Preparation And Properties Of Nanocrystalline High Entropy Alloy By Laser Evaporation Condensation

As a new multi-component alloy material,high entropy alloy breaks the traditional design concept of binary and ternary alloys.The unique high entropy effect,lattice distortion effect,slow diffusion effect and cocktail effect make it have the remarkable characteristics of high strength,high ductility and high temperature stability.The strength,hardness and wear-resisting properties of high entropy alloy are further improved by the nanocrystallization of high entropy alloy.Inert gas condensation（IGC）is an effective method to prepare high-performance nanocrystalline metal materials.However,the traditional thermal resistance evaporation method is difficult to obtain ideal alloy composition due to the anisotropy of alloy target components,which is usually limited to metal elemental and binary alloys.The preparation and properties of nanocrystalline high entropy alloys by inert gas condensation have not been reported.The research on the preparation methods and properties of new nanocrystalline high entropy alloys is of great significance to the development and application of high entropy alloys.In this work,bulk nanocrystalline high entropy alloys and high entropy alloy composites were prepared by self-developed laser evaporation inert gas condensation system（Laser-IGC）.The development law between microstructure evolution,mechanical properties and magnetic properties during annealing of high entropy alloys was deeply discussed,and systematic study of its mechanical strengthening mechanism and magnetic mechanism.The main research results are as follows:（1）（CoCrFeNi）93.5Mn6.5 nc-HEA with an average grain size of 25 nm was prepared by inert gas condensation（IGC）with a laser evaporation source.Different from the same system nc-HEA prepared by other methods,Laser-IGC nc-HEA has shown the ultra-high hardness,excellent thermal stability and tunable ferromagnetic properties.An unprecedented increment in hardness was found from 484 HV to 791HV after annealing at 600°C for 1 h.More surprisingly,the original high hardness is maintained even after annealing at 1100°C.After annealing at 900°C,the tensile yield strength and fracture strength are 592 and 836 MPa respectively,which are 218%and 59.5%higher than those of the as-cast HEA with the same composition,respectively.Meanwhile,the Laser-IGC nc-HEA exhibits shows unique ferromagnetic behavior and the Curie temperature（Tc）increased nearly 10 times compared to any CoCrFeNi Mn HEAs prepared by various other methods.In addition,the saturation magnetization（Ms）and Tc of the Laser-IGC nc-HEA can be controlled via heat treatment.The mechanism includes:grain boundary relaxation induced hardening,multiple precipitates,the synergistic effect of heterostructure and high-density annealing twins.（2）Al/CoCrFeNi nanocrystalline high-entropy alloy composite（nc-HEAC）with FCC structure was prepared by Laser-IGC.By controlling the diffusion process between the Al and CoCrFeNi nanocrystals,we can continuously regulate the phase structure of the Al CoCrFeNi nc-HEA,which affect its mechanical and magnetic properties,and finally obtain the bulk BCC Al CoCrFeNi HEA with the highest hardness reported at present..At the early stage of annealing,grain boundary relaxation promotes the hardening of the HEA.Then,the heat treatment was used to trigger the interdiffusion between Al and CoCrFeNi nanocrystals and form a FCC-Al CoCrFeNi phase,which improves the interface adhesion of the alloy,The Co Ni precipitates provide ferromagnetism.With the increase of the annealing temperature,element diffusion intensifies,and the Al CoCrFeNi phase undergoes a phase transition from FCC to BCC structure further improves the hardness and magnetic properties.The hardness and saturation magnetization of 40%-Al CoCrFeNi HEA increase with the increase of annealing temperature,and reach the peak at500℃,1124 HV and 35.9 Am²kg^-1 respectively.The mechanical and magnetic mechanisms are the synergistic effect of the nano precipitates,diffusion induced phase transition and coherent strengthening effect during annealing.（3）The dual-phase（FCC+BCC）Al CoCrFeNi high entropy alloy（DP-HEA）with excellent strength-plastic equilibrium properties were prepared via thermal diffusion induced phase transition of Al/CoCrFeNi nanocrystalline high-entropy alloys composite.The yield strength and tensile strength of 16%-Al CoCrFeNi dual-phase high entropy alloy annealed at 800℃reach 1586 MPa and 1579 MPa,and the elongation is 1.9%.The yield strength and tensile strength of the DP-HEA annealed at 900℃are 1338 MPa and 1363 MPa,and the elongation is about 12.1%.The yield strength and tensile strength of the DP-HEA annealed at 1000℃are 981 MPa and1122 MPa,and the elongation is 17.6%.The tensile properties of DP-HEA are affected by the synergistic strengthening effect of dual-phase heterogeneous structure,high-density stacking fault and annealing twins.The high yield strength is contributed by the large back stress formed by"soft and hard"dual-phase（FCC+BCC）under stress,the blocking effect of high-density stacking fault and twins on dislocation.The back stress hardening effect caused by gradient strain and the nucleation and accommodation of stacking faults and twins on dislocation provide considerable ductility.