Composition,microstructure And Mechanical Properties Of FCC-Structure High Entropy Alloys

High entropy alloys tend to form simple solid solution structures due to their high mixingentropy.The high entropy alloy of face centered cubic structure has many slip systems and a large number of dislocations can be produced during the process of deformation,so the alloys have good plasticity.Otherwise,it is a problem that the yield strength of FCC structure alloys is low and the microstructure evolution of the material in the process of deformation is still to be studied.The FCC structure high entropy alloy has high plasticity,but the yield strength is low,and the microstructure evolution during the deformation process needs to be studied.In view of these properties of high entropy alloy with FCC structure,this paper designs many series?the equal-molar ratio and the non-equal-molar ratio?alloys with different composition of Ni_xCoCrFeMo_0.1,Ni₂CoCrFeMo_x and VMn Fe Co Ni with FCC structure by using a variety of parameters such as valence electron concentration,electronegativity ??,atomic radius difference ? and mixing enthalpy ?Hmix,which based on the existing design rules of high entropy alloy.The composition,microstructure and mechanical properties of the Ni₂CoCrFeMo_x alloy with single FCC structure were studied systematically.By means of X ray diffraction,SEM scanning electron microscope,EDS energy spectrum analysis and TEM transmission electron microscope,the microstructure of the material was changed by cold rolling and room temperature stretching,and the deformation behaviors of Ni₂CoCrFeMo_0.2alloy under different rolling pressure conditions such as 10%,30%,50% and 70% was studied.The main conclusions of the study are as follows:?1?A series of FCC structure high entropy alloys are designed by the parameters of valence electron concentration VEC,electronegativity ??,atomic radius difference ? and mixing enthalpy ?Hmix.The NiCoCrFeVMo series of alloys are single-phase FCC structure,and the dendrite structure does not have obvious segregation.The plasticity of alloys is good,and the compression rate of all components can reach above 50%.But the hardness is only about 150 HV.Because of the different element composition,the performance of VCrMnFeCoNi series of alloys is more violent.The second phase appears besides the FCC matrix.The plastic property of the single phase VMn Fe Co Ni is excellent,and a large number of Taylor lattice are found in the samples after the deformation,while the VCr Mn Fe Ni in same system hardly can be compressed.The hardness of alloys fluctuates from 168.62 HV to816.8HV,which is directly associated with the compressive yield strength.It can be found that NiCoCrFeMo series of alloys have the most stable performanceand are more suitable for further exploration as single phase FCC structure among both of series of alloys.?2?all the structures in the 0.1 series of alloys are single phase FCC solid solution with good plasticity and compression fracture strain beyond 50%.With the increase of Ni content,the hardness and yield strength of the alloy decrease,and the Ni₁CoCrFeMo_0.1 alloy has the largest compressive yield strength 260 Mpa among series of alloys and hardness is 151.02 HV.The Ni_xCoCrFeMo_0.1 alloy showed a higher microstructure stability at 1200?,and the properties and microstructure of the alloy did not change significantly after heat treatment.The structure of Mo_0.2,Mo_0.4,Mo_0.6,Mo_0.8 and Mo_1.0 in Ni₂CoCrFeMo_x alloy system is a single FCC solid solution phase or FCC matrix +Cr₉Mo₂₁Ni₂₀ second phase.In addition to Mo_1.0,which compressive rate is almost 40%,other Ni₂CoCrFeMo_x alloys are more than50%,so these alloys have good plasticity.With the increase of Mo content,the hardness and compressive yield strength of the alloy increased gradually,and the compressive yield strength of the highest Mo_1.0 alloy reached 830 Mpa.The whole alloy series has the potential to find an alloy not only with high strength but also with high plasticity.The mechanical properties of different components vary greatly.?3?the Ni₂CoCrFeMo_0.2 alloy exhibits excellent plasticity.The tensile elongation of the cast alloy is over 47.5%,the tensile yield strength is 325 Mpa,and the comprehensive mechanical properties are better.The fracture surface after tensile fracture is surrounded by round and deep dimples,and the grains are elongated obviously.Rolled material showed that the microstructure of the alloy changed with the increase of rolling deformation,and the grain was elongated along the rolling direction until it was broken and then refined.The hardness of the alloy gradually increased,from 149.16 HV to 395.93 HV,which was linearly proportional to the deformation.The linear trend line expression of hardness is?10??28?113x523HVY?HV?.From the microstructure of the deformed alloy,it can be concluded that dislocation slip was the main cause of the plastic deformation.The dislocation moves along a certain direction to form Taylor lattice,which changes to the cellular dislocation with the increase of deformation.