| As a new material,high entropy alloys（HEAs）have received considerable attention due to their extraordinary properties caused by the following four core effects:high entropy effect in thermodynamics,lattice distortion effect in crystallography,sluggish diffusioneffect in dynamics and“cocktail”effect.As a kind of large plastic deformation method,friction stir processing（FSP）is widely used which can realize the refinement,densification and homogenization of microstructure and obtain the large fraction of high angle grain boundaries.In this study,as-cast CoCrFeNiCu HEA was firstly processed by using two different stirring tools with flat shoulder and convex shoulder,respectively.It was found that both stirring tools significantly improved the microstructure,mechanical properties and corrosion resistance of CoCrFeNiCu HEA.This study provides a new method to significantly refine the microstructure and improve the strength of cast CoCrFeNiCu HEAs.After FSP with flat shoulder,the Cu element segregation in the cast CoCrFeNiCu HEA was almost eliminated,and the coarse two-phase structure（several micrometers）was changed into a near-nano single-phase structure（150 nm）with a large fraction of high angle grain boundaries and nanoscale deformation twins.This unique microstructure was mainly resulted from the severe plastic deformation during FSP,the sluggish diffusion effect in dynamics and the lattice distortion effect in crystallography for HEAs.Furthermore,FSP clearly improved the hardness and yield strength（YS）of the CoCr FeNiCu HEA with high values of 380 HV and 1150 MPa,respectively,which were 1.5 times higher than those of the base material（BM）.The strengthening effect after FSP was mainly attributed to the significant grain refinement with large lattice distortion and twin grain boundaries.In order to obtain deeper processing zone（PZ）,the convex shoulder was used to FSP of CoCrFeNiCu HEA.The microstructure,mechanical properties and corrosion resistance of CoCrFeNiCu HEA were improved by FSP.The average dendritic size of the BM was about6μm in the as-received condition.After FSP,grain size was significantly refined to 320 nm,and a large fraction of high angle grain boundaries and a large number of nanometer twins were obtained.In addition,the hardness,YS and ultimate tensile strength（UTS）were significantly enhanced by FSP.The hardness and YS of the BM were 150HV and 430MPa,respectively.The hardness was 285HV and the YS was 860MPa after FSP,which were about twice of the BM.The excellent mechanical properties after FSP were attributed to refinement strengthening,solid solution strengthening,severely lattice distortion and twin boundary strengthening.The intrinsic hardness was 104 HV,and Hall-Petch relationship coefficient was determined to be 103 HVμm1/2.Such a high Hall-Petch coefficient means that grain refinement strengthening is a very important strengthening mechanism for the FSP CoCrFeNiCu HEA.The PZ shows excellent corrosion resistance compared to the BM by immersion test and electrochemical test.The immersion test showed that the corrosion depth of the BM was about 43μm.However,the corrosion depth of the PZ was only 6μm.The electrochemical tests showed that the PZ has a significant passivation interval.The outstanding corrosion resistance was mainly due to the dissolution of Cu element into the matrix,resulting in a weakened potential difference. |
