Study On The Mechanical Properties And Corrosion Resistance Of Al_xCoFeNiCr_1-x High-Entropy Alloys

High-entropy alloys in the design concept have broken the shackles of traditional alloys and also provide a new idea for the design and preparation of new material.Al and Cr elements in the AlCoFeNiCr HEA mainly constitute BCC phase and FCC phase,respectively.Additionally,these two elements display good corrosion resistance due to the form of passivation film easily.In this paper,the Al_xCoFeNiCr_1-x（0.1≤x≤1.0）HEAs were fabricated by using arc-melting under high-purity argon atmosphere.In order to investigate the effect of different Al and Cr in Al_xCoFeNiCr_1-x-x HEAs on the relationships of microstructure evolution,mechanical properties and preservative,the corresponding performance characteristics were analyzed by XRD,SEM,EDS,HV-1000,Instron3382 and CHI660 D.The result are as follows:（1）With the Al content increase and Cr content decrease,the crystal structures of Al_xCoFeNiCr_1-x-x HEAs shifted from FCC phase to FCC+BCC phase to FCC+BCC+B2 phase,and then to BCC phase.The Al element can promote the formation of BCC structure,while the reduced Cr element can induce the disappearance of FCC phase.（2）Owning to the existence of mutual exclusion and attraction between different elements in these alloys,the matrix phase of Al_0.7CoFeNiCr_0.3.3 HEA changed from FCC phase（rich in Fe,Cr）to BCC phase（rich in Al,Ni）,accompanied segregation phase from BCC phase to FCC phase.By comparing the behaviors of alloys with types of matrix phase,these alloys with FCC matrix phase generally show a better plasticity and lower strength,however,the properties of the others with BCC matrix phase illustrate opposite characteristics.Furthermore,the wire mesh-like structure in Al_0.7CoFeNiCr_0.3.3 and Al_0.8CoFeNiCr_0.2.2 HEAs optimized mechanical properties of above alloys.（3）In 3.5%NaCl corrosion solution,the lattice distortion energy of alloys,with a single FCC phase or BCC phase,gradually enhance,and the lattice defects become more serious leading to worse corrosion resistance with the increase of large radius Al element and reduce of Cr element.For alloys with FCC+BCC dual phase,due to the effect of corrosive thermocouple,the larger of Volume fraction of segregated phase,the worse the corrosion resistance of this alloy.The obvious intermittent passivation area can be noticed in the potential polarization curve of as-cast Al_x CoFeNiCr_1-x-x HEAs,implying the outstanding corrosion resistance of these alloys.Meanwhile,based on the previous research,the Al_0.6CoFeNiCr_0.4.4 HEAs show a significant performance advantages in balancing strength and ductility.Therefore,the influence of different annealing temperatures on its microstructure,mechanical properties and corrosion resistance were explored.The main results are as follows:（4）With the annealing temperature increase from 550℃to 850℃,the crystalline phase of these alloys change from FCC+BCC to FCC+BCC/B2 and then to B2+FCC structure.The columnar dendrites FCC phase in grain boundaries and intercrystalline can observe the coarse disordered BCC phase.（5）Partly disordered BCC phase in 650℃annealing alloy transforms into spherical ordered B2 phase and it reveal the highest vickers hardness and yield strength.Additionally,apart from the B2 phase nucleating and accumulating inside the disordered BCC phase,the750℃和850℃annealing alloy present Zigzag-like decomposition,which are composed of FCC structure and needle-shaped B2 phase.The mechanical properties of these alloys are mainly combined with FCC phase decomposition and ordered B2 phase.The proper decomposition structure can improve comprehensive mechanical properties of alloys,while excessive decomposition structure may greatly deteriorated its performance.（6）The corrosion resistance of 550℃and 650℃annealing alloys are greater than that of as-cast alloy.With the heat temperature increase,a large amount of B2 phase enriched with Al element is formed by segregation precipitation and decomposition.The corrosion resistance of these HEAs are gradually decrease because the porous Al₂O₃ film may be easily eroded and destroyed by Cl-ion compared with Cr₂O₃ passivation film.