Study On The Compositional Design,Microstructure And Mechanical Properties In The Heavily Distorted Medium/High Entropy Alloys

Medium and high entropy alloys are one of the fastest-developing new materials in the field of metallic materials.The emergence of medium and high entropy alloys breaks the design concept of traditional alloys,and the development of components in the phase diagram of high principal element alloys extends from the past terminal solid solution alloy to the area in the center of the phase diagram.Compared with traditional diluted solid solution alloys,medium and high entropy alloys have more metastable states,so they have a wider range of state parameters and performance control,which may break through the performance limits of traditional metallic materials and give full scope to their advantages in service under extreme conditions.In this work,the binary alloys（Ni Pd）,medium entropy alloys（MEAs）（Ni Co Pd,NiCrPd,and NiFePd）,and high entropy alloy（HEA）（Ni Co Cr Fe Pd）with fully-recrystallized microstructure were investigated to understand the kinetics of grain growth and solid solution hardening effect,and mechanical behavior of NiCrPd and NiFePd ternary MEA at room temperature and cryogenic temperature.Additionally,Al_0.3CoCrFeNi HEA high-temperature tensile creep behavior and its strengthening mechanism was explored in detail.The results of the study are as follows:（1）In the temperature range of 800～900℃,the grains grew slowly with the number of principal elements increasing in alloys,while at 1000℃ and above,NiCrPd and Ni Co Cr Fe Pd alloys presented the inverse trend.Among all the studied MEAs,NiCrPd alloy exhibited superior thermal stability with the highest activation energy and the slowest grain growth.Ni Co Cr Fe Pd and Ni Co Pd alloys have the highest hardness and the lowest hardness values,respectively,indicating that the strengthening effect in solid solution alloys cannot be predicted simply by the number of elements.In the MEAs,the hardness of NiCrPd based on the estimation of modulus mismatch and electronegativity difference does not stand out at all,indicating that the type of alloying element played a pronounced role.The yield strength of NiCrPd at 673K is 477MPa.When the temperature is increased to 873K,the yield strength of the alloy decreases less,still reaching 436.1MPa,and the elongation is about 35%.It has higher high-temperature yield strength than reported ternary and quaternary alloys.（2）At 293 K,the deformation mechanism of NiFePd HEA is mainly dominated by ordinary dislocation slip.As the temperature decreases to 20K,extensive deformation twins,stacking faults（SFs）,and amorphous bands（ABs）formed in NiFePd HEA,which can hinder dislocations motion,promoting dislocation accumulation and contributing to strain hardening.Consequently,the dominant deformation mechanism changes from a dislocation slip to deformation twin,SFs,and ABs cooperation mechanism.A decrease in temperature leads to a decrease in stacking fault energy（SFE）,leading to the easier activation of SFs and deformation twins.（3）The face-centered cubic solid solution of non-equiatomic Al_0.3CoCrFeNi is unstable under the coupling of applied load and temperature for a long time,and the face-centered cubic matrix will decompose into Cr-rich phase,L1₂phase,and slat-like B2 phase.Especially when alloy creeps at 973K for a long time,the face-centered cubic matrix will decompose into a large number of L1₂ phases,which is contrary to the previous studies that this alloy can only produce L1₂ phase at 893K and below.It is reported that Al_0.3CoCrFeNi alloy is unstable and usually decomposes into B2,rather than L1₂ phase at 973K,indicating that the formation of L1₂ phase is closely correlated with the applied load during the creep deformation.In addition,through the systematic characterization of the deformed samples,it is found that the L1₂ and B2 precipitated phases with different morphologies interact with defects such asdislocations and delamination,including dislocation bypass,accumulation,and wind,etc.,and the high-density precipitated phase can form a strong pinning effect on the movable dislocations,which significantly improves the creep strength of the alloy,and the stress factor（3～6.5）and activation energy（390～580k J/mol）of the alloy are significantly greater than the creep parameters of the reported high-entropy alloy.It was shown that the excellent creep resistance of the alloy was attributed to the self-precipitation strengthening effect and low fault energy of Al_0.3CoCrFeNi alloy.