A Study On The Microstructure Control And Strengthening-toughening Mechanisms Of High Strength CoCrFeNiMn High Entropy Alloy

CoCrFeNiMn high entropy alloy（HEA）has attracted extensive attention in recent years due to its stable FCC phase,excellent mechanical properties at cryogenic temperature and resistance to hydrogen embrittlement.However,its yield strength（YS）at room temperature is not satisfactory.When melting and casting are used to produce it,the differences in melting points of constituent metals,elements segregation and evaporation of Mn prove challenging.Based on these problems,this study fabricates CoCrFeNiMn HEA consisting of ultrafine grains and nanoparticles by solid state powder metallurgy route and thus enhances its mechanical properties by the synergistic strengthening-toughening effect of ultrafine grains and nanoparticles.Firstly,this study focuses on the preparation of CoCrFeNiMn HEA by high energy mechanical milling and thermomechanical consolidation,and addresses several key process issues,including oxygen content control and extrusion crack elimination.Then,further study suggests that high energy mechanical milling of Co,Cr,Fe,Ni and Mn powders for 66 h achieve alloying,grain refinement and also introduction of C and O.Hot extrusion of the milled powders at 1000°C leas to the full-density consolidation of these milled powders and attains in-situ Cr-rich M₂₃C₆ and（Cr,Mn）₃O₄ particles.As a result,CoCrFeNiMn HEA consisting of ultrafine grains and nanoparticles is prepared.To refine the fairly large Cr-rich M₂₃C₆ particles,the strategy of microalloying learnt from the research of traditional alloys is adopted and the microstructure control is further studied.Results show that the addition of Nb refines M₂₃C₆ particles by forming Nb C particles with an average size of 139 nm,which are mainly located at the grain boundaries,while the addition of Ti changes M₂₃C₆ and（Cr,Mn）₃O₄ particles simultaneously by forming randomly distributed Ti O（C）nanoparticles with an average size of 57 nm.Quantitative analysis of the driving and drag forces for grain growth during hot extrusion reveals that the pining force from nanoparticles is the main force to inhibit grain growth.The addition of Nb and Ti increases this force effectively,leading to the grain size reduction.It also suggests that increasing the degree of deformation and refining the prior powder particles could also render similar drag force to refine grains as nanoparticles.Based on the problem that strengthening mechanism models in coarse-grained metals could not apply to ultrafine-grained metals,the interactions between grain boundary strengthening and nanoparticle strengthening are clarified.A model to estimate the superposition of strengthening mechanisms of metals consisting of ultrafine grains and nanoparticles is established.It reveals that intragranular particles strengthen by the Orowan mechanism,while the intergranular particles influence the YS by changing the Hall-Petch coefficient of grain boundary strengthening,which depends on the dislocations emitted from particle/matrix interfaces.Meanwhile,the study on the plastic deformation behavior suggests when the grains of CoCrFeNiMn HEA are refined to certain size,the low mobile dislocation density and high strain rate sensitivity lead to the transition of plastic deformation behavior.Yield drop,Lüders deformation,work hardening and then necking are observed.When the average grain size is lower than the critical size and the upper YS is higher than the maximum flow stress,necking occurs immediately after yield drop without Lüders deformation or work hardening.Therefore,the elongation decreases dramatically.For nanoparticles,this study reveals that intergranular nanoparticles become sources of cracks during deformation and have negative effects on the ductility,while intragranular nanoparticles pin dislocations to inhibit dislocation annihilation at grain boundaries and thus improve work hardening ability.Introducing intragranular nanoprecipitates through heat treatment reduces valid spacing between nanoparticles and inhibit dislocation annihilation more effectively.This suggests that introducing intragranular nanoprecipitates is an effective strategy to enhance YS and work hardening ability of HEAs with reasonable grain size（about 500 nm in this study）,leading to the high strength which is comparable to those of finer grain materials,and also better ductility.Furthermore,back stress is introduced to optimize comprehensive mechanical properties based on the idea of multilevel structural heterogeneity by controlling grain size distribution and nanoparticle location through Spark Plasma Sintering（SPS）.SPS before hot extrusion introduces two different regions,including Ti O（C）nanoparticles reinforced ultrafine-grained regions and Ti O（C）nanoparticles free coarse-grained regions.It has a high YS of 1298 MPa,a uniform elongation of9.4%and an elongation to fracture of 13.1%.Such comprehensive mechanical properties are much better than those without the multilevel structural heterogeneity.Analysis futher reveals that nanoparticles in ultrafine-grained regions enhance Young’s modulus difference between the two regions,produce extra back stress through strain gradient and increase resistance to the forward stress of the ultrafine-grained regions.As a result,such structure renders the higher back stress than those without nanoparticles and better comprehensive mechanical properties of CoCrFeNiMn HEA is achieved.Finally,nanotwins are introduced to further“refine”ultrafine grains and the YS enhanced to ultrahigh level,being 1507 MPa,which is 4-5times as much as those of traditional CoCrFeNiMn HEA by melting and casting.It exceeds that of the nanocrystalline CoCrFeNiMn alloy with an average grain size of only 10 nm.Meanwhile,a good tensile ductility is maintained with a uniform elongation of 4.7%and an elongation to fracture of 7.0%.Analysis shows that in such structure consisting of nanoparticles,ultrafine grains and nanotwins,they are main contributors to the YS.Particularly,the ultrafine grains are divided into finer domains by twin boundaries,which refines the grains effectively.Their synergistic strengthening effect can be estimated by the Hall-Petch relationship as long as the grain size is replaced by the domain size.Meanwhile,the pinning of dislocations by nanoparticles,the storing of dislocations by nanotwins and inhibiting of thread dislocation movement by nanoparticles and nanotwins with different orientations work together to enhance the work hardening ability.