Mechanical Properties And Deformation Mechanisms Of Single-and Dual-phase Structured NiFeCo Series High Entropy Alloys

High-entropy alloys(HEAs)generally contain at least five elements as the principal components with atomic percentage between 5%and 35%.The HEAs break through the conventional design concept that has one or two principal elements as main components,and provide a totally new research field for alloys.The unique atomic structures of the HEAs result in high entropy effect,lattice distortion effect,slow diffusion effect and cocktail effect,performing high strength,high hardness,high plasticity,high cryogenic toughness and excellent thermal stability.Therefore,HEAs possess the potential of industrial applications and great value of theoretical research.The present work focused on the tensile properties and deformation mechanisms of HEAs.Firstly,we prepared AlCoCrFeNi_2.1eutectic high-entropy alloy(EHEA),by using the conventional eutectic alloy concept.Based on the tensile property at room temperature and characterizations of the microstructures,the deformation mechanisms,strengthening and toughening mechanisms in the EHEA were systematically investigated.Secondly,we designed and prepared Cr₂₆Mn₁₈Fe₂₂Co₂₀Ni₁₄HEA by appropriately changing the content of each element on the basis of equiatomic Cr Mn Fe Co Ni HEA to the lower stacking fault energy(SFE).Based on the tensile behavior and microstructural evolutions over a wide temperature range from 293 to 4.2 K,the effects of temperature on deformation mechanisms,strengthening and toughening mechanism were systematically analyzed and discussed.Finally,we taken the single FCC structured Ni₂Fe Co V_0.5Mo_0.2HEA as a model material,the dislocation structures,reactions and the slip mechanisms of the dislocations and the stacking faults(SFs)in the HEA were systematically analyzed and discussed,by means of TEM characterization,deformation kinetics and first-principles calculation.The main contributions of this dissertation are as following:(1)The AlCoCrFeNi_2.1EHEA solved the poor casting ability of HEAs,and possessed the combination of high strength and high ductility,possessing the great potential for industrial applications.Excellent mechanical properties resulted from the 3-D regular FCC(L1₂)/BCC(B2)lamellar structure and the unique deformation mechanisms of the two phases.Post-deformation analyses revealed a planar slip with characteristics of high density of parallel dislocation arrays and dense long straight{111}slip traces as well as massive SFs in the FCC(L1₂)phase,contributing to the high strain hardening and ductility of AlCoCrFeNi_2.1EHEA.Moreover,a high density of dislocations pinned by the Cr-enriched nano-precipitates lying on the two{110}slip bands was found in the BCC(B2)phase,contributing to the high strength.In addition,the back stress effect caused by the dual-phase lamellar structure also played the important role for the improvement of strength and ductility.(2)The Cr₂₆Mn₁₈Fe₂₂Co₂₀Ni₁₄HEA possesses high strength and high ductility over a wide temperature range from 293 to 77 K,more importantly,the strength and ductility increase simultaneously with the decrease of temperature.The yield strength(YS)increases by 215%from 143 to 450 MPa,ultimate tensile strength(UTS)increases by 137%from 746to 1022 MPa,and the ductility increases by 30%from 72%to 95%.With the decrease of temperature,SFs become the dominant deformation mechanism,replacing the unit dislocation activity which is suppressed at cryogenic temperature.The twinning-induced plasticity(TRIP)effect was enhanced,while the transformation-induced plasticity(TWIP)effect was weakened.With further decreasing the temperature to 4.2 K,the YS and UTS are continuously increased by 362%to 660 MPa,197%to 1283 MPa,respectively.While the tensile ductility decreased to 66%,and the HEA exhibits marked sudden slips,or stress-drops,called“serrations”behavior.At 20 and 4.2 K,the transformation governs deformation mechanism,while the long SFs and twins are difficult to find.The single TRIP effect contribute the limited strain hardening,resulting the decrease of ductility,and the continuous nano-transformation is the main reason for the“serrations”behavior.(3)The Ni₂CoFeV_0.5Mo_0.2HEA features superior combination of high strength and ductility at room temperature.The as-cast sample exhibits a YS of 240 MPa,and an UTS of980 MPa,as well as the tensile ductility of 90%,breaking the conventional strength-ductility rule of pure Ni and its alloys.The excellent mechanical properties resulted from the ultra-high work hardening ability,and the corresponding average work hardening rate n reaches 0.65,which is the highest reported value.The planar slip with with parallel dislocation arrays and SFs is the single deformation mechanism,and the strengthening mechanism is also dominated by the accumulation of high density dislocations in the HEA.During the slip process,heavy dislocation interactions happens between the 1/2<110>{111}dislocations,such as,12[-1 10](10)1 2[10-1](28)12[0 1-1]reacted between the dislocations on the same(111)slip plane and forming the hexagonal dislocation network,and12[01 1](10)1 2[10-1](28)12[110]reacted between the different dislocation arrays on(111)and(11?1)slip planes,forming massive Lomer sessile dislocations lying on<110>direction.Deformation kinetics and first-principles calculation reveal the extremely slow dislocation planar slip mechanism with nano-scale mean free path in the HEA.The activation volume ?V_ap^* in the as-cast coarse grain sample decreases from 100b³to 60b³,and the mean free path of dislocation slip (?) is only 2.5 nm,comparable to the value of the nanostructured and ultrafine grained materials.The first-principles calculation results show that the SFE of the{111}slip plane is about 50 m J/m²,which belongs to the medium-high SFE.What's more,the unstable stacking fault energy(USFE)for the HEA{111}<110>slip is not an unique value,but varies continuously in a broad range and forms peak profile.The results indicate that different energy barriers need overcome in slip and the dislocations might be obstructed by high energy barriers,resulting in the heavy accumulation of dislocations and the nano-scale planar slip,which is the fundamental reasons for the ultra-high work hardening ability and high strength,as well as high ductility of the HEA.