Effect Of Rolling And Heat Treatment On Microstructure And Mechanical Properties Of AlxCoCrFeNi High Entropy Alloys

High entropy alloys greatly broaden the range of the alloys and has many excellent properties.It has broad application prospects in many fields such as industrial production and has high research value.Segregation is easy to occur in the high entropy alloy suction casting process.Most of the as-cast high entropy alloys have a dendritic structure,and the plasticity is good at room temperature while the strength is low.Therefore,it is necessary to increase the strength of the as-cast high entropy alloy and optimize the properties of the alloy to broaden its application range.In this work,the AlxCoCrFeNi（x=0.3,0.5,0.6）high entropy alloys were prepared by vacuum arc furnace melting.The alloy was cold rolled and heat treated at 873 K,973 K and1073 K for 1 h.The effects of rolling and heat treatment on the structure and mechanical properties of AlxCoCrFeNi high entropy alloys were studied by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy dispersive spectrometer（EDS）,transmission electron microscopy（TEM）.The results show that:（1）Al_0.3CoCrFeNi as-cast high entropy alloy has FCC single-phase structure,Al_0.5CoCrFeNi and Al_0.6CoCrFeNi as-cast high entropy alloys are FCC+BCC dual-phase structure,The content of BCC in the Al_0.3CoCrFeNi,Al_0.5CoCrFeNi and Al_0.6CoCrFeNi casting high entropy alloy is 0,11.86%and 33.81%,respectively.With the increase content of Al,the content of BCC in the as-cast AlxCoCrFeNi high entropy alloy increases,meanwhile the strength of the high entropy alloy increases,the plasticity decreases,and the microstructure also changes from equiaxed（x=0.3）to dendritic（x=0.5,0.6）.（2）The phase structure of as-cast Al_XCoCrFeNi（x=0.3,0.5,0.6）high entropy alloy has not changed after cold rolling with 75%total deformation.After rolling,the alloys produced a large number of dislocations.With the increase of Al content,the strength of the alloys increases and the plasticity decreases.Due to the work hardening effect,the cold-rolled samples showed a significant increase in strength and decreased plasticity compared with the as-cast samples.（3）BCC phase were precipitated in air-cooled Al_0.3CoCrFeNi and Al_0.5CoCrFeNi high-entropy alloys after 873 K heat treatment.Compared with cold-rolled samples,the strength increased and the plasticity decreased.BCC and L1₂ phases were precipitated from air-cooled Al_0.6CoCrFeNi high entropy alloy after 873 K heat treatment.（4）Al_0.3CoCrFeNi high entropy alloy heat treatment at 973 K and 1073 K temperature,and Al_0.5CoCrFeNi high entropy alloy heat treatment at 973 K,when the composition and temperature are the same,the furnace cold sample is better than the air-cooled sample.Because of the increases of the precipitation,twin size and density phase after furnace cooling.（5）The air-cooled Al_XCoCrFeNi（x=0.3,0.5,0.6）high entropy alloys treated by 1073 K are all FCC+BCC dual-phase structures.As the Al content increases,the strength of the alloy increases and the ductility decreases.Due to precipitation phase strengthening,fine grain strengthening and twinning,the high entropy alloy in this state has a significantly improved strength compared with the as-cast sample,and has good comprehensive mechanical properties,which broadens the application range of high entropy alloys.The tensile strength of Al_0.3CoCrFeNi high entropy alloy is 1216 MPa（as cast:719 MPa）,and the maximum elongation is 28.2%.