Microstructure And Properties Of Al_xCoCrFeNi High Entropy Alloys Prepared By Sintering And Arc-melting Methods

High-entropy alloys have high research value because of their unique various effects and various properties which that the traditional alloys do not have.The vacuum arc melting has a mature process to prepare high-entropy alloys and the obtained alloy samples have many advantages such as high density,so it is widely used to prepare high-entropy alloys.The spark plasma sintering as a new process for preparing metal materials has high efficiency and this method is easy to form and environmentally friendly,which has attracted wide attention.This thesis uses vacuum arc melting and spark plasma sintering to prepare Al_xCoCrFeNi series high-entropy alloys,using optical metallurgical microscope（OM）,X-ray diffractometer（XRD）,scanning electron microscope and equipped energy dispersive spectrometer（SEM/EDS）,micro-hardness tester,electrochemical workstation to test the microstructure composition,mechanical properties and corrosion resistance of Al_xCoCrFeNi high entropy alloys.The following conclusions are obtained:The microstructure of the as-cast Al_xCoCrFeNi（x=1.0,0.7,0.4）high-entropy alloys shows that the AlCoCrFeNi alloys contain equiaxed dendrites and the Al_0.7CoCrFeNi alloys contain lath dendrites.Al_0.4CoCrFeNi alloy is columnar crystal.With the decrease of the Al element content:Al_xCoCrFeNi（x=1.0,0.7,0.4）high-entropy alloys phase structure changes from BCC phase to FCC phase structure.The microhardness value decreases with the decrease of hard phase BCC phase content,single BCC phase The hardness value of AlCoCrFeNi alloys is the highest,reaching 572 HV.The corrosion resistance of the alloys is improved as the reduction of Al element.The sintered Al_xCoCrFeNi（x=0.7,1.0）high-entropy alloys have a micro-structure with round spherical grain boundaries.The grain boundaries contain finer grains and the phase structure is composed of FCC and BCC phases.The higher content of the Al element,the more content of the BCC phase and the higher sintering temperature,the higher height of the corresponding phase peak.When the sintering temperature is the same,the higher content of the Al element,the higher content of the BCC phase,the greater the hardness value.When the alloys element composition is the same,the higher the sintering temperature,the higher the alloy’s density,the greater the hardness value.The hardness value of the AlCoCrFeNi alloys at 1200℃is 545 HV,because the sintered alloys undergo a phase transformation during sintering,a new phase FCC phase is generated and the content of the hard phase BCC phase is reduced,so the microhardness value is lower than the as-cast AlCoCrFeNi alloys.The corrosion resistance is also affected by the sintering temperature and the Al element content,the higher the sintering temperature,the lower the Al element content,and the better corrosion resistance of the alloys.Electrochemical experiments show that in 0.5M H₂SO₄ solution,Al_xCoCrFeNi（x=1.0,0.7）high-entropy alloys prepared by two preparation processes both occurred irreversible electrode reactions,capacitive,arc resistance and obviously passivation phenomenon,Al_0.4CoCrFeNi high-entropy alloys showed capacitance-resistant arc and diffusion tail,Al_xCoCrFeNi high-entropy alloys under both preparation processes only showed capacitance-resistant arc in simulated seawater solution.The corrosion resistance of the as-cast Al_xCoCrFeNi（x=0.4,0.7,1.0）high-entropy alloys is AlCoCrFeNi<Al_0.7CoCrFeNi<Al_0.4CoCrFeNi high-entropy alloys and the corrosion resistance of the sintered Al_xCoCrFeNi（x=0.7,1.0）is Al_0.7CoCrFeNi（900℃）<AlCoCrFeNi（900℃）<AlCoCrFeNi（1200℃）<Al_0.7CoCrFeNi（1200℃）.