Research On Microstructure And Mechanical Properties Of FeMnNiCr Type High Entropy Alloy And Its Composites

In this paper,the Fe_xMnNi_2-xCr type high entropy alloy and its composites were prepared by vacuum induction melting.The alloy and its composites with annealing and alloying were characterizated by in-situ stretching,scanning electron microscopy,transmission electron microscopy,electron back scattering diffraction,X-ray diffraction and so on.The microstructure morphology,structure,mechanical properties and fracture modes of the enhanced high-entropy alloy and its composites were studied.The main experimental contents and results are as follows:In the Fe_xMnNi_2-xCr high entropy alloy,the equal atomic ratio alloy has a dual-phase structure of FCC+BCC,with FCC as the main phase.Increasing the content of Fe and reducing Ni caused increasing of BCC phase.The increase of the stronger BCC phase makes the Fe_1.2MnNi_0.8Cr tensile strength up to 662 MPa,elongation rate is 24%.While,after reducing the content of Fe,the Fe_0.5MnNi_1.5Cr alloy becomes a single-phase FCC structure.Due to the high uniform deformability of FCC,the elongation of Fe_0.5MnNi_1.5Cr is as high as47%.After adding Nb to Fe_0.5MnNi_1.5Cr,dendrites appear inside the material.At the same time,with the increase of Nb,the Fe₂Nb Laves phase appearred which improved the strength of the material.In Fe_1.2MnNi_0.8Cr V_yC_y,V and C were added to Fe_1.2MnNi_0.8Cr at the same time.C and Cr formed C₂₃C₆,V was dissolved in the alloy.The increase in the tensile strength（989 MPa）of Fe_1.2MnNi_0.8Cr V_0.21C_0.21 was attributed to the combined effect of grain refining,solid solution strengthening and precipitation strengthening.In the Ti-C type Fe_1.2MnNi_0.8Cr composites,TiC particles were generated in-situ.The best strength plastic matching was acquired when the volume fracture is 10%（the ultimate tensile strength is 856 MPa,elongation is 34%）.Not only were Ti and C formed TiC,but also dissolved in the matrix.Solid solution C reduced the stacking fault energy of the material and increased the work hardening rate.The tensile strength of 10 vol.%Ti-C composite annealing at 600℃-1 h increased to 1173 MPa,but the plasticity dropped rapidly to 12%.The Ti-B type Fe_1.2MnNi_0.8Cr composites generated Fe Cr B whiskers and Ti B particles in the FCC phase.The strength of the composite increased but the plasticity decreased as the volume fraction of Ti-B increased.5 vol.%Ti-B composite was found to possess the highest values in ultimate tensile strength,which is 924 MPa,and the hardness is 678 HV.After annealing at 600°C for 2 h,its strength further increased to 1153 MPa,and the plasticity only decreased slightly（11%）.The BCC phase growed up and spheroidized after annealing,which reduced the stress concentration inside the material and delay the crack initiation.In the Ti-C-B type Fe_1.2MnNi_0.8Cr composites,TiC particles and Fe Cr B whiskers were produced at the same time while Ti B particles were not observed.Compared with Fe_1.2MnNi_0.8Cr,the strength and plasticity of the composite were improved,but its comprehensive mechanical property was lower than 10 vol.%Ti-C composites.As the content of Ti and C decreases,the plasticity decreases.In Ti-C-B composites,the toughening effective of Ti and C was obvious but the enhancement effect of Ti and B was suppressed.