Research On The Microstructures And Properties Of AlCoCuFeNi_x High Entropy Alloys

High entropy alloys（HEAs）are novel metallic materials with simple phase structure,excellent mechanical properties,special functional performances and outsanding high-temperature behaviors.Extensive researches have been paid on these alloys.In this paper,we researched on the structure and properties of as-cast AlCoCuFeNi_x HEAs,studied the influence of cold rolling,recovery,recrystallization on the AlCoCuFeNi_3.0 HEAs with the best plasticity,and investigated the oxidation meachinsm of AlCoCuFeNi_x HEAs at elevated temperature.Alloy ingots of AlCoCuFeNi_x HEAs were prepared by arc-melting method.The phase constitutions of these as-cast samples were characterized by X-ray diffraction（XRD）and transmission electron microscope（TEM）.Microstructure and component distribution were carried out by optical microscope（OM）,scanning electron microscopy（SEM）,energy dispersive spectrometer（EDS）and X-ray photoelecgon spectroscopy（XPS）.Mechanical properties of specimens were examined by Vickers microhardness tester and electron universal testing machine at room temperature.The thermal behaviours were characterized by differential scanning calorimeter（DSC）.The magnetic properties were measured by Vibrating Sample Magnetometer（VSM）.The results show that:（1）For AlCoCuFeNi_x（x=0.5,0.8,1.0,1.5,2.0 and 3.0）HEAs,the addition of Ni element can lead to the breakdown of BCC and formation of FCC,retard the elemental segregation.AlCoCuFeNi_3.0 HEA possesses the best plasticity and the worst hardness and strength.AlCoCuFeNi_1.5 alloy achieves a better balance of fracture strain（35.9%）and yield strength（1725 MPa）.Atomic size difference（δ）is utilized as the critical parameter to explain the lattice distortion and phase transformation induced by Ni addition.AlCoCuFeNi_x HEAs could be applied as structure materials,soft magnetic materials and super alloys due to their mechanical properties,soft magnetic properties and phase stability.（2）With the increase of cold rolling reduction,the hardness of AlCoCuFeNi_3.0HEAs increases from 279.6 HV（as-cat alloy）to 464.8 HV（70%reduction）due to the synergy of dislocation,etc.After annealing treatment for these 70%reduction AlCoCuFeNi_3.0 HEAs,the broken dendrite microstructure will disappear and the recrystallized grains will form.The grain size increases with annealing temperature increasing.When deformed alloys annealed at the temperature of 1100℃for 2 h,the grain size can reach 167.1μm,annealing twins can be observed,dislocation density and residual stress were partialy eliminated,the hardness was also softened to 328.1HV accordingly.（3）AlCoCuFeNi_x HEAs exhibit good oxidation resistance at 1000℃.After oxidation for 100 h,a slight increase in weight gain for these five alloys can be observed due to oxide reaction.The oxide layers form on the surface and provide a good protection for substrate metal.However,the oxidation mechanism of AlCoCuFeNi_x HEAs varies with the Ni content.The oxidation resistance becomes poorer while the Ni content incereases.With the element concentration gradient between oxide layers and substrate metal,the Al-depleted FCC phase form beneath the oxide layers.At the meantime,the oxidation behavior of AlCoCuFeNi HEAs at different temperature are investigated.The oxidation resistance and phase stability are excellent at 900 and1000℃.However,when AlCoCuFeNi HEA is oxidized at 1100℃,as the result of differences in the thermal expansion coefficients amog BCC,FCC and multiple oxides,both the oxide layers and the substrate metal are easy to crack.Finally,this ignot is oxidized completely within 20h.Therefor,it can be concluded that the work temperature of AlCoCuFeNi HEA should not up to 1100℃.