| High entropy alloy is a solid solution composed of multiple elements in equimolar or non-equimolar ratios,which not only breaks the design idea of traditional alloy with one or two components as the matrix,but also presents excellent mechanical properties such as high strength,high plasticity,high hardness,corrosion resistance and high thermal stability.It was found that the face-centered cubic structure high entropy alloys have high plasticity but unsatisfactory strength.For example,(Fe50Mn30Co10Cr10)97C2Mo1 high-entropy alloy is FCC single-phase after homogenization,with a plasticity of about 64%and yield and tensile strengths of only 239 MPa and 574 MPa.In order to improve the strength of the alloy,this paper uses the method of Post-deformation annealing(PDA)to refine the grains in order to achieve the improvement of the mechanical properties of the alloy.The amount of plastic deformation in cold deformation and the temperature and time of annealing significantly affect the mechanical properties of the alloy.In order to obtain the best mechanical properties,the effect of different rolling processes on the microstructure and properties of homogenized(Fe50Mn30Co10Cr10)97C2Mo1 high-entropy alloy was investigated in this paper,and the main experimental results are as follows:(1)Homogenized(Fe50Mn30Co10Cr10)97C2Mo1 high-entropy alloy is cold rolled at 20%,40%,60%,80%and 90%plastic deformation,the grains are extended along the rolling direction and compounds of C,Mo and Mn are precipitated in the grains.After 20%cold rolling,dislocations are randomly distributed inside the grains,and a large number of nanoscale deformation twins are formed inside the grains with an average width of about 26 nm.With the increase of cold deformation,the volume fraction of deformation twins gradually increases to saturation,the gap between deformation twins is extremely small,and they are arranged in parallel inside the grains,and dislocations are rearranged and mainly gathered at the grain boundaries and inside the grains of deformation twins.After 40%cold rolling,shear bands start to form and increase with the amount of plastic deformation,which are internally deformed twins of different sizes and lengths.After 80%and 90%of large deformation,the organization of the alloy consists of a large number of shear bands,the tensile strength increases from the initial 574 MPa to 1926 MPa,and the plasticity decreases significantly to only 2%,which is due to the rise in strength and decrease in plasticity caused by work hardening,and a large number of shear bands also cause the alloy to decrease in plasticity.(2)After cold rolling 90%of(Fe50Mn30Co10Cr10)97C2Mo1 high entropy alloy at annealing temperature 400?,500?,600?,700? and 800?,respectively for 1h,the alloy underwent obvious phase separation phenomenon.Cr-rich(?)phases were formed inside the matrix after annealing at 500? to 800?,and Cr-rich BCC phases were formed after annealing at 700?.The Cr(?)-rich phase is a white elliptical particle,formed at the shear zone position,and its volume fraction increases gradually with increasing annealing temperature.After annealing at 700?,the volume fraction is the largest,and a large amount of Cr(?)-rich phase forms the Cr-rich BCC phase.The alloy undergoes partial recrystallization behavior after annealing at 800?,when the Cr(?)-rich phase precipitates at the grain boundaries.During the tensile process,the strength of the specimens annealed at 400? and 500? increased abnormally from 1926 MPa to 2078 MPa and 2060 MPa,respectively,and the strengthening effect was stronger for the specimens annealed at 400?,which was attributed to the precipitation of a small amount of round nanoparticles with an average diameter of about 11.5 nm due to low-temperature annealing.(3)Cold-rolled 70%(Fe50Mn30Co10Cr10)97C2Mo1 high-entropy alloy recrystallized after annealing at 900?,950? and 1000?.After annealing at 900?,the grain size was not uniform and there were white round Cr(?)-rich phase particles precipitated on the grain boundaries.After short annealing at 950?,there are still a small amount of small-sized grains and Cr(?)-rich phase particles present,and after extending the annealing time,the recrystallization degree further deepens,forming equiaxed crystals and dissolving Cr(?)-rich phase.Equiaxed crystals have been formed after annealing at 1000? for 5 min,and the grain size is about 7.9 ?m,and the grain size gradually increases after extending the annealing time.1000? for 5 min.The yield strength and tensile strength were increased to 384 MPa and 886 MPa,respectively,while the plasticity was slightly increased.The deformation mechanism was analyzed using EBSD,and it was found that planar dislocation slip was the main deformation mechanism at the initial stage of plastic deformation,and after the formation of deformation twins,the deformation mechanism changed to the proliferation of deformation twins. |
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