Effect of High-Entropy Dodecaboride Reinforcements on the Properties of Magnesium Alloys

With the rapid development of marine equipment technology, there is an increasing demand for lightweight marine equipment. Magnesium alloys possess advantages such as light specific gravity, high specific strength, and good castability. They are widely used in transportation, aerospace, and other fields and are considered one of the most promising materials. However, magnesium alloys have low hardness and poor corrosion resistance, which severely limits their application in marine environments. In this paper, high-entropy twelve boride is used as the reinforcing phase to improve the performance of magnesium alloys and make their use in marine environments more extensive.

Activate high-entropy ceramics with hydrofluoric acid. Prepare xHECBs/AZ31 composites by melting in a crucible resistance furnace. During the whole melting process, pass a mixed protective gas of 1 vol.% SF6 + 99 vol.% CO2. Observe and analyze the microstructure of the cast alloy using a metallographic microscope. Later, use an X-ray diffractometer to identify the phases of the alloy. Select a SEM scanning electron microscope (Zeiss GeminiSEM300 Oxford energy dispersive field emission electron microscope) for EDS energy spectrum analysis. For the corrosion performance test of the alloy, use the Reference 600+ electrochemical workstation of Gamry Company in the United States. Conduct tensile mechanical property tests using a tensile testing instrument (CMT6103 tensile instrument). Test the hardness of the alloy using a micro Vickers hardness tester (HVS-100 manual turret digital micro Vickers hardness tester).

1. After adding the HECB phase high-entropy ceramic, the three strong peaks shift to the right. The high-entropy ceramic phase is successfully integrated into the magnesium alloy. By comparing the peaks, it can be known that there are no obvious impurity phase peaks in the XRD pattern of the new magnesium alloy. The high-entropy ceramic phase causes a decrease in peak height, proving that the crystallinity becomes worse. The broadening of the peak width also proves that the crystallinity becomes worse, but the number of grains increases and the grains become smaller. The grain size decreases from 125-200 μm to 100-150 μm. 2. After adding the high-entropy ceramic phase to AZ31 magnesium alloy, its self-corrosion potential increases. The corrosion current density is lower than that of AZ31 magnesium alloy without adding high-entropy ceramic. And the corrosion current density of activated high-entropy ceramic/AZ31 magnesium alloy is lower than that of unactivated high-entropy ceramic/AZ31 magnesium alloy. The self-corrosion potential is increased to -1.398V, and the self-corrosion current density is decreased to 4.958×10 μA/cm. Its performance has a maximum increase of 0.5V in self-corrosion potential. In the impedance test, the impedance of activated high-entropy ceramic/AZ31 magnesium alloy is larger than that of unactivated high-entropy ceramic/AZ31 magnesium alloy. Compared with AZ31 magnesium alloy without adding high-entropy ceramic, the resistivity is increased by 92%. The radius of the Nyquist plot is 5-6 times larger. 3. The average values of the micro Vickers hardness of alloys with unactivated high-entropy ceramics (1%, 2%, and 5%) are 75.86HV, 85.68HV, and 92.24HV, respectively. After adding the phase high-entropy ceramic and activating treatment, compared with the unactivated high-entropy ceramic magnesium alloy, the hardness slightly decreases, which are 72.265HV and 83.415HV (the high-entropy ceramic content is 1% and 2%, respectively). For its tensile properties, after adding unactivated high-entropy ceramics to magnesium alloy, the yield strength stress will increase significantly, and the ultimate tensile strength will decrease significantly. The yield stress is increased from 29.6 MPa to 46.6 MPa, and can reach up to 671.3 MPa at most, but the elongation will decrease from 26.84% to 4.5% with the increase of high-entropy ceramic content. After activating the high-entropy ceramic, the elongation of the activated high-entropy ceramic reinforced magnesium alloy shows a significant increase compared with the unprocessed high-entropy ceramic reinforced magnesium alloy, and the yield stress is also significantly enhanced. Compared with before activation, the yield stress can be increased by 47.72% - 54.45%.

High-entropy ceramics can reduce the grain size of AZ31 magnesium alloy from 125-200 μm to 100-150 μm. When the content of high-entropy ceramics is 2%, the size of high-entropy ceramic powder is about 10-15 μm. After adding high-entropy ceramics, there are no obvious impurity phases. Adding high-entropy ceramics can effectively inhibit the Cl- pitting corrosion of magnesium alloy in 3.5% NaCl solution. In electrochemical tests, the addition of high-entropy ceramics shifts the corrosion potential of the material to the positive direction and reduces the corrosion current density. The 2% activated high-entropy ceramic reinforced magnesium alloy has the best corrosion resistance effect. Based on the combination of electrochemistry, analyzing the 2% content high-entropy ceramic reinforced phase, after adding high-entropy ceramics, the Vickers hardness and yield stress of magnesium alloy increase, while the ductility and ultimate stress will decrease. However, the ductility of the activated high-entropy ceramic reinforced phase is relatively better. In general, the 2% content activated high-entropy ceramic reinforced magnesium alloy has the best performance.