| As a new type of alloy,multi-principal High entropy alloy(HEA)is composed of at least five principal elements with each volume fraction at 5-35%.Owing to the high entropy effect,lattice distortion effect and sluggish diffusion effect caused by the multi-principal nature,HEAs are prone to form simple solid solutions including face-centered cubic(FCC),body-centered cubic(BCC),or the mixed types,other than the complicated intermetallic compounds.In addition,nano-precipitates might form in the matrix under some heat treatment conditions.The distinctive structure leads to solution strengthening and precipitation strengthening,which contributes to the remarkable mechanical properties of HEAs,such as the good tensile strength and ductility,the excellent high temperature creep resistance and thermal stability.However,the intrinsic mechanism of high temperature deformation and fracture for HEAs is still lack of comprehensive interpretation and need to be further investigated.In the present study,the CoCrFeNiMn HEA was prepared by vacuum electromagnetic induction melting and drop casting method.To investigate the effect of thermomechanical processing on the microstructural evolution and the mechanical properties,some procedures including homogenization,cold rolling and recrystallization annealing were performed.Tensile tests of the thermomechanically treated alloy at room and elevated temperatures were conducted and the mechanism of serrated flow behavior was analyzed with the combination of microstructural characterizations and theoretical calculations.Moreover,the high temperature tensile creep behavior was investigated.To examine the effect of Al addition on the microstructures and mechanical properties of the alloy,AlxCoCrFeNiMn(x=0.4,0.5,0.6)HEAs were synthesized using the same processing route.The phase constitutions,microstructures and tensile properties of the alloys with different Al contents were characterized.Additionally,the tensile creep behavior of Al0.5 alloy at high temperatures was investigated.The results showed that,the microstructure of the CoCrFeNiMn alloy evolved from coarse dendritic and interdendritic structures into homogenous and refined equiaxed structures after thennomechanically treated.Only simple FCC phase was detected and no extra second phase formed.Higher rolling ratio or lower recrystallization temperature resulted in more refined grain size.The most refined alloy with the average grain size of 25 ?m had the ultimate tensile strength of 580 MPa and the elongation of 56%at room temperature(RT).The alloy exhibited excellent dynamic strain aging(DSA)ability in the intermediate temperature range,with the highest strain hardening exponent of 0.42 at 500 ?.Meanwhile,prominent serrations were observed at 300-600 ? and evolved in the sequence of A?A+B?B?B+C?C with increasing temperature or decreasing strain rate during plastic deformation.The largest serration amplitude appeared in type C at 3×10-4 s-1/600?(?6.7 MPa)and type B+C at 500?/1×10-5 s-1(?8.9 MPa).Dislocation substructures revealed low density of short and straight pile-ups at grain boundaries during the initial stages of plasticity(?1%strain)and high density of dislocations tangled to form cell substructures during severe deformation stage(?20%strain).Numerous bowing and kinks of dislocations were observed at 400 and 600 ?,manifesting the pinning process by solutes.Based on the solute drag model and the quasi-static aging model,the activation energy for serrated flow was evaluated to be 116 kJmol-1 at 300-500?,indicating the pipe diffusion is responsible for the pinning process of dislocations.In contrast,a higherQ(295 kJmol-1)between 500 and 600 ?was obtained,suggesting the cooperative lattice diffusion mechanism was dominated and the most sluggish diffusion species(i.e.Ni)was rate controlling.The tensile creep behavior of the 25 ?m grain-sized alloy exhibited two distinct regions with a stress-dependent transition.In low stress region(LSR),stress exponents were in the range of 5-6,and average activation energy was evaluated to be 268 kJmol-1.In contrast,high stress region(HSR)showed much larger values of 8.9-14 and 380 kJmol-1,respectively.Microstructural examinations of the interrupted samples revealed the evidence of a number of jog configurations,indicating that dislocation climb occurred during the creep process.In addition,remarkable dynamic recrystallization was observed at higher stress levels,and dynamic precipitates identified as M23C6 or Cr-rich ? phase were predominantly formed along grain boundaries.Therefore,it was proposed that,in both stress regions,the creep deformation was dominated by stress-assisted dislocation climb controlled by lattice diffusion.However,the larger stress exponents in HSRs were ascribed to the coordinative contributions of dynamic recrystallization and precipitation-inducing boundary obstacle stress.Microstructural examinations of the thermomechanical processed AlxCoCrFeNiMn(x=0.4,0.5,0.6)HEAs revealed that the volume fractions of AlNi-rich bcc phase increased with increasing Al content,while the average grain sizes showed an inverse variation.Due to the bcc phase strengthening effect and the"Hall-Petch" relationship for grain size,the Al0.6 alloy exhibited excellent tensile properties with yield strength and ultimate strength at room temperature(RT)up to 348MPa and 801 MPa,respectively.Similar to the CoCrFeNiMn alloy,notable DSA ability was observed at 300-600? for Alx alloys,which was ascribed to the serrated behavior at the same temperature range.The creep behavior of the Al0.5 alloy showed a temperature-dependent transition.At 500? and 550?,the stress exponents were in the range of 2.6-3,and the average activation energy was evaluated to be 201 kJmol-1,indicating that the creep process was dominated by viscous glide of dislocations controlled by pipe diffusion.Distinctly,the stress exponents at 600? and 650?were in the range of 4.6-5.4,and the average activation energy was 411 kJmol-1,suggesting a dislocation-climb mechanism which was controlled by solute lattice diffusion. |
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