Test And Analysis On Mechanical Properties Of Al_xCoCrFeNi Alloys

The concept of high-entropy alloys (HEAs) has been considered as a novel alloy design idea. Research demonstrates some HEAs have a variety of excellent properties, such as high thermal stability, great strength, superior corrosion resistance, outstanding wear resistance, hydrogen storage and so on. Therefore, high-entropy alloys are supposed to have great potential in industrial production and application. While in the aspect of engineering application, mechanical properties are very important parameters of materials design. After summarizing the current papers about high-entropy alloys, it is found that there are very few reports on the tensile mechanical properties of high-entropy alloys at room temperature and elevated temperature. Besides, the reported HEAs exhibit a wide range of Young’s modulus and most data are from compressive tests. There is no report demonstrating whether the test methods influence the experimental values of elastic modulus. Still, no theoretical method has been raised to estimate the elastic modulus of HEAs from their compositions and the constitute elements. Thus AlxCoCrFeNi (in molar ratio, x=0.15,0.4,0.6,0.8, denoted as Al0.15, Al0.4, Al0.6, Al0.8respectively) alloys with simple microstructure and excellent mechanical properties were studied in this paper.The as-cast alloys exhibit typical dendrites microstructure. Al0.15and Al0.4alloys are single FCC solid-solution. BCC phase starts to form and the quantity is growing as Al content increases. Al0.6and Al0.8alloys are duplex FCC+BCC phase. Through thermodynamic calculation and solid-solution formation rule of HEAs, the AlxCoCrFeNi alloys in this paper are indeed HEAs.The hardness of AlxCoCrFeNi alloys increases as Al content increases and the Vickers hardness of Al0.8is390.0±5.1HV. Compressive tests demonstrate that Al0.15and Al0.4alloys have good ductility. Yield strength and compressive strength increase with the increasing Al content. The compressive strength of Al0.8alloy reaches2.885GPa with21.3%plastic strain. The fracture of the alloys was observed to be in brittle cleavage or quasi-cleavage failure mode. Tensile tests demonstrate that the yield strength, tensile strength and plastic strain of Al0.15at room temperature are241.6MPa,798.3MPa and55.4%. The yield strength and tensile strength of Al0.15decrease at elevated temperature, but ductility is still good. The tensile strength of Al0.4at room temperature reaches906.4MPa. As temperature increases, its yield strength increases while tensile strength and plastic strain decrease. The yield strength of Al0.6alloy is the highest470.9MPa, and basically both strength and ductility decrease with increasing temperature. From the perspective of fracture mode, the AlxCoCrFeNi alloys exhibit ductile type to brittle type and transgranular fracture to transgranular+intergranular fracture.The test methods have a great influence on experimental values of elastic modulus of AlxCoCrFeNi alloys, but the content of Al doesn’t. The tensile and ultrasonic results are very close, while compressive tests may cause relatively big error and fluctuation. The "rule of mixture" is utilized to estimate the Young’s modulus of HEAs. The calculated elastic modulus of AlxCoCrFeNi fit well with the tensile and ultrasonic data. It is suggested this rule could be applied to HEAs for estimation of their elastic modulus. It is found that some reported experimental data are roughly in agreement with the theoretical results. However, still some have a big discrepancy which is maybe due to inaccurate deformation measurement. Therefore, it is suggested to describe test method clearly when publishing elastic modulus of HEAs so as to provide more reliable reference data.