Co_1-xAl_xCrFeMnNi HEAs Study On Mechanical Properties And Creep Properties At High Temperature

The study on high temperature deformation of high entropy alloy(HEA),a new superalloy,has attracted more and more attentions.However,the creep behavior of HEAs,which occurs in practical application,is less studied while relatively more work on the tensile property at elevated temperatures is reported.Moreover,the limited work on the creep of HEA is usually carried out in compressive mode with a nanoindenter.It cannot fully represent the macroscopic creep behavior of HEAs due to its limited deformation region.In addition,the difference between the high temperature creep behavior of HEA and that of conventional alloys and nanocrystalline materials is also one of the important scientific problems to be clarified.In this dissertation,three kinds of Co_1-xAl_xCrFeMnNi HEAs with Al contents of 0,0.5 and 1 were prepared by vacuum induction melting technique.Both the room temperature tensile and high temperature creep properties are systematically studied.Combined with XRD,SEM+EDS,EBSD and TEM observation,the effect of Al content on the mechanical properties is analyzed in detail.The crystal structures of three Co_1-xAl_xCrFeMnNi HEAs with x=0,0.5 and 1 are FCC,BCC+FCC and BCC,respectively.Significantly Mn-rich in inter-dendrite and Mn-deficient in dendrite is observed for x=0 alloy while relatively homogeneous distribution of elements exists for x=1 alloy.However,apparent composition fluctuation can be found in different dendrite for x=1 alloy.Both hardness and brittleness of as-cast alloy increase significantly with x.The tensile test of x=1 alloy cannot be carried out.After annealing at 673-1273 K for 6 h,the dendrite structure gradually disappeared with increasing temperature while the element segregation decreased obviously.However,a phase decomposition of x=0.5 alloy occurs obviously after annealing at 1073 K.The tensile properties at room temperature show that only x=0 alloy has excellent tensile plasticity.After annealing,the yield strengths_0.2decreases,tensile strengths_bincreases,and elongationdincrease significantly.Thes_0.2,s_banddof CoCrFeMnNi annealed at 1273 K+6 h are 264±12 MPa,557±30 MPa and46.3±9.9%,respectively.However,for Co_0.5Al_0.5CrFeMnNi alloy,the increasedstrength accompanied by the decreasingdcan only be obtained after annealing at 673K.With further increasing annealing temperature,the tensile property decreases markedly due to the phase decomposition.No tensile test can be completed after annealing at 1073 K since of the high brittleness.Although thedof CoCrFeMnNi is high at room temperature,the apparent crack during cold-rolling can be observed once the the area reduction ratio exceeds 10%.It indicates that a significant cold work hardening occurs during cold-rolling.The SEM observation on tensile fracture shows that many dimples along with the second phase particles can be observed for x=0 alloy while several fracture facets with regular particles appear for x=0.5 one.The EDS results of x=0 alloy show that apparently higher Mn content can be obtained at both dimples and particles in the bottom of simples.It suggests that the dislocation viscous slip dragged of Mn atoms should be the main tensile deformation mechanism.For the x=0.5 alloy,obvious fluctuation of composition at fracture can observed,similar to the undeformed sample.The creep behavior of CoCrFeMnNi annealed at 1273 K is studied systematically in 773–973 K and steady state creep rate of about 10^-9?10^-5S^-1since a significant tensile plasticity can only be observed in this alloy.Different with the primary creep stage of conventional polycrystalline metals or alloys,a novel stage with almost constant creep rate can be observed at the beginning of creep.Accordingly,the primary creep is divided into the primary I stage(constant creep rate)with duration is less than 60 s and primary II(decreasing creep rate)one with several hours.The corresponding strain is represented bye_PIande_PII,respectively.The total straine_p(=e_{p I}+e_PII)in the first stage depends strongly upon temperature and stress,which can be as high as 40%;moreover,almost alle_pis contributed bye_{p I}.The primary I creep should be associated with the dislocation slip without solute atom drag.The systematic study on the relationship between steady state creep rate?_sand stresssand temperature shows that the steady state creep of CoCrFeMnNi HEA still obeys the exponential law approximately.In the temperature range of 823-973 K,the n is about 3 in low?_sregion while it is about 6 in high?_sregion while n is about4.8 at 773 K.In addition,a transition of the creep activation energy Q occurs between823 to 873 K.the Q varies from 134 k J/mol at low temperature region to 302 k J/mol at high temperature one,which is consistent with the results in literatures.If the true steady state creep rate is used instead of the nominal creep rate?_s,the n in both low and high temperature regions is obviously decreased.Different with the conventional alloy,the Q obtained by stress jump creep test is obviously lower than that by constant stress creep test.It may be associated with the existence of largee_{p I}.Combined with the apparent Mn-rich in both dimples and particles in the bottom of dimples,the creep mechanism of CoCrFeMnNi HEA should be the dislocation viscous slip dragged by Mn atoms in low temperature and low?_sregion while it be dislocation climb involved Mn atom diffusion in high temperature and high?_sregion.In summary,the effects of Al content on the microstructure and mechanical properties of Co_1-xAl_xCrFeMnNi high entropy alloy are studied systematically.It is found that only CoCrFeMnNi owns excellent tensile plasticity whether it is annealed or not.A short creep stage with approximately constant creep rate can be observed.No fracture occurs even that the strain associated with this short stage can be as high as 38%.It indicates that the alloy has a potential application in the field of energy absorption.With increasing creep temperature,the steady state creep mechanism will change from the dislocation viscous slip dragged by Mn atoms to the dislocation creep involved Mn atom diffusion.