Effects Of Two Strengthening Strategies On Microstructure And Mechanical Properties Of CoCrFe₂Ni High Entropy Alloy

High entropy alloys?HEAs?are a series of new alloys which broke up the conventional principle of design.The research results indicate that HEAs possess the excellent physical,chemical and mechanical properties.Moreover,HEAs exist the important potential prospect of industry and theory significance of research.HEAs with FCC structure possess good ductility with low strength.However,heat treatment and deformation would change the properties of the HEAs to get an outstanding comprehensive property.The aim of this study is to design a series of HEAs with FCC structure by theory of composition design and experimental study.Meanwhile,the HEAs possessed the excellent ductility.Then heat treatment and other processes were used to strengthen the strength of FCC structure HEAs with the decrease of ductility.Finally,the designed HEAs obtain an excellent comprehensive property with the perfect match between strength and ductility.this study was regard CoCrFeNi alloy as a base alloy using the model of VEC????X??H to get a new series of HEAs.The series of alloys is CoCrFe_xNi?x=2,3,4?,CoCrFeNi_x?x=2,3,4?,CoCrFe_xNi_x?x=2,3,4?alloys.Analyzing the structure,microstructure and properties of the series of alloys,CoCrFe₂Ni alloy was regard as most ideal alloy which possesses the low cost and excellent comprehensive properties.Then this study would do the study to strengthen CoCrFe₂Ni alloy.Adding different element into CoCrFe₂Ni alloy would strengthen the strength.The study indicated that Mo element possesses the bigger atomic radius and better strengthening effect.The CoCrFe₂Mo_xNi?x values in molar ratio,x=0,0.2,0.4,0.6,0.8,1.0?alloys was prepared.And the effects of Mo element on microstructure and properties of CoCrFe₂Ni alloy were investigated.The conclusion as follow:?1?The structure of CoCr Fe₂Mo_xNi alloys changes from a primary FCC phase to FCC+FeMo-type?phase,then to FCC+?+Fe₇Mo₆-type?phase with the increase of Mo content,and the morphology changes from columnar grain to dendritic,then to a near eutectic morphology.?2?When the Mo content increases,the strength and hardness of alloys increase,with the decrease of ductility.In conclusion,CoCrFe₂Mo_0.6Ni alloy possesses the greatest comprehensive properties.The hardness and compressive yield strength value of this alloy are254 HV and 542 MPa.And the compressive sample of CoCrFe₂Mo_0.6Ni alloy didn't fracture when compressive ratio reached 50%.Reasonable process is a common strategy to strengthen the properties of alloys.Cold deformation was carried on CoCrFe₂Ni alloy based on work hardening.Meanwhile,the effects of shape variables on microstructure and properties of CoCrFe₂Ni alloy were investigated.The results as follow:?1?When the variable is 20%,the grain size is smaller than casted alloy.When the variable is 40%,the grain is lengthened.Then the grain of CoCrFe₂Ni alloy is lengthened further,when the variable is 80%.?2?The hardness of CoCrFe₂Ni alloy is increased with the increase of variable.And the variation trend is linear.Fine-grain strengthening is the only strengthening strategy which doesn't loss strength.This study used recrystallization annealing to refine the grain of CoCrFe₂Ni alloy to strengthen the alloy.Due to the uncertainty of holding temperature during heat treatment,7group tests were prepared.The holding temperature was 800?C,900?C,950?C,1000?C,1050?C,1100?C,1200?C,respectively.The results were listed as follow:?1?CoCrFe₂Ni alloy is in recovery or initial recrystallization stage at 800?C and 900?C,and the grain morphology doesn't change,obviously.At 950?C,the alloy is in final recrystallization stage,and the grain size diminishes,severely.Moreover,amount of twin exists.Then,with the increase of holding temperature,the alloy is in grain growth stage,and the grain size enhances.Therein,the size of twin enhances,similarly,and the quantity decrease.The structure of CoCrFe₂Ni alloy doesn't change with the change of holding temperature.It is still single FCC structure;the lattice parameter doesn't change either.?2?The hardness of alloy decreases with the increase of holding temperature.The trend divides into two parts.It is linear below 1050?C,firstly.Then it tends to be gentle.Tensile test was done at 950?C,1000?C and 1050?C.The results indicate that the strength of CoCrFe₂Ni alloy increases after recrystallization annealing.However,the ductility doesn't decrease as regular phenomenon,even increases,partly.Furthermore,the strength decreases with the increase of holding temperature,and the ductility increases.The comprehensive analysis indicates that CoCrFe₂Ni alloy possesses greatest comprehensive properties when the holding temperature is 1000?C.At 1000?C,the value of hardness,tensile yield strength,tensile fracture strength and ductility are 136 HV,194 MPa,549 MPa and 70%.