingly important. Although there are numerous studies on the mechanical properties of high-entropy alloys under uniaxial loading, research on the mechanical properties of CoCrFeNi high-entropy alloys remains limited, particularly regarding their biaxial mechanical properties which are almost entirely unexplored. Characterizing the mechanical properties of CoCrFeNi high-entropy alloys through quasi-static and dynamic biaxial tensile tests holds significant scientific research value.

     In 2024, a research team from Northwestern Polytechnical University published an article titled "Biaxial Tensile Behavior of CoCrFeNi High-Entropy Alloy under Dynamic and Proportional Loadings" in the Chinese Journal of Aeronautics. The researchers designed a biaxial dynamic cross-shaped test specimen suitable for large plastic deformation, investigated the effects of strain rate and stress state on the mechanical properties of the alloy, and validated the applicability of the von Mises yield criterion and the Hill48 yield criterion.

    This paper mainly introduces the quasi-static biaxial tensile part related to the research field of our company. It is worth noting that the experimental research conducted by the researchers was completed using our company's equipment.

https://doi.org/10.1016/j.cja.2024.06.003

Li H, Gao L, Qi L, et al. Biaxial tensile behavior of CoCrFeNi high-entropy alloy under dynamic and proportional loadings [J]. Chinese Journal of Aeronautics, 2024, 37(9): 475-89.

Test material,experimental material 

    The research team smelted high-purity metal raw materials (Co, Cr, Fe, Ni) in a vacuum induction furnace, then poured the molten alloy into molds to produce cast bars with a diameter of 100mm and a height of 420 mm. The cast bars were heated to 1150°C for forging, annealing, wire cutting, and polishing, resulting in HEA sheets with a diameter of 160 mm and a thickness of 2.5 mm, as shown in Figure 1.

                 Figure 1 CoCrFeNi high-entropy alloy finished sheet

    The researchers prepared a CoCrFeNi high-entropy alloy and sectioned its central region. After inlaying, polishing, and etching with FeCl3 solution for 15 seconds, the grain boundaries became clearly visible. Microscopic examination revealed that clear sections were photographed from the sample center. All grain sizes within each image were statistically analyzed, and the average grain size was calculated as 174.16 μm, as shown in Figure 2.

Figure 1 Grain structure of CoCrFeNi high-entropy alloy

    The researchers characterized the atomic ratios of individual elements using energy dispersive spectroscopy (EDS) under a scanning electron microscope (SEM). The results from both surface and spot scans demonstrated that the four elements in the prepared CoCrFeNi high-entropy alloy were uniformly distributed with nearly equal atomic ratios, as shown in Figure 3.

Figure 3 EDS energy spectrum analysis of CoCrFeNi high-entropy alloy under scanning electron microscopy

Single Axis Test Piece Design

    The researchers designed the quasi-static uniaxial test specimens in accordance with the national standard GB/T 228.1-2021 "Tensile Test of Metallic Materials-Part 1: Test Method at Room Temperature", while the dynamic uniaxial specimens were designed based on GB/T 30069.1-2013 "High Rate of Strain Tensile Test of Metallic Materials-Part 1: Elastic Rod System", as illustrated in Figure 4.

Figure 4 Geometric configuration of uniaxial test specimen

Design of Double Axis Cross-shaped Test Piece

    To minimize the impact of geometric errors on experimental results, researchers used identical cross-shaped specimens for both dynamic and quasi-static biaxial tensile tests. Drawing on previous experience, they designed a biaxial tensile cross-shaped specimen universally applicable for testing the biaxial mechanical properties of quasi-isotropic materials, with detailed parameters shown in Figure 5. The specimen measures 150mm in total length, 2.5mm in thickness, and features four arms with a 15mm width and 50mm clamping end length. A single thinning treatment is applied in the central region, where a 10mm×10mm rounded square gauge section (0.7mm thick) is located. Four 10mm slits are cut into the four arms, each ending in a 1.2mm diameter stress relief hole to enhance deformation during loading and improve stress-strain uniformity within the gauge section. Additionally, to reduce the specimen's failure load and better match the testing equipment, a transition arc was designed to decrease the cross-sectional area of the central gauge section.

        During the geometric design phase of the test specimen, researchers first verified the uniformity of stress distribution in the central region of the cross-shaped specimen. The method involved selecting element bodies along two paths (X-direction and Y-direction) within the gauge length of the cross-shaped specimen, with the integration paths shown in Figure 6. Within a relative distance of 0–0.6 along both paths, the relative Mises stress values differed by no more than 5%, as illustrated in Figure 7. When subsequently applying DIC technology for strain measurement in the central region, the selected points were also located near the center. Thus, variations in the edge region did not affect the calculation of central strain.

 Figure 7 shows the distribution of relative Mises stress values along the X and Y paths when the central equivalent plastic strain (PEEQ) is 0.15: (a) 1:1 loading, (b) 4:3 loading, (c) 2:1 loading, and (d) comparison of the three stress concentration coefficients.

Loading equipment 

    The quasi-static biaxial tensile test was performed on the Herculi multi-channel loading testing machine, with a load range of 20-500kN and a biaxial four-directional displacement synchronization error of ≤0.5%. The strain field was analyzed using the accompanying Herculi 3D DIC cloud computing program, as shown in Figures 8 and 9.

  Figure 8: Herculi Multi-Channel Loading Testing Machine and 3D DIC System

Test Results and Analysis

    The researchers found that the load-time and load-displacement curves of the three-directional specimens processed at 0°,45°, and 90° under uniaxial tensile loading at a strain rate of 0.01% per second showed excellent coincidence, as shown in Figure 10. It can be concluded that the CoCrFeNi high-entropy alloy processed along the disk plane exhibits tensile isotropy, with a yield strength of 292 MPa, a fracture strain of 0.86, and a tensile strength of 960 MPa.

   Figure 10 Load-displacement and true stress-strain curves of CoCrFeNi high-entropy alloy under quasi-static uniaxial tensile

     At a 1:1 loading ratio, the yield strength in the X-axis direction is 328 MPa, and the yield strength in the Y-axis direction is 330 MPa. The average strain in the central region at fracture is 0.085, and the tensile strength is 726 MPa. When the loading ratio is 4:3, the yield strength in the X-axis direction remains 328 MPa, and the yield strength in the Y-axis direction is 330 MPa. The average strain in the central region at fracture is 0.085, and the tensile strength is 726 MPa, as shown in Figure 11. Both the yielding and fracture of the specimen occur first in the central gauge length segment, further validating the feasibility of the cross-shaped specimen design, as illustrated in Figure 12.

    Figure 11 Real stress-strain curves of CoCrFeNi high-entropy alloy under quasi-static biaxial tensile loading

   Figure 12 DIC results and fracture locations of CoCrFeNi high-entropy alloy under quasi-static biaxial tensile loading

    The study calculated the plastic work corresponding to true plastic strains of 0.01,0.03, and 0.05 under uniaxial tensile loading, selected the equivalent plastic work calculated from biaxial tensile data, and compared the stress values corresponding to the same unit volume plastic work on stress-strain curves under different loading ratios. The experimental data points were fitted using either the Mises yield criterion or the Hill48 yield criterion, with the latter's simplified form presented in Formula 1. Results showed that the yield trajectory plotted using the Mises criterion deviated significantly from the measured values. However, when the anisotropy index R=3.5, the Hill48 yield criterion better described the yield behavior of CoCrFeNi high-entropy alloy, with its yield trajectory more closely matching the experimental measurements, as illustrated in Figure 13.

                 Formula 1

  Figure 13 Comparison of yield curves of CoCrFeNi high-entropy alloy under quasi-static and dynamic loading with different true plastic strains

    The high strain test with strain rate of 2000 was carried out by using the two-axis four-directional electromagnetic Hopkinson rod, and the results were compared with the quasi-static data.

    Lnnovations of this study

1. A cross-shaped specimen was designed for the biaxial tensile test to be suitable for large plastic deformation. The feasibility of the specimen design was verified by numerical simulation and test.

2. The single and double axis tests with different strain rate, loading form and loading ratio were carried out, which provided the key mechanical property database for the engineering application of CoCrFeNi high entropy alloy.

3. The von Mises criterion is used to estimate the yield stress of the alloy, and the Hill 48 criterion is used to describe the yield and hardening behaviors of the alloy.