Microstructure Characteristics And Properties Of Al_xCrFe₂Ni₂ High-entropy Alloy With BCC And FCC Structure Under Cryogenic Treatment

High-entropy alloys(HEAs),a new type of alloy with some excellent properties that traditional alloys cannot match,become one of the potential new materials in the future.But usually face-centered cubic(FCC)structure HEAs have good ductility but low strength,while body-centered cubic(BCC)structure HEAs are the opposite,with high strength but low ductility.It has always been a challenge to obtain HEAs with excellent comprehensive mechanical properties of high strength and high ductility.At present,scholars mostly try to optimize its comprehensive mechanical properties by using traditional heat treatment and alloying methods,but the effect is not ideal.Cryogenic treatment plays a positive role in improving the comprehensive performance of traditional alloys such as aluminum alloys.Cryogenic treatment is expected to further control the microstructure of HEAs and optimize their performance.In this paper,the Al1.4CrFe2Ni2 HEAs with BCC structure and Al0.6CrFe2Ni2 HEAs with FCC structure were prepared by vacuum arc furnace smelting.They were subjected to cryogenic treatment at 77 K for different times(4,8,12,24,48 h).The effects of cryogenic treatment on microstructure and properties of AlxCrFe2Ni2 HEAs with different structures were investigated by means of X-ray diffraction,scanning electron microscope,transmission electron microscope,electronic universal testing machine and hardness tester.The results show that with the increase of the cryogenic treatment time,the size of the nanophase of the Al1.4CrFe2Ni2 HEAs with BCC structure first decreases and then increases,the number of nanophases first increases and then decreases,the dislocation density increases significantly and the malformation gradually evolved from a short straight line to a long curve and a ring.In particular,when the cryogenic treatment time is 24 h,the average size of the nanophase is reduced from 220.1 nm(as cast)to 126.1 nm(24 h),which is 42.7%thinner,and the number of nanophases is the largest at this time.The alloy has the most excellent comprehensive mechanical properties when the cryogenic treatment time is 24 h,the hardness reaches 480 HV,the compressive fracture strength is as high as 3107 MPa,which is 24.33%higher than the as-cast state,and the compressive plastic strain is as high as 40.8%.The wear resistance is significantly improved,and the average friction coefficient is changed from 0.725(as cast)to 0.409(24 h).The Al0.6CrFe2Ni2 HEA with FCC structure has an equiaxed crystal structure.With the increase of cryogenic treatment time,the size of equiaxed crystal grains decreases,the number of spherical nanophases at the grain boundary increases,the alloy hardness and compression yield Strength and wear resistance continue to improve.When the cryogenic treatment is 48 h,a large number of stacking faults appear in the alloy.The hardness and yield strength of the alloy have been increased from 152 HV and 280 MPa(as cast)to 193 HV and 390 MPa(48 h),respectively.Compared with the as-cast state,which is an increase of 21.33%and 28.21%,respectively.The average friction coefficient is also reduced from 0.75(as cast)to 0.6(48 h).To sum up,the cryogenic treatment will shrink the volume of HEAs with BCC structure or FCC structure,this generates internal stress in the alloy,which results in an increase in the number of nanophases and dislocations and also changes in dislocation morphology,which will significantly improve the overall performance of the alloy.This research will lay a solid theoretical foundation for the control of microstructure and properties of HEAs.