Study On Recovery,recrystallization And Creep Behavior Of CoCrFeMnNi High Entropy Alloy

As a new type of structural material,multi-principal high entropy alloy（HEA）has attracted extensive attention in the world since it was proposed.In recent years,there is a growing consensus that HEA is less likely to replace traditional structural materials,despite its excellent properties.The application of HEA in some specific fields,such as high temperature and corrosion resistance,is more realistic.Therefore,the high temperature properties of HEA and its composites have attracted much attention.As one of the most representative HEAs,CoCrFeMnNi with good plasticity and strength is also one of the most ideal matrix materials for composite materials and has been widely studied.However,there are relatively few studies on the recovery and recrystallization behavior of CoCrFeMnNi.In addition,obvious disagreements exist in the creep behavior at intermediate-high temperature,which needs further clarification.In this thesis,equiatomic CoCrFeMnNi HEA was prepared by vacuum levitation melting.The alloy ingot was annealed at 1273 K for 6 h,cold-rolled with a thickness reduction rations R（=20,40,60 and 80%）and annealed at different temperatures.The recovery,recrystallization and grain growth behaviors are systematically studied.Furthermore,to obtain the effect of testing method on high temperature deformation behavior,the tensile creep of as-annealed alloy was tested by constant strain rate,constant stress（load）and stress decrement methods,respectively.（1）The dendrite structure of as-cast alloy disappears while homogeneous distribution of elements can be obtained after annealing at 1273 K for 6 h.However,apparent element redistribution occurs during cold-rolling.For low R,such as 20%and 40%,obvious Mn enrichment and slight Ni enrichment can be observed along grain boundaries,accompanied the corresponding deficiency of Co,Cr and Fe.At R=60%,the amount of Mn and Ni enrichment decreases while enrichment range obviously increases.With further increasing R to 80%,the amount of Mn and Ni enrichment increases markedly accompanied by the reducing range.The element enrichment caused by cold-rolling disappears gradually with increasing annealing temperature.（2）The hardness and tensile tests of both cold-rolled alloy and subsequently annealed one at different temperatures for 2 h was conducted.The microhardness of the 80%cold-rolled alloy is 390.4±6.2h V,the yield strength is 836±46 MPa,the tensile strength is 1256±119 MPa and the elongation is 7.5±0.4%.After annealing at 773 K,the hardness,yield strength and tensile strength increased to 420±10 HV,808±23 MPa and 1325±16 MPa,respectively,and the elongation decreased to 6.0±0.1%,which should be associated with the solute segregation to stacking faults and the formation of short range ordering.After annealing at 873-1073 K,the hardness decreases and the elongation increases markedly,indicating that the recovery occurs significantly.After annealed at 1073 K and higher temperature,the decreasing trend of hardness becomes slowly,suggesting that recrystallization and grain growth occur in this temperature region.（3）The recovery kinetics of cold-rolled alloys is systematically studied only for R=60%and 80%since nonuniform deformation of grains can be observed for R of40%and lower.The hardness of cold-rolled alloys was conducted after annealing at923-1023 K for 0-2 h with an interval of 25 K.The relationship between hardness and annealing time at different temperatures can be obtained.Different from the conventional method based on residual processing rate,the hardness as a function of annealing time can be directly nonlinear fitted.The results show that the recovery activation energies of 60%and 80%cold-rolled alloy are 148.9±7.6 and 155.1±5.0k J/mol,respectively.It suggests that the recovery is mainly controlled by dislocation（viscosity）sliding.In addition,the fitted equilibrium hardness at infinite annealing time decreases with the increasing annealing temperature,but it is still some higher than that of alloy without cold-rolling.It suggests that the microstructure formed during cold-rolling cannot return back to that of initial stage by recovery alone.（4）The cold-rolled alloy with R=80%was annealed at 1073-1273 K for 2 h with an interval of 50 K.The relationship between grain size and annealing temperature was obtained by EBSD analysis.The grain size as a function of temperature was also nonlinear fitted.The results show that the grain growth index is2 while the activation energy is 254±46 k J/mol.It indicates that the grain growth of alloy should be controlled by grain boundary motion,which is in well agreement with the classical theory of grain growth.In addition,the recrystallization temperature of alloy with R=80%should be between 1073 K and 1123 K.（5）The results of tensile tests at constant strain rate show that the alloy has no steady-state behavior in the range of 773-973 K and 10^-4-10^-3/s.The zigzag rheological curves can be observed for all tensile tests except the low-speed tension at973 K,which indirectly confirms the existence of dislocation viscous sliding.The creep test method has a great influence on the steady creep rate,but a relatively small effect on the creep mechanism.As a whole,the activation energy of creep is 130-150 k J/mol while the stress exponent n is 2–3 at low stress and low rate,indicating that it should be controlled by dislocation viscous sliding.At intermediate-high stress,the activation energy is about 340 k J/mol while n is 5-6,suggesting that it should be controlled by dislocation climb.With further increasing stress,the creep power law breaks.In addition,no matter at room temperature or intermediate-high temperature,EDS analyses at the fracture surface as well as line scanning near cracks show that apparent Mn enrichment can be observed.It indicates the redistribution of Mn atom should occur significantly during deformation.（6）the recovery of cold-rolled CoCrFeMnNi alloy at 923-1023 K is mainly controlled by dislocation（viscosity）sliding while the grain growth at 1073-1273 K is controlled by grain boundary motion.The creep test method has obvious influence on steady creep rate,but has relatively small influence on mechanism.Mn enrichment can be observed at the fracture for all deformation temperatures,indicating that dislocation viscous sliding should be one of the main mechanisms.