Effect Of Surface Treatment On Mechanical Properties And Microstructure Of Medium/High Entropy Alloys

In recent years,medium-entropy alloys(MEAs)and high-entropy alloys(HEA)(referred to as medium/high-entropy alloys in this paper)are a class of novel multi-element alloys that have broken through the traditional alloy design concept.Due to their inherent four core effects,medium/high-entropy alloys show their unique structural characteristics and outstanding mechanical performance different from traditional alloys,which have attracted the wide attention of scientific researchers.In this paper,CoCrFeMnNi HEA and Fe49.3Co23Ni23C0.85Mn1Si2.85 MEA were selected as the research objects,respectively,according to the design concept of medium/high entropy alloys.Then the microstructure evolution and mechanical properties of their homogenized states under different surface treatment conditions were studied.The corresponding research approach is to prepare alloy samples by vacuum arc melting and suction casting,to homogenize them by a vacuum annealing process,and to treat their surfaces through an industrial shot blasting machine and vacuum electron beam remelting equipment,respectively.Subsequently,the phase composition and structural evolution were studied by means of an X-ray diffractometer,scanning electron microscope,electron backscattering diffraction,and transmission electron microscope.The mechanical properties of these samples were studied by a universal mechanical testing machine and Vickers hardness tester.Firstly,the effects of the surface shot blasting process on the microstructures and mechanical properties of the CoCrFeMnNi HEA were studied.After homogenization annealing at 1373 K for 24 h,the single face-centered cubic(FCC)coarse crystals govern the microstructure of the CoCrFeMnNi HEA.After shot blasting,the crystal structure is still a single FCC phase,but the surface grains are refined,forming gradient nanostructures with different scales from the surface to the inside.With the increase of shot blasting time,the degree of surface grain refinement gradually increases.The best mechanical properties can be achieved after shot blasting for 20 min,and the near-surface structure shows residual stress of 372±11 MPa.The yield strength of the sample after shot blasting is increased from 153 ± 15 MPa to 426 ± 15 MPa,the tensile strength is increased from 477 ± 10MPa to 577 ± 10MPa,and the elongation remains 36%.The improvement in properties should be attributed to nanocrystallization,dislocation strengthening,and heterogeneous structures.Secondly,the effects of the vacuum electron beam surface remelting treatment on the microstructure and mechanical properties of the CoCrFeMnNi HEA were studied.Homogenized CoCrFeMnNi HEA has a single FCC structure.After vacuum electron beam surface remelting treatment,the surface grains are obviously refined and maintain a single FCC structure due to rapid solidification.With the increase of the electron beam remelting beam,the surface grains decrease first and then increase,and the remelting depth gradually increases.Mechanical experiments show that under reasonable remelting parameters,the yield strength can be increased from the initial homogenization of 146 ± 5 MPa to 280±5 MPa,the tensile strength can be slightly increased to 535 ± 2 MPa,and the plasticity is not greatly reduced.It was found that the grain boundary strengthening mechanism plays an important role in the deformation process.Thirdly,the influences of the surface shot blasting process on the microstructure and mechanical properties of the Fe49.3Co23Ni23C0.85Mn1Si2.85 ME A were studied.The sample after homogenizing annealing at 1273 K for 12 h exhibits coarse grains,and the corresponding crystal structure is a dual-phase(FCC+BCC)structure.After surface shot blasting treatment,the surface grains are obviously refined,and a multi-layer gradient structure is formed,showing up to a high residual stress of 1000.9 MPa and a high-density dislocation of 5.79×1016,and the crystal structure still maintains a dual-phase structure.The mechanical experiments show that the surface hardness of the homogenized sample is gradually increased from 3.4 GPa to 9 GPa,the yield strength is increased from 507 ± 5 MPa to 641 ± 5 MPa,and the tensile strength is increased to 898 ± 2 MPa.The strengthening mechanism is attributed to dislocation strengthening,precipitation strengthening,grain boundary strengthening,and back stress brought by heterostructure.Fourthly,the effects of vacuum electron beam surface remelting treatment on the microstructure and mechanical properties of the Fe49.3Co23Ni23C0.85Mn1Si2.85 MEA were studied.After homogenizing annealing at 1273 K for 12 hours,the sample has coarse grains and a dualphase structure.After electron beam surface remelting treatment,the samples obtained under melting currents of 5 mA and 10 mA transform to a single FCC structure,and others are still FCC+BCC structures.During the low beam density remelting process,the depth of the sample remelting layer is small,which is mainly due to rapid heat treatment,while the high beam density remelting treatment is strongly associated with rapid solidification,which eventually leads to the transformation from two-phase structure to single-phase structure.Mechanical experiments show that the best performance combination is achieved under a melting current of 15 mA.The yield strength is increased from 528±10 MPa to 567±13 MPa,and the tensile strength is increased from 806 ± 15 MPa to 823 ± 18 MPa.With the increase of beam density,the surface hardness is greatly improved,and the overall yield strength first increases and then decreases,which depends on the competition between grain refinement and defect formation during rapid solidification.