Effect of C content on microstructure and properties of Al_1.5CoCrFeNiNb_0.2C_x high-entropy alloys by plasma cladding

Al_1.5CoCrFeNiNb_0.2C_x (x = 0, 0.01, 0.02, 0.05, and 0.2) high-entropy alloys with different C contents were prepared by plasma cladding on the surface of low-carbon steel. The effect of C content on the microstructure and mechanical properties of cladding layers was studied by using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), microhardness tester, and wear testing machine. The results indicate that when x = 0 and 0.01, the Al_1.5CoCrFeNiNb_0.2C_x high-entropy alloys are composed of a BCC phase and a small amount of Nb-rich Laves phase. When x = 0.02~0.2, the NbC phase can be synthesized in situ. Moreover, the precipitation amount of NbC gradually increases with the increase of C content. When x = 0 and 0.01, the microstructure of high-entropy alloys is a dendritic crystal. The dendrite is the BCC phase, and the interdendritic is an eutectic texture consisting of the BCC phase and Nb-rich Laves phase. When x = 0.02~0.2, in-situ NbC is precipitated on the dendritic matrix. With increasing C content, the precipitation amount and size of NbC gradually increase, and its morphology is gradually transformed from granular at low C contents (x = 0.02 and 0.05) to granular and cross-dendritic at high C content (x = 0.2). TEM shows that the interface between the high-entropy alloy matrix and the NbC reinforcement phase is smooth and clean, without any defects or impurities. As the C content rises from x = 0 to x = 0.2, the hardness increases from 560.1 HV to 762.2 HV, while the weight loss rate decreases from 24.69 mg/min to 4.70 mg/min.